java数据域初始化

1、在声明中赋值

/**
 * Created by N3verL4nd on 2016/11/19.
 */

class Test{
    private String str = "Hello World";
    public void show(){
        System.out.println(str);
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }
}

2、在构造器中设置值

/**
 * Created by N3verL4nd on 2016/11/19.
 */

class Test{
    private String str;
    Test(){
        str = new String("Hello World");
    }
    public void show(){
        System.out.println(str);
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }
}

3、使用初始化块

/**
 * Created by N3verL4nd on 2016/11/19.
 */

class Test{
    private String str;
    {
        str = new String("Hello World");
    }
    public void show(){
        System.out.println(str);
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }
}

只要构造类的对象，初始化块就会被执行。

/**
 * Created by N3verL4nd on 2016/11/19.
 */

class Test{
    {
        str = new String("111");
    }
    private String str = "222";
    Test(){
        str = new String("333");
    }
    public void show(){
        System.out.println(str);
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }
}

以上代码，调整初始化顺序，单步调试发现，构造函数最后被执行。

在声明中赋值与在初始化块中设置值则按照顺序执行。

/**
 * Created by N3verL4nd on 2016/11/19.
 */

class Test{
    Test(){
        str = new String("333");
    }

    {
        str = new String("111");
    }
    private String str = "222";

    static {
        System.out.println(555);
    }

    public void show(){
        System.out.println(str);
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }
}

static块在类加载的时候被执行，也就是优先于main函数执行。顺序：进入main函数-->static块-->返回main函数。

无main函数的Hello World

public class HelloWorld {
    static {
        System.out.println("Hello World");
        System.exit(0);
    }
    /*public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }*/
}