Which you add during initialization to change values. The Java language has a wide range of modifiers, the main ones are:

• access modifiers;
• class, method, variable, and thread modifiers not used for access.

To use a modifier in Java, you must include its keyword in the definition of a class, method, or variable. The modifier must come before the rest of the operator, as shown in the following examples:

public class className ( // ... ) private boolean myFlag; static final double weeks = 9.5; protected static final int BOXWIDTH = 42; public static void main(String arguments) ( // method body )

Access Modifiers

Java provides a number of access modifiers to set access levels for classes, variables, methods, and constructors. There are four accesses:

• Visible in the package (default and no modifier required).
• Visible only to the class (private).
• Visible to all (public).
• Visible to the package and all subclasses (protected).

Default access modifier - no keyword

Default access modifier- means that we do not explicitly declare an access modifier in Java for a class, field, method, etc.

A variable or method declared without an access control modifier is available to any other class in the same package. Fields in an interface are implicitly public, static, final, and methods in an interface are public by default.

Example

Variables and methods can be declared in Java without any modifiers, as shown in the following example:

String version = "1.5.1"; boolean processOrder() ( return true; )

private access modifier

private modifier- Methods, variables and constructors that are declared private in Java can only be accessed within the declared class itself.

The private access modifier is the most restrictive access level. Class and interfaces cannot be private.

Variables declared as private can be accessed outside the class if the public methods that receive them are present in the class (see example and explanation below).

Using the private modifier in Java is the main way to hide data.

Example

The following class uses private access control:

Public class Logger ( private String format; public String getFormat() ( return this.format; ) public void setFormat(String format) ( this.format = format; ) )

Here the variable format class logger is private, so there is no way for other classes to get and set its value directly.

So to make this variable available to everyone, we have defined two public methods: getFormat(), which returns a value format, And setFormat(String), which sets its value.

public access modifier

public modifier- class, method, constructor, interface, etc. declared as public can be accessed from any other class. Therefore, fields, methods, blocks declared inside the public class can be accessed from any class belonging to the "universe" of Java.

However, if we are trying to access a public class in another package, then the public class must be imported.

Thanks to class inheritance, in Java, all public methods and variables of a class are inherited by its subclasses.

Example

The following function uses public access control:

Public static void main(String arguments) ( // ... )

Method main() must be public. Otherwise, it cannot be called from the java interpreter to run the class.

protected access modifier

protected modifier- Variables, methods, and constructors that are declared protected in a superclass can only be accessed by subclasses in another package, or by any class in the protected class package.

The protected access modifier in Java cannot be applied to classes and interfaces. Methods and fields can be declared protected, but methods and fields in an interface cannot be declared protected.

Protected access gives a subclass the ability to use a helper method or variable, preventing an unrelated class from attempting to use it.

Example

The following parent class uses protected access control so that its child class overrides the method openSpeaker():

Class AudioPlayer ( protected boolean openSpeaker(Speaker sp) ( // implementation details ) ) class StreamingAudioPlayer ( boolean openSpeaker(Speaker sp) ( // implementation details ) )

However, if we define a method openSpeaker() as protected, it will not be accessible from any class other than AudioPlayer. If we define it as public, then it will become available to everyone. But our intention is to only expose this method to the subclass, which is why we used the protected modifier.

Access Control and Inheritance Rules

The following rules in Java apply to inherited methods:

• Methods declared public in a superclass must also be public in all subclasses.
• Methods declared protected in a superclass must either be protected or public in subclasses; they cannot be private.
• Methods declared as private are not inherited by everyone, so there is no rule for them.

Class, method, variable, and thread modifiers not used for access

Java provides a number of modifiers not for access, but for implementing many other functionalities:

• modifier static used to create methods and class variables;
• modifier final used to complete the implementation of classes, methods, and variables;
• modifier abstract needed to create abstract classes and methods;
• modifiers synchronized And volatile are used in Java for threads.

static modifier

static modifier- used to create methods and class variables.

static variables

The static keyword is used to create variables that will exist independently of any instances created for the class. Only one copy of a static variable exists in Java, regardless of the number of instances of the class.

Static variables are also known as class variables. In Java, local variables cannot be declared static.

static methods

The static keyword is used to create methods that will exist independently of any instances created for the class.

In Java, static or static methods do not use any instance variables of any class object, they are defined. The static methods take all the data from the parameters and some of these parameters are evaluated without reference to the variables.

Class variables and methods can be accessed using the class name followed by a period and the name of the variable or method.

Example

The static modifier in Java is used to create class methods and variables, as shown in the following example:

Public class InstanceCounter ( private static int numInstances = 0; protected static int getCount() ( return numInstances; ) private static void addInstance() ( numInstances++; ) InstanceCounter() ( InstanceCounter.addInstance(); ) public static void main(String arguments ) ( System.out.println("Starting from " + InstanceCounter.getCount() + " instance"); for (int i = 0; i

The following result will be obtained:

Starting from instance 0 Created 500 instances

final modifier

final modifier- used to complete the implementation of classes, methods and variables.

final variables

A final variable can only be initialized once. A reference variable declared as final can never be assigned to refer to another object.

