What is java static

java static includes static variables, static methods, and static initialization blocks, the following is a detailed explanation of static. (Recommended tutorial: java tutorial)

1. What is a static variable

As we all know, we can create multiple such variables based on one class Class objects, each object has its own members, independent of each other.

However, sometimes, we prefer that all objects of this class share the same member. This is when static comes into play!

Members modified by static in Java are called static members or class members. It belongs to the entire class, not to an object, that is, it is shared by all objects of the class.

Static members can be accessed directly using the class name, or they can be accessed using the object name. Of course, in view of the particularity of its role, it is recommended to use class names to access

Use static to modify variables, methods and code blocks.

public class Test1 {
    // static修飾的變量為靜態(tài)變量，所有類的對象共享hobby
    static String hobby = "Tedu";
    
    public static void main(String[] args) {
        // 靜態(tài)變量可以直接使用類名來訪問，無需創(chuàng)建對象
        System.out.println("通過類名訪問hobby：" + Test1.hobby);
        // 創(chuàng)建類的對象
        Test1 t1 = new Test1();
        // 使用對象名訪問靜態(tài)變量
        System.out.println("使用對象名訪hobby:" + t1.hobby);
        // 使用對象名的形式修改靜態(tài)變量的值
        t1.hobby = "cjj";
        // 再次使用類名訪問靜態(tài)白變量，值已經(jīng)被修改
        System.out.println("通過類名訪問hobby：" + Test1.hobby);
    }
}

Running result:

通過類名訪問hobby：Tedu
使用對象名訪hobby:Tedu
通過類名訪問hobby：cjj

Note:

Static members belong to the entire class. When the system uses the class for the first time, it will allocate memory space until the Resources will be recycled only after the class is unloaded!

2. Static methods used by static

Like static variables, we can also use static modified methods, called static methods or class methods.

In fact, the main method we have been writing before is a static method

public class Test01 {
    //使用static關(guān)鍵字聲明靜態(tài)方法
    public static void print() {
        System.out.println("歡迎你：Cjj!");
    }
    
    public static void main(String[] args) {
        //直接使用類名調(diào)用靜態(tài)方法
        Test01.print();
        //也可以通過對象名調(diào)用，當(dāng)然更推薦使用類名調(diào)用
        Test01 t1 = new Test01();
        t1.print();
    }
}

Copy code

歡迎你：Cjj!
歡迎你：Cjj!

Note:

1. Static methods can directly call similar methods Static members in , but non-static members cannot be called directly. For example:

If you want to call non-static variables in a static method, you can create an object of the class and then access the non-static variables through the object. For example:

2. In ordinary member methods, you can directly access non-static variables and static variables of the same type, such as:

3. Non-static methods cannot be directly called in static methods. You need to Access non-static methods through objects. For example:

public class Test01 {
    String name = "Cjj"; //靜態(tài)成員變量
    static String hobby = "study"; //非靜態(tài)成員變量
    
    //普通成員方法
    public void print1() {
        System.out.println("普通成員方法~~~"); 
    }
    
    //靜態(tài)成員方法
    public static void print2() {
        System.out.println("靜態(tài)成員方法~~~");
    }
    
    public static void main(String[] args) {
        //普通成員方法必須通過對象來調(diào)用
        Test01 t1 = new Test01();
        t1.print1();
        //可以直接調(diào)用靜態(tài)方法
        print2();
    }
}

3. Static initialization block used by static

In Java, data can be assigned through initialization blocks. For example:

public class Test02 {
    String name; // 定義一個成員變量
    //通過初始化塊為成員變量賦值
    {
        name = "cjj";
    }
}

Execute these code blocks in sequence. If an initialization block is modified with static, it is called a static initialization block.

Note:

The static initialization block is only executed when the class is loaded, and will only be executed once. At the same time, the static initialization block can only assign values ??to static variables and cannot initialize ordinary member variables.

Let’s take a look at a piece of code:

public class Test02 {
    int num1;
    int num2;
    static int num3;
    public Test02() {// 構(gòu)造方法
        num1 = 11;
        System.out.println("通過構(gòu)造方法為變量num1賦值");
    }
    { // 初始化塊
        num2 = 22;
        System.out.println("通過初始化塊為變量num2賦值");
    }
    static {// 靜態(tài)初始化塊
        num3 = 33;
        System.out.println("通過靜態(tài)初始化塊為靜態(tài)變量num3賦值");
    }
    
    public static void main(String[] args) {
        Test02 t1 = new Test02(); //創(chuàng)建對象
        System.out.println("num1:" + t1.num1);
        System.out.println("num2:" + t1.num2);
        System.out.println("num3:" + num3);
        
    }
}

Running results:

通過靜態(tài)初始化塊為靜態(tài)變量num3賦值
通過初始化塊為變量num2賦值
通過構(gòu)造方法為變量num1賦值
num1:11
num2:22
num3:33

By outputting the results, we can see that the static initialization block is executed first when the program is running, and then The normal initialization block is executed, and the constructor method is executed last. Since the static initialization block is only executed once when the class is loaded, the static initialization block is not executed when the object hello2 is created again.

The class is loaded the first time it is used, and will not be removed after loading.

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