JAVA基础

static关键字

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概述

• static 变量：static变量由该类的所有对象共享，不需要创建对象也可使用。
• static 方法：允许直接访问，不需要创建对象也可被调用。如 main 方法。
• static 初始化块：在创建类的第一个对象前自动执行（先执行静态初始化块，再执行初始化块）。
• static 内部类：外部类对象共享，只能访问外部类的静态成员。

静态成员（字段和方法）属于类本身，而非类的实例。因此，可以通过类名直接访问和调用静态成员，无需创建对象。静态成员在内存中只有一份拷贝，被所有对象所共享。

另外，静态成员不依赖于对象的创建而存在，所以它们可以在没有创建对象的情况下被访问。然而，静态成员不能访问非静态成员，因为非静态成员需要通过对象才能够访问。

静态成员与加载顺序

public class FatherClass {

    // static
    public static String STATIC_FIELD = "father static field";

    static {
        System.out.println("father static block");
    }


    {
        System.out.println("father non static block");
    }


    public FatherClass() {
        System.out.println("father class construction");
    }

    public static void staticMethod() {
        System.out.println("father static method");
    }

    // 内部类
    public static class StaticInnerClass {
        static {
            System.out.println("FatherClass.StaticInnerClass static block");
        }

        {
            System.out.println("FatherClass.StaticInnerClass non static block");
        }

        public StaticInnerClass() {
            System.out.println("FatherClass.StaticInnerClass class construction");
        }

        public static void fatherStaticInnerClassStaticMethod() {
            System.out.println("FatherClass.StaticInnerClass static method");
        }
    }
}


public class ChildClass extends FatherClass {
    public static String STATIC_FIELD = "child static field";

    static {
        System.out.println("child static block 1");
    }

    {
        System.out.println("child non static block 1");
    }

    static {
        System.out.println("child static block 2");
    }

    {
        System.out.println("child non static block 2");
    }

    public ChildClass() {
        System.out.println("child class construction");
    }

    public static void staticMethod() {
        System.out.println("child static method");
    }

    // 内部类
    public static class StaticInnerClass {

        static {
            System.out.println("ChildClass.StaticInnerClass static block");
        }


        {
            System.out.println("ChildClass.StaticInnerClass non static block");
        }


        public StaticInnerClass() {
            System.out.println("ChildClass.StaticInnerClass class construction");
        }

        public static void childStaticInnerClassStaticMethod() {
            System.out.println("ChildClass.StaticInnerClass static method");
        }

    }

    /**
     * 类初始化
     * 1)静态变量(代码块)static只会初始化(执行)一次。
     * 2)当有父类时，完整的初始化顺序为：父类静态变量(静态代码块)->子类静态变量(静态代码块)->父类非静态变量(非静态代码块) ->父类构造器 ->子类非静态变量(非静态代码块)->子类构造器 。
     * 3)当有多个代码块时,按顺序加载.
     * <p>
     * father static block
     * child static block 1
     * child static block 2
     * father non static block
     * father class construction
     * child non static block 1
     * child non static block 2
     * child class construction
     * test start
     * test end
     */
    @Test
    public void testInit() {
        System.out.println("test start");
        System.out.println("test end");
    }

    /**
     * father static block
     * child static block 1
     * child static block 2
     * father non static block
     * father class construction
     * child non static block 1
     * child non static block 2
     * child class construction
     * test start
     * father non static block
     * father class construction
     * child non static block 1
     * child non static block 2
     * child class construction
     * test end
     */
    @Test
    public void testNewObj() {
        System.out.println("test start");
        ChildClass childClass = new ChildClass();
        System.out.println("test end");
    }

    /**
     * father static block
     * child static block 1
     * child static block 2
     * father non static block
     * father class construction
     * child non static block 1
     * child non static block 2
     * child class construction
     * test start
     * FatherClass.StaticInnerClass static block
     * ChildClass.StaticInnerClass static method
     * test end
     */
    @Test
    public void testFatherStaticClass1() {
        System.out.println("test start");
        FatherClass.StaticInnerClass.fatherStaticInnerClassStaticMethod();
        System.out.println("test end");
    }

    /**
     * father static block
     * child static block 1
     * child static block 2
     * father non static block
     * father class construction
     * child non static block 1
     * child non static block 2
     * child class construction
     * test start
     * FatherClass.StaticInnerClass static block
     * FatherClass.StaticInnerClass non static block
     * FatherClass.StaticInnerClass class construction
     * ChildClass.StaticInnerClass static method
     * test end
     */
    @Test
    public void testFatherStaticClass2() {
        System.out.println("test start");
        FatherClass.StaticInnerClass staticInnerClass = new FatherClass.StaticInnerClass();
        staticInnerClass.fatherStaticInnerClassStaticMethod();
        System.out.println("test end");
    }

    /**
     * father static block
     * child static block 1
     * child static block 2
     * father non static block
     * father class construction
     * child non static block 1
     * child non static block 2
     * child class construction
     * test start
     * ChildClass.StaticInnerClass static block
     * ChildClass.StaticInnerClass static method
     * test end
     */
    @Test
    public void testChildStaticClass1() {
        System.out.println("test start");
        ChildClass.StaticInnerClass.childStaticInnerClassStaticMethod();
        System.out.println("test end");
    }

    /**
     * father static block
     * child static block 1
     * child static block 2
     * father non static block
     * father class construction
     * child non static block 1
     * child non static block 2
     * child class construction
     * test start
     * ChildClass.StaticInnerClass static block
     * ChildClass.StaticInnerClass non static block
     * ChildClass.StaticInnerClass class construction
     * ChildClass.StaticInnerClass static method
     * test end
     */
    @Test
    public void testChildStaticClass2() {
        System.out.println("test start");
        StaticInnerClass staticInnerClass = new StaticInnerClass();
        staticInnerClass.childStaticInnerClassStaticMethod();
        System.out.println("test end");
    }
}