However, the data inside the object can be changed. Thus, the state of the object can be changed, but not the reference.

With variables in Java, the final modifier is often used with static to make a class variable a constant.

Example

public class Test( final int value = 10; // Here are examples of constant declarations: public static final int BOXWIDTH = 6; static final String TITLE = "(!LANG:Manager"; public void changeValue(){ value = 12; //будет получена ошибка } } !}

final methods

The final method cannot be overridden by any subclass. As mentioned earlier, in Java, the final modifier prevents a method from being modified by a subclass.

The main intention of making a method final would be that the content of the method should not be changed by the side.

Example

The declaration of a method that uses the final modifier in a class declaration is shown in the following example:

Public class Test( public final void changeName()( // method body ) )

final class

The main purpose in Java of using a class declared as final is to prevent the class from being subclassed. If a class is marked final, then no class can inherit any function from the final class.

Example

public final class Test ( // class body )

abstract modifier

abstract modifier- used to create abstract classes and methods.

abstract class

The abstract class cannot be instantiated. If a class is declared abstract, then its only purpose is to be extended.

A class cannot be both abstract and final, because a final class cannot be extended. If a class contains abstract methods, then it must be declared abstract. Otherwise, a compilation error will be generated.

An abstract class can contain both abstract methods as well as ordinary ones.

Example

abstract class Caravan( private double price; private String model; private String year; public abstract void goFast(); //abstract method public abstract void changeColor(); )

abstract method

An abstract method is a method declared with any implementation. The method body (implementation) is provided by the subclass. Abstract methods can never be final or strict.

Any class that extends an abstract class must implement all the abstract methods of the superclass, unless the subclass is an abstract class.

If a class in Java contains one or more abstract methods, then the class must be declared abstract. An abstract class is not required to contain abstract methods.

An abstract method ends with a semicolon. Example: public abstract sample();

Example

public abstract class SuperClass( abstract void m(); //abstract method ) class SubClass extends SuperClass( // implements abstract method void m()( ......... ) )

synchronized modifier

synchronized modifier

The synchronized keyword is used to indicate that a method can only be accessed by one thread at a time. In Java, the synchronized modifier can be applied with any of the four access level modifiers.

Example

public synchronized void showDetails()( ....... )

transient modifier

An instance variable marked transient tells the Java Virtual Machine (JVM) to omit the specified variable when serializing the object containing it.

This modifier is included in the statement, which creates a variable of the variable's predecessor class or data type.

Example

public transient int limit = 55; // will not persist public int b; // will be saved

volatile modifier

volatile modifier- are used in Java for threads.

In Java, the volatile modifier is used to let the JVM know that a variable access thread should always merge its own copy of the variable with the main copy in memory.

Accessing a volatile variable synchronizes all cached copied variables in RAM. Volatile can only be applied to instance variables that are of type object or private. A reference to a volatile object can be null.

Example

public class MyRunnable implements Runnable( private volatile boolean active; public void run()( active = true; while (active)( // line 1 // some code here) ) public void stop()( active = false; / / line 2 ) )

Typically, run() is called on one thread (this is the first time you start using Runnable in Java) and stop() is called on another thread. If the cached value active is used on line 1, then the loop cannot stop until you set active to false on line 2.

In the next lesson, we will discuss the basic operators used in the Java language. This section will give you an overview of how you can use them during application development.

Here we will try to consider almost all cases of using access modifiers. The only exception is their use for nested ( nested) and internal ( inner) classes, as well as for interfaces, since we have not yet considered these topics.

Classes and packages, used in conjunction with access modifiers, serve as a means of encapsulation, that is, a means of hiding implementation details behind a simple interface.

Access modifiers can be applied to both classes and their members - fields and methods. There are four access modifiers in total, and here we will give a brief description of them, then we will consider each in detail.

• public- any component declared as public, accessible from any code
• protected- allows access to the component within the package and classes to descendants
• private- allows access to components within the class
• default(no keyword) - allows access to components within the package

Descendant classes are classes inherited from a class. Inheritance we have not yet studied.

Access to classes

By default, top-level classes are available in the package in which they are defined.. However, if the top-level class is declared as public, then it is available everywhere (or wherever the package itself is available). We have limited this statement to top-level classes because classes can be declared as members of other classes. Because these inner classes are members of the class, they are subject to the access control rules for class members..

Access to class members

Class members are always available within the class body. Default class members are also available in the package in which the class is defined.

public modifier

For a non-nested class, only one of two possible access levels can be specified: given default And public . When the class is declared as public, it should be the only one public class declared in the file, and the file name must match the class name.

How public classes, fields, methods and constructors can be declared.

protected modifier

We will discuss this modifier in detail in the topic of class inheritance. If inheritance is not used, then this modifier works, just like the default modifier.

