Java Initialization

程式範例

public class Parent { 
    static int staticVariable;
    int nonStaticVariable;   

    //類別初始化時，只會被執行一次。
    static {
        System.out.println("Parent static initialization block.");
        staticVariable = 6;
    }

    //物件初始化時，每次都會被執行一次。
    {
        System.out.println("Parent object initialization block.");
        nonStaticVariable = 6;
    }

    public Parent() {
        System.out.println("Parent constructor.");
    } 
}

public class Child extends Parent {
    //類別初始化時，只會被執行一次。
    static {
        System.out.println("Child static initialization block.");
    }

    //物件初始化時，每次都會被執行一次。
    {
        System.out.println("Child object initialization block.");
    }

    public Child() {
        System.out.println("Child constructor.");
    } 
}

public class Other {
    public static void main(String args[]) {
        System.out.println("First test.");
        System.out.println("====================================");
        new Parent();
        new Parent();
        System.out.println();

        System.out.println("Second test.");
        System.out.println("====================================");
        new Child();
    }
}

執行結果：

First test.
====================================
Parent static initialization block.
Parent object initialization block.
Parent constructor.
Parent object initialization block.
Parent constructor.

Second test.
====================================
// Parent static initialization block. // 在First test已初始過，這裡就不執行。
Child static initialization block.
Parent object initialization block.
Parent constructor.
Child object initialization block.
Child constructor.

參考網站

https://stackoverflow.com/questions/3987428/what-is-an-initialization-block