我最近遇到了这些问题,在StackOverflow上找不到答案;
- Java类变量初始化的顺序是什么?
- 还有一个相关的问题,重新排列变量是否会更改类的行为?
- 为什么?
根据Meta的建议,我将发布这个问题的答案。
在Java中,类变量的初始化顺序如下:
1和2只在类第一次实例化时执行。
因此,给定以下代码:
class Test
extends TestSuper
{
final int ti1;
final int ti2 = counter ++;
{ ti1 = counter ++; }
static final int ts1;
static final int ts2 = counter ++;
static { ts1 = counter ++; }
public static void main(String[] argv) {
Test test1 = new Test();
printTest(test1);
Test test2 = new Test();
printTest(test2);
}
private static void printTest(Test test) {
System.out.print("ss2 = " + test.ss2);
System.out.print(", ss1 = " + test.ss1);
System.out.print(", ts2 = " + test.ts2);
System.out.println(", ts1 = " + test.ts1);
System.out.print("si2 = " + test.si2);
System.out.print(", si1 = " + test.si1);
System.out.print(", ti2 = " + test.ti2);
System.out.println(", ti1 = " + test.ti1);
System.out.println("counter = " + test.counter);
}
}
class TestSuper
{
static int counter = 0;
final int si1;
final int si2 = counter ++;
{ si1 = counter ++; }
static final int ss1;
static final int ss2 = counter ++;
static { ss1 = counter ++; }
}
ss2 = 0, ss1 = 1, ts2 = 2, ts1 = 3
si2 = 4, si1 = 5, ti2 = 6, ti1 = 7
counter = 8
ss2 = 0, ss1 = 1, ts2 = 2, