What are java static variables

In Java, static variables refer to variables of classes modified by static; static variables are shared by all class instance objects and have only one copy in memory, if and only if the class is loaded for the first time. is initialized.

The operating environment of this tutorial: windows7 system, java8 version, DELL G3 computer.

1. JVM runtime data area

• The Java heap stores almost all object instances and is shared by all threads
• The method area (permanent generation), like the Java heap, is a memory area shared by each thread. It is used to store class information, constants, static variables, etc. that have been loaded by the virtual machine.

2. static variables

Variables of a class modified by static are also called static variables. Non-static variables are also called instance variables

The difference between static variables and instance variables is:

• Static variables are shared by all class instance objects, and there is only one in memory A copy if and only if it is initialized when the class is first loaded.
• Instance variables are owned by the instance object. They are initialized when the object is created. There are multiple copies. The copies owned by each instance object do not affect each other.

public class MyStatic {
    // 静态变量
    private static int index = 0;

    // 实例变量
    private int count = 0;

    public void inc() {
        index++;
        count++;
    }

    public String  get() {
        return "静态变量index = " + index + ",  实例变量count = " + count;
    }
}

/**
 * index是静态变量
 * count是实例变量
 * */
public class StaticVariableTest {

    public static void main(String[] args) {
        MyStatic myStatic1 = new MyStatic();
        MyStatic myStatic2 = new MyStatic();
        MyStatic myStatic3 = new MyStatic();

        myStatic1.inc();
        System.out.println("myStatic1的index、count增加1：");
        System.out.println("myStatic1的" + myStatic1.get());
        System.out.println("myStatic2的" + myStatic2.get());
        System.out.println("myStatic3的" + myStatic3.get() + "\n");

        myStatic2.inc();
        System.out.println("myStatic2的index、count增加1：");
        System.out.println("myStatic1的" + myStatic1.get());
        System.out.println("myStatic2的" + myStatic2.get());
        System.out.println("myStatic3的" + myStatic3.get() + "\n");

        myStatic3.inc();
        System.out.println("myStatic3的index、count增加1：");
        System.out.println("myStatic1的" + myStatic1.get());
        System.out.println("myStatic2的" + myStatic2.get());
        System.out.println("myStatic3的" + myStatic3.get());
    }
}

Running results:

It can be seen that:

• The static variable index is shared by multiple object instances
• The instance object count is multiple Object instance private

myStatic1的index、count增加1：
myStatic1的静态变量index = 1,  实例变量count = 1
myStatic2的静态变量index = 1,  实例变量count = 0
myStatic3的静态变量index = 1,  实例变量count = 0

myStatic2的index、count增加1：
myStatic1的静态变量index = 2,  实例变量count = 1
myStatic2的静态变量index = 2,  实例变量count = 1
myStatic3的静态变量index = 2,  实例变量count = 0

myStatic3的index、count增加1：
myStatic1的静态变量index = 3,  实例变量count = 1
myStatic2的静态变量index = 3,  实例变量count = 1
myStatic3的静态变量index = 3,  实例变量count = 1

3. Misunderstanding of static variables

3.1 Static cannot act on local variables

• static in C/C can scope local variables
• static in Java is not allowed to modify local variables.
  Don't ask why, this is a requirement of Java grammar.
  

3.2 this can access static variables

Although static variables are independent of objects, it does not mean that they cannot be accessed through object instances. , all static methods and static variables can be accessed through object instances (as long as the access rights are sufficient).
This represents the current object instance, and static variables are shared by all objects, so static variables can be accessed through this

public class StaticVariable {
    public static int index = 1;

    public static void main(String[] args) {
        StaticVariable staticVariable = new StaticVariable();
        staticVariable.get();
    }

    private void get() {
        int index = 2;
        System.out.println(this.index);
    }
}

Running results:

3.3 static will not change the access permissions of class members

The static keyword in Java will not affect the scope of variables or methods. In Java, the only keywords that can affect access rights are private, public, and protected (including package access rights)

public class MyStatic {
    public static int index1 = 1;

    // 静态变量
    private static int index2 = 0;
}

public class StaticVariableTest {

    public static void main(String[] args) {
        System.out.println(MyStatic.index1);
        System.out.println(MyStatic.index2);
    }
}

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