The only thing that can now be briefly said is that for components declared as protected, will have access any child class from any package or any class from the same package.

How protected fields, methods, constructors, nested classes, and nested interfaces can be declared.

protected .

private modifier

This is the most restrictive access modifier. Elements declared as private available only within the same class and not to anyone outside the class.

How private fields, methods, constructors, nested classes, and nested interests can be declared.

Top-level classes and interfaces cannot be declared as private .

In essence, access modifiers are a simple topic, but we will return to it later. For now, it was just an introduction. And now for some practice...

I created the Mod02.java, DefMod.java, ProMod.java and PrvMod.java classes which belong to the pro.java.pkg002 package, and the PubMod.java class which belongs to the pro.java.pkg003 package. Next, I’ll just give screenshots of these classes and the result of the program:

5

I saw some discussions on StackOverflow about this topic, but I don't see anything that helped me understand the following point:

I come from a C++ background and recently I started learning Java. In C++ when protected, only the subclass that can access the member is used (analogous to a field in Java).

There are also "friend" classes in C++ that can access private/secure class chambers that give "friendship". This is a bit like the "package" field modifier in Java (the default field modifier), except that in C++ friendship gives access to all private members, but in Java, access from classes in the same package is class field specific,

What I can't figure out, assuming I only want to grant access to subclasses, is what I can do in C++ by declaring protected members in a class that doesn't "give" friendliness.

But in Java, I don't know how to do this, since with the "protected" field modifier - I also give access to all classes in the package. The only way I find to do this is to declare a protected field and isolate the class in your package.

From this I conclude that the grouping of classes into one package should be done on the basis of "friendship" between classes. Is this really the leading factor when grouping packages?

One more thing I don't understand, In Java, assuming I have two fields in class A: b, c. I want to give B access to b but not to, and I want to give C access to c but not b. and to "World" I want b, c to hide. How can I do that? I assume that B, C must be in the same package as A. , but by declaring b, c with package modifier I allow B, C access to both b and c. Is there a way in Java to do this?

Hope for some explanation of this question

11

A better question if less useful to you would be more narrow and specific. The general question "everything about privacy in Java and C++ and how they differ" is more than too broad. Can you ask a more specific question about a more specific problem? - Yakk 04 Mar 15 2015-03-04 16:38:58

• 4 answers

Sorting:

Activity

2

In C++, when protection is used, only a subclass can access the element (analogous to a field in Java).

Access specifiers are also for member functions/methods, not just member variables.

In C++ there are also "friend" classes that can have access to the private/protected mambers of the class, which gives "friendship". This one is a bit like the "package" field modifier in Java (the default field modifier), except that in C++ friendship gives access to all private members, but in Java, access from classes in the same package is class field specific.

There are not only friend classes, but also functions.

It is true that access to Java private parts is similar, but it is not a complete replacement. It is better to say that these two functions have subset the problems they solve. There are problems that can be solved by friend but not package-private, and vice versa.

What I couldn't figure out, assuming I want to only grant access to subclasses, is what I can do in C++ by declaring users protected in a class that doesn't "give" friendship.

But in Java, I don't know how can I do it,

Answer: You can't.

since with the "protected" field modifier - I also grant access to all classes in the package.

The only way I find is to declare a protected field and have the class isolated in its package.

Technically, yes. But this creates other problems. Your class will no longer be able to access package private parts of its previous package. Let's say your BaseClass was in com.example.one . You will move it to com.example.two . Now it will no longer be able to access other package-private classes of com.example.one .

Is this really the leading factor when grouping packages?

Yes, Java is designed that way. You may try fight the rules of the language, but it's a losing battle in any programming language.

Another thing I don't understand is in Java, assuming I have two fields in class A: b, c. I want to give B access to b but not to, and I want to give C access to c but not b. and in "World" I want b, c to hide. How can I do that?

It can't be done in a clean way (clean I mean: without any hacks that would require you to check the runtime call stack and throw exceptions).

If you're concerned about this scenario, since you're developing a public API, a low-tech solution that usually works great is to create one or more *.internal packages and clearly document the fact that they shouldn't be used in client code.

1

That's quite a bunch of questions...

But in Java, I don't know how I can do this, as by virtue of using the "protected" field modifier - I also grant access to all classes in the package.

Indeed, there is no way to grant access only to subclasses and not to classes in the same package. It was a design decision made centuries ago...

The only way I find to do this is to declare a protected field and isolate it in my package.

This is technically correct, although it will be of little use. Class packaging is for grouping related classes, where "related" means "classes that fulfill a particular relationship", i.e. they belong to the same use case, belong to the same architectural level, are in the same entities, etc.

From this I conclude that the grouping of classes into one package should be done on the basis of "friendship" between classes. Is this really the leading factor when grouping packages?

I believe I already answered this in the previous paragraph: packaging is for grouping related classes according to some specific criteria.

