本文转自此处
源代码:
public class Teststaticblock { public Teststaticblock() { this("second"); System.out.println("begin constructor"); System.out.println(s_a); //111 System.out.println(s_b); //222 System.out.println(c); //333 System.out.println(d); //d的值为什么是4,却不是444?? //因为程序会按照初始化块和实例Field生命语句的先后顺序来执行, //先赋给d的值444会被后赋给d的值4覆盖掉,所以d的最终值为4. // this("second");//call to this must be first statement in constructor s_a=1111; s_b=2222; c=3333; d=4444; System.out.println(s_a); //1111 System.out.println(s_b); //2222 System.out.println(c); //3333 System.out.println(d); //4444 System.out.println("end constructor"); } public Teststaticblock(String s) { System.out.println("begin second constructor"); System.out.println("end second constructor"); } public static void main(String args[]) { System.out.println("begin main"); System.out.println(s_a); System.out.println(s_b); //s_b的值为什么是2,却不是22?? //因为程序会按照静态初始化块和静态Field声明语句的先后顺序来执行,先赋给s_b的值22 //会被后赋给s_b的值2覆盖掉,所以s_b最终的值为2. // System.out.println(c); //non-static variable c cannot be referenced from a static context // System.out.println(d); //non-static variable c cannot be referenced from a static context s_a=11111; s_b=22222; // c=33333;//non-static variable c cannot be referenced from a static context // d=44444;//non-static variable c cannot be referenced from a static context System.out.println(s_a); //11111 System.out.println(s_b); //22222 // System.out.println(c); //non-static variable c cannot be referenced from a static context // System.out.println(d); //non-static variable c cannot be referenced from a static context System.out.println("before new class object"); Teststaticblock t = new Teststaticblock(); System.out.println("end new class object"); System.out.println(s_a);//1111 System.out.println(s_b); //2222 // System.out.println(c); //non-static variable c cannot be referenced from a static context // System.out.println(d); //non-static variable c cannot be referenced from a static context s_a=111111; s_b=222222; // c=333333;//non-static variable c cannot be referenced from a static context // d=444444;//non-static variable c cannot be referenced from a static context System.out.println(s_a); //111111 System.out.println(s_b); //222222 // System.out.println(c); //non-static variable c cannot be referenced from a static context // System.out.println(d); //non-static variable c cannot be referenced from a static context System.out.println("end main"); } static int s_a=1; int c=3; { System.out.println("begin block"); System.out.println(s_a); //11111 System.out.println(s_b); //22222 System.out.println(c); //3 // System.out.println(d);//illegal forward reference s_a=111; s_b=222; c=333; d=444; System.out.println(s_a); //111 System.out.println(s_b); //222 System.out.println(c); //333 // System.out.println(d);//illegal forward reference System.out.println("end block"); } static { System.out.println("begin static block"); //begin static block System.out.println(s_a);//1 //为什么不能输出s_b的值,却可以给s_b赋值?? // System.out.println(s_b);//illegal forward reference // System.out.println(c); //non-static variable c cannot be referenced from a static context // System.out.println(d); //non-static variable c cannot be referenced from a static context s_a=11; s_b=22; System.out.println(s_a); //11 // System.out.println(s_b);//illegal forward reference // System.out.println(c); //non-static variable c cannot be referenced from a static context // System.out.println(d); //non-static variable c cannot be referenced from a static context System.out.println("end static block"); } int d=4; static int s_b=2; }
运行结果:
/* begin static block 1 11 end static block begin main 11 2 11111 22222 before new class object begin block 11111 22222 3 111 222 333 end block begin second constructor end second constructor begin constructor 111 222 333 4 1111 2222 3333 4444 end constructor end new class object 1111 2222 111111 222222 end main */
结果分析:
1、在类第一次加载时候,会执行静态成员变量(静态Field)初始化语句和静态初始化块(用static{}包含的部分)。
注意:
a、不管静态成员变量声明语句的实际位置在哪儿,当第一次加载类的时候都会首先对它初始化为缺省值(0,false,null等)。
b、即使静态成员变量声明中使用了显式初始化语句(比如:int x=3),第一次加载类的时候也会先把它初始化为缺省值(此时x为0),然后再按照下面说的要点c来执行赋值语句(x=3)。
c、对于静态成员变量的显式初始化语句和静态初始化块,按照在类中代码出现的先后顺序执行。
因此,在上面的例子程序中,我们看到
static int s_a=1;
static
{
s_a=11;
s_b=22;
}
static int s_b=2;
对s_a,s_b会有不同的效果。
类加载时候,s_a,s_b都被初始化为0,然后由于依照代码顺序执行了s_a=1;s_a=11;s_b=22;s_b=2;
结果s_a、s_b分别变成了11和2。
2、当构造类实例时候,会先对实例成员变量初始化为缺省值,然后执行初始化块(用{}括起来的部分),然后执行构造方法。其中:
a、如同1中一样,如果有实例成员变量的显式初始化语句,程序仍然是先将该成员变量初始化为缺省值,然后按照代码在类中出现的先后顺序执行初始化语句或者初始化块。
如果初始化块位置在初始化语句前面,即使它改变了该成员变量的值,也会被随后执行的初始化语句改回去。
b、在进入构造方法后,如果构造方法第一句是使用this(...)调用另一构造方法的话,则先执行另一构造方法,然后再执行本构造方法的方法体。这种用法必须让this(...)位于第一句。