For your A, B and C classes, for example with attributes:

I think B, C should be both in the same package as A. a declaring b, with packaging modifier I let B, C access both b and c. Is there a way in Java to do this?

The answer is no, there is no simple and clean way to do this. You could achieve this with some hacks or more advanced techniques, but again, this was part of the decisions made by the language designers a long time ago...

0

Short answer: there is no way to do this.

If you're worried about intrusion from injecting class clients in a package to gain unauthorized access, you can move the sensitive code in a separate package, and make the package sealed in the jar you deliver it to: http://docs.oracle.com/javase/tutorial /deployment/jar/sealman.html

1

It is implicitly assumed that all classes in a package "know" each other (because they were written by the same person/company/organization). So they either don't get access to protected fields, or if they do, they know how to do it properly.

Classes in the same package are supposed to be more related to each other than a parent is to a derived class, because the derived class might actually be written by someone else. So they decided that private protection was more limited than secure.

So I guess you don't have to worry about how classes in the same package can access each other's fields. In general, I just don't use this feature except when I'm writing iterators.

If you have two fields, you can make them inner classes so that they have access to private fields (again, logic: if a class is inside another class, it knows about the semantics of that class) and can provide that access to their derived classes through protected methods.

Of course you could come up with a complex token exchange protocol to make this field only available to B/C instances, but that would be a wonderful overhead and another object could still use reflection to access all private members if you don't disable it with security policies, which is usually not the case, but again, security policies are ultimately decided by the owner of the JVM.

So, ultimately the preferred way to do what you say in Java is to either put them in the same package, or write B and C as inner classes of A so they can directly access A's private members and expose them to them derived classes.

Public class A ( public static abstract class B ( protected Whatever getWhatever(A a) ( return ab; ) protected void setWhatever(A a, Whatever value) ( ​​ab = value; ) ) public static abstract class C ( protected Whatever getWhatever(A a) ( return ac; ) protected void setWhatever(A a, Whatever value) ( ​​ac = value; ) ) private Whatever b; private Whatever c; )

once again, you always assume that the classes in the same package will never do anything wrong.

First, let's deal with access modifiers. There are only four of them:

• private class members are only accessible within the class
• package-private or default (default) class members are visible inside the package
• protected class members are available inside the package and in derived classes
• public class members are available to everyone

During inheritance, it is possible to change access modifiers towards GREATER visibility.

The access modifier for constructors, methods and fields can be any, but with classes and their blocks, everything is not so simple. A class can only be either public or default, and only one public class can be in one file. A block can have only one modifier - default.

static, abstract, and final modifiers

Static

• Applies to inner classes, methods, variables, and logic blocks
• Static variables are initialized at class load time
• Static variables are the same for all objects of the class (the same reference)
• Static methods only have access to static variables
• Static methods and variables can be accessed via the class name
• Static blocks are executed at class load time
• Non-static methods cannot be overridden as static
• Local variables cannot be declared as static
• Abstract methods cannot be static
• Static fields are not serialized (only when implementing the Serializable interface)
• Only static class variables can be passed to a constructor with parameters, called via the word super(//parameter//) or this(//parameter//)

Abstract

• Applies to methods and classes only
• Abstract methods do not have a method body
• It is the opposite of final: final class cannot be inherited, abstract class must be inherited
• A class must be declared abstract if:

1. it contains at least one abstract method
2. it does not provide implementation of inherited abstract methods
3. it does not provide an implementation of the methods of the interface it declared to implement
4. it is necessary to prohibit the creation of instances of the class

Final

• Fields cannot be changed, methods are overridden
• Classes cannot be inherited
• This modifier only applies to classes, methods and variables (also local variables)
• Method arguments marked as final are read-only, attempting to change will result in a compile-time error
• Final variables are not initialized by default, they must be explicitly assigned a value when declared or in the constructor, otherwise a compilation error
• If a final variable contains a reference to an object, the object can be changed, but the variable will always refer to the same object.
• This is also true for arrays because arrays are objects - an array can be changed and a variable will always refer to the same array.
• If a class is declared final and abstract (mutually exclusive), a compilation error will occur
• Since a final class cannot be inherited, its methods can never be overridden.

Constructor cannot be static, abstract or final

strictfp, transient, volatile, synchronized, native modifiers

Strictfp

• Applies to methods and classes
• Provides operations on float and double (floating point) numbers according to the IEEE 754 standard

Transient

• Only applies to class-level variables (local variables cannot be declared transient)
• Transient variables need not be final or static.
• Transient variables are not serialized

Volatile

• Only used with variables
• Can be used with static variables
• Not used with final variables - The value of a variable declared as volatile, changed by one thread, changes asynchronously for other threads
• Used in multi-threaded applications

Synchronized

• Applies only to methods or parts of methods
• Used to control access to important parts of the code in multithreaded programs

native

• Only used for methods
• Indicates that the method is written in another programming language
• Classes in Java use many native methods to improve performance and access to hardware
• It is possible to betray/return Java objects from native methods
• Method signature must end with ";", curly braces will cause compilation error

Features in interfaces

• Methods are always public and abstract, even if not declared
• Methods cannot be static, final, strictfp, native, private, protected
• Variables are only public static final even if it is not declared
• Variables cannot be strictfp, native, private, protected
• Can only extend another interface, but not implement an interface or class (implements).

Putting all the modifiers together:

	Class	inner class	Variable	Method	Constructor	Logic block
public	Yes	Yes	Yes	Yes	Yes	Not
protected	Not	Yes (except local and anonymous classes)	Yes	Yes	Yes	Not
default	Yes	Yes		Yes	Yes	Yes
private	Not	Yes (except local and anonymous classes)	Yes	Yes	Yes	Not
final	Yes		Yes (and for local variable)	Yes	Not	Not
abstract	Yes	Yes (except anonymous classes)	Not	Yes	Not	Not
static	Not	Yes (except local and anonymous classes)	Yes	Yes	Not	Yes
native	Not	Not	Not	Yes	Not	Not
transient	Not	Not	Yes	Not	Not	Not
synchronized	Not	Not	Not	Yes	Not	Yes (only as part of a method)
volatile	Not	Not	Yes	Not	Not	Not
strictfp	Yes	Yes	Not	Yes	Not	Not

Modifier class
The Modifier class encodes all modifiers
used in type declarations, in the form
constants:
ABSTRACT, FINAL, INTERFACE, NATIVE,
PRIVATE, PROTECTED, PUBLIC, STATIC,
STRICT, SYBCHRONIZED, TRANSIDENT,
VOLATILE.
Each of the constants corresponds to a request method of the form
isMod(int modifier) ​​(here Mod is one of the above
given names, for example, isPublic),
which returns true if the modifier
mod is present in the type declaration.

Consider an example. Let there be
field declaration
public static final int s=10;
then the return value of the method
getModifiers of the corresponding object
of the Field class will look like
Modifier.PUBLIC | Modifier.STATIC |
Modifier.FINAL
The strictfp modifier appears
constant STRICT.
Request methods can be used in
following form

Modifier.isPrivate(field.getModifiers());
this is equivalent to the following condition
(field.getModifiers()&Modifier.PRIVATE)!=0
Field class
The Field class implements methods
to request information about
field type, as well as read and set it
meaning.
Consider some methods of the Field class
1. getType() - returns a class object
Class corresponding to the type of the current field.
For example, for a field of type int, we get
int class

2. Methods set and get - allow you to read
the current value of the field, as well as set a new one.
Consider an example:
public static void printField(Object o,
String name) throws
NoSuchFieldException,
IllegalAccessException(
field field = o.getClass().getField(name);
Short value = (Short) field.get(o);
System.out.println(value);
}
Those. the get method returns the value that
refers to the corresponding field or object
shell class.
An example of using the set method looks like this:

public static void setField(Object o, String name,
short nv) throws
NoSuchFieldException,
IllegalAccessException(
Field field = o.getClass().getField(name) ;
field.set(o,new Short(nv));
}
To store nv in the field of this object
wrapper classes must be used.
There are also methods that look like
getPrimitiveType (e.g. getInt) and
setPrimitiveType. These methods can
use to change fields in a class,
having a primitive type. For example,
field.setShort(o,nv);

Method class
Means of the Method class - allow you to get
complete information regarding
declarations of methods of a certain class,
and call these methods if needed
context of the given objects.
Consider the methods of the Method class.
1. public Class getReturnType() - returns
the Class object corresponding to the type
the value returned by the current method.
If instead of the return type in
method declaration specified service
the word void, the method in question will return
void.class object.

2. public Class getParameterTypes() - returns

parameters that are set in the declaration
current method. Objects are entered into an array in
in the order in which the parameters are listed in
method declaration. If the method does not
parameters, an empty array is returned.
3. public Class getExceptionTypes() - returns
an array of Class objects corresponding to the types
exceptions that are specified in the proposal
throws declaration of the current method. Objects
are entered into the array in the order in which
exception type names are listed in
method declaration.

4. public Object invoke(Object onThis, Object args)
throws IllegalAccessException,
IllegalArgumentException,
InvocationTargetException
Calls a method defined by the current object
Method, in the context of an onThis object with a job
the argument values ​​passed by the args array.
For non-static methods, the choice of implementation
performed based on the actual type
the object specified by the onThis parameter. For
static methods onThis is not accepted during
attention and may be null.
The length of the args array must match the number
parameters in the method declaration, and the array element object types must be assignable
corresponding types of method parameters - in
otherwise an exception will be thrown
IIlegalArgumentException.

10.

If as part of an object defined
onThis parameter, no type, member
which is the current method,
an exception is thrown
IllegalArgumentException.
If the value of onThis is null and the method is not
static, throws an exception like
NullpointerException.
If the execution of the called method
terminates abnormally, thrown away
an exception of type InvocationTargetException.

11.

Consider an example. Let's call the means
reflection method return str.indexOf(".", 8)
then we have
try(
Сlass strClass = str.getClass();
Method indexM = strClass.getMethod("indexOf",
new Class(string.class, int.class));
Object result = indexM.invoke(str, new object (
".", new lnteger(8)));
return ((Integer)result).intValue();
}
catch (NoSuchMethodException e) ( …….. )
catch (invocationTargetException e) (…….. )
catch (illegalAccessException e) (……)

12.

Constructor class
To create new instances (objects)
type method can be used
newInstance of the Class object,
corresponding to this type.
The method calls the constructor with no arguments,
owned by the type and returns a reference
to a newly created object of class Object,
which must be explicitly converted to
required type.
Consider an example.

13.

static double testData = ( 0.3,1.3e-2, 7.9, 3.17 );

try(
for(int arg = 0; arg< args.length; arg++){
String name = args;
Class classFor = Class.forName(name);
SortDouble sorter =
(SortDouble)classFor.newInstance();
SortMetrics metrics = sorter.sort(testData);
System.out.println(name + ": " + metrics);
for(int i =0; i< testData.length; i++)
System.out.println(" " + testData[i]); ) )
catch(Exception e) ( System.err.println(e); ) )

14.

newlnstance method when incorrect
application is capable of throwing out a large
number of different exception objects
types.
InstantiationException - if class, object
which should be created, does not possess
constructor with no arguments, or
defined as abstract, or in
reality is an interface,
or executing the creation procedure
object is interrupted by some other
reasons.
IllegalAccessException - if the class is either its
constructor with no arguments are not available.

15.

SecurityException - if the current policy
security prohibits the creation of new objects
ExceptionInInitializerError - thrown when
class initialization.
There are other methods defined in the Constructor class.
public Сlass getParameterTypes()

corresponding to the types of parameters that
defined in the declaration of the current constructor.
public Class getExceptionTypes()
Returns an array of Class objects,
corresponding to the types of exceptions that
defined in the throws clause of the declaration
current constructor.

16.

public Object newInstance(Object args)
throws InstantiationException,
IllegalAccessException,
IllegalArgumentException,
InvocationTargetException
Uses the constructor represented by the current
a Constructor object to create and initialize
new instance of the class, in which the constructor
declared, with the given arguments passed.
Returns a reference to the newly created and
initialized object. args array length
must match the number of parameters in
constructor declaration, and array element object types must be assignable
corresponding types of constructor parameters -
otherwise an exception will be thrown
IllegalArgumentException.

17.

Consider an example:
classMyclass(
private int a;
public Myclass(int k)(a=k;)
public int func(int a,int b)(return a+b;)
}
public class Main(
public static void main(String args)(
try(
Stringname="myclass";
Class mycl=Class.forName(name);
Class d=(int.class);
Constructor c=mycl.getConstructor(d);
Myclass ob=(Myclass)c.newInstance(new Object(
new Integer(10)));
System.out.println(ob.func(3,5)); )
catch(Exception e)();
}}

18.

AccessibleObject class
The Field, Constructor and Method classes are
derived from the AccessibleObject class,
which makes it possible to permit or
disable level access flag checking
languages ​​such as public and private.
The AccessibleObject class has methods
1. public void setAccessible(boolean flag)
Sets the object's access flag to
according to argument value: true
means the object is no longer obeyed
access rules set at the level
language (and will always be available), a false
forces the object to maintain the specified
access level.
If the permissions to change the access flag
is not enough, an exception is thrown like
SecurityException

19.

2. public static
void setAccessible(AccessibleObject array,
boolean flag)
Allows you to set the access flag
objects passed as an array.
If in the process of processing the next
object throws an exception of type
SecurityException, objects located
in the array earlier, save the newly set
access level values, and all other
objects remain the same.
3. public boolean isAccessible()
Returns the current value of the access flag
to the object

20.

Array class
The Array class is used to create an array
means of reflection.
Two forms of method are used to create arrays
newInstance.
public Object newInstance(Class compType, int length)
Returns a reference to a new array of type compType
given length length.
public Object newInstance(Class compType, int dim)
Returns a reference to a new multidimensional array of type
compType whose dimensions are given by the values
elements of the array-parameter dim.
If the array dim is empty or has a length greater than
allowed number of dimensions (usually 255),

legalArgumentException.

21.

Consider examples.
Example1. Let's form an array of type byte
ba = (byte)
Array.newInstance(byte.class,13);
This is equivalent
ba = new byte;
Example2.
int dims = (4, 4);
double matrix =(double)
Array.newInstance(double.class, dims);
This is equivalent
double matrix = new double;

22.

The Array class has get and set methods.
Let an array xa of values ​​of type int be given; then
expression xa[i] will match:
Integer n=Array.get(xa, i)
You can assign a value to an array element like this:
xa[i] = 23; is the same as
Array.set(xa, i, new Integer(23));
Package class
Calling the getPackage method of the Class class allows
get an object of class Package containing
Description of the package that contains the
class (the Package class itself is located in the package
java.lang).
The getName() method of the Package object returns
the full name of the current package.

23.

Proxy class
The Proxy class allows you to dynamically create
classes that implement one or more
interfaces.
Suppose we have class A,
implementing some interfaces.
The Java machine at runtime can
generate a proxy class for the given
class A, i.e. a class that
implements all interfaces of class A, but
replaces calling all methods of these
interfaces to invoke the invoke method,
interface InvocationHandler, for
which you can define your
implementation.

24.

A proxy class is created with a method call
Proxy.getProxyClass which takes a ClassLoader and
an array of interfaces (interfaces), and returns an object
class java.lang.Class which is loaded with
of the passed ClassLoader and implements the passed array
interfaces.
There are a number of restrictions on the passed parameters:
1. All objects in the interfaces array must be
interfaces. They cannot be classes or
primitives.
2. There can't be two identical ones in the interfaces array
objects.
3. All interfaces in the interfaces array must be
loaded by the ClassLoader that is passed to the method
getProxyClass.
4. All non-public interfaces must be defined
in the same package, otherwise generated proxy class
cannot implement them all.

25.

5. No two interfaces can have
method with the same name and
parameter signature, but with different
return types.
6. The length of the interfaces array is limited
65535 interfaces. No Java class
cannot implement more than 65535
interfaces.

26.

Dynamic Proxy Properties
1. The proxy class is public, provided
final modifier and is not abstract.
2. Default proxy class name is not
defined, but starts with Proxy. Everything
namespace starting with Proxy
reserved for proxy classes
(This is not required in recent versions of Java.)
3. The proxy class is inherited from
java.lang.reflect.Proxy.
4. The proxy class implements all interfaces,
transferred at creation, in order of transfer.

27.

5. If the proxy class implements a non-public
interface, then it will be generated in that package,
which defines this most non-public
interface. In general, a package that
proxy class undefined will be generated.
6. Method Proxy.isProxyClass returns true for
classes created with
Proxy.getProxyClass and for object classes,
created with Proxy.newProxyInstance and
false otherwise.
This method is used by the subsystem
Java security and you need to understand that for
a class simply inherited from
java.lang.reflect.Proxy it will return false.

28.

The properties of the created instance of the proxy class are as follows:
1. The object of the proxy class is cast to all interfaces,
passed in the interfaces array. If IDemo is one of
of the passed interfaces, then the operation proxy instanceof
IDemo will always return true, while the (IDemo) proxy operation
will end correctly.
2. Static method Proxy.getInvocationHandler
returns the call handler passed on creation
instance of the proxy class. If transferred to this
method object is not an instance of the proxy class, then
an IllegalArgumentException exception will be thrown.
3. The call handler class implements the interface
InvocationHandler, which defines the invoke method,
having the following signature:
public Object invoke(Object proxy, Method method,
Object args) throws Throwable

29.

Consider an example:
packagejavaapplication3;
interface Account(
double getBalance();
void changeBalance(int sum);
void percent(double per);)
class MyAccount implements Account(
private double balance;
public MyAccount()( balance=0.0; )
public double getBalance()( return balance; )
public void changeBalance(int sum)(
balance+=sum;)
public void percents(double per)(
balance+=balance*per/100; ); )

30.

class MyAccountProxy implements
InvocationHandler(
private Account ac;
public MyAccountProxy(Account acc)( ac=acc; )
public static Account newInstance(Account da)(
return (Account)Proxy.newProxyInstance(
da.getClass().getClassLoader(),
da.getClass().getInterfaces(),
new MyAccountProxy(da));
}

31.

public Object invoke(Object proxy,
Method method, Object args)
throws Throwable(
if(method.getName()=="percents")(
double d=((Double)args).doubleValue();
if (d<0) d=0;
if(d>30) d=30;
args=new Double(d);

else(
return method.invoke(ac, args); )
}
}

32.

public class Main(
public static void main(String args)(
MyAccountma=new MyAccount();
account
a=(Account)MyAccountProxy.newInstance(ma);
a.changeBalance(150);

a.percents(20);
System.out.println(a.getBalance());
a.percents(35);
System.out.println(a.getBalance());) )

33.

Loading classes
The runtime system loads classes as needed.
the need for them.
Functional features of boot procedures
classes essentially depend on
implementation of the Java virtual machines, but in
most cases to find classes,
addressed by the application, but not downloaded
executive system, the mechanism is applied
viewing the class search path.
To create an application that can
load classes in ways other than
provided by default, should
use an object of class ClassLoader,
able to get the bytecode of the implementation of the desired
class and load it into the runtime
systems.

34.

The ClassLoader class is an abstract class.
To create your own classloader,
you need to create a class that inherits from
ClassLoader and override method
protected Class findClass(String name) throws
ClassNotFoundException
Which finds the bytecode of a class with a given
named name and loads the data into the environment
virtual machine, returning a Class object,
representing the found class.
The loader object is able to delegate
permissions to load classes "parent"
class loader (parent class loader).
The "parent" classloader can be
given as a class constructor argument
class loader.

35.

protected ClassLoader()
Creates a ClassLoader object, implicitly
using as "parent"
class loader system loader
(which can be obtained through
calling the getSystemClassLoader method).
protected ClassLoader(ClassLoader parent)
Creates a ClassLoader object using
the given "parent" classloader.
The main part of the ClassLoader class
is the loadClass method

36.

public Сlass loadClass(String name) throws
ClassNotFoundException
returns the Class object for the class with the given
name and optionally loads this
Class. If the class cannot be loaded,
an exception of the type is thrown
ClassNot FoundException.
The class loading scheme offered by the method
loadClass by default and usually not
override looks like this:
1. check by method call
findLoadedClass of class ClassLoader, not
whether the given class was loaded before; as part of
ClassLoader provided object table
Class for all classes loaded by means
the current class loader; if the class was
loaded before, findLoadedClass method
will return a reference to an existing Class object;

37.

2. if the class was not loaded, it is called
loadClass of the "parent" loader
classes; if the current bootloader is not
has a "parent", is used
system class loader;
3. if the class is still not loaded,
the findClass method is called, which performs
search and class loading.
Thus, it is necessary to implement
own versions of the following methods
classloader:

38.

protected synchronized Class
loadClass(String name,boolean resolve)

protected Class findClass(String name)
throws ClassNotFoundException
protected java.net.URL findResource(String name)
protected java.util.Enumeration
findResources(String name) throws IOException
(The abstract class ClassLoader represents
only an implementation of the loadClass method based on
on protected methods - findLoadedClass and findClass).

39.

Consider an example.
class PlayerLoader extends ClassLoader(
public Class findClass(String name) throws
classNotFoundException(
try(
byte buf = bytesForClass(name);
return defineClass(name, buf, 0, buf.length);
}
catch(IOException e)(
throw new ClassNotFoundException(e.toString());
}
}
// ... Declarations of the bytesForClass method and others
methods
}

40.

The findClass method usually does two things
functions.
First, it must detect the bytecode
of the given class and store it in an array
byte type - this duty in the example
assigned to the bytesForСlass method.
Second, it uses the applied method
defineClass to do the actual
loading the class defined by the bytecode.
The defineСlass method looks like

41.

protected final Class defineClass(String name,
byte data, int offset, int length) throws
ClassFormatError
Returns the Class object for the class with the given name
name; the binary representation of the class is passed to
as an array data.
Only bytes are used to load the class,
contained in the elements of the data array with indices
from offset to offset+length. If the bytes from the specified
gaps do not meet the required format
class declarations, an exception object is thrown
of type ClassFormatError.
The method is responsible for maintaining the reference to the object
Class for loaded class in loaded table
classes looked up by the findLoadedClass method.

42.

Consider the bytesForClass method.
protected byte bytesForClass(String name) throws
lOException, ClassNotFoundException(
FileInputStream in = null;
try(


if (length == 0) throw new ClassNotFoundException(name);
byte buf = new byte;

return buffer;
}
finally(
if (in!=null) in.close();
}
}

43.

So the complete code looks like:
import java.lang.reflect.*;
import java.io.*;
class MyClassLoader extends ClassLoader(
public classfindClass(String name) throws
classNotFoundException(
byte buf=ReadFromBuffer(name);
if(name.equals("MyInterface1"))(

) else if(buf==null) (
return findSystemClass(name);
) else (
return defineClass(name,buf,0,buf.length);
}
}

44.

protected byte ReadFromBuffer(String name) throws
classNotFoundException(
FileInputStream in = null;
try(
in = new FileInputStream(name + ".class");
int length = in.available(); // number of available bytes
if (length == 0) throw
new ClassNotFoundException(name);
byte buf = new byte;
in read(buf); // Read bytes
return buffer;
}
catch(FileNotFoundException e)( return null;)
catch(IOException e)(return null;)
finally(
try( if (in!=null) in.close(); )
catch(IOException e)( )
}
}

45.

protected synchronized Class
loadClass(String name,boolean resolve) throws
classNotFoundException(
class result= findClass(name);
if (resolve) resolveClass(result);
return result;
}
}

46.

public class Main1(
public static void main(String args) (
try(
Stringname="myclass";
ClassLoader ld=new MyClassLoader();
Class cl=Class.forName(name, true, ld);
Constructor s=cl.getConstructor(int.class);
MyInterface1
ob=(MyInterface1)s.newInstance(
new Integer(8));
System.out.println(ob.func(3,5));
)catch(Exception e)( );
}
}

47.

public interface MyInterface1(
public int func(int a,int b);
}
public class Myclass implements MyInterface1 (
private int a;
public Myclass(int k) ( a=k; )
public int func(int a,int b)( return a+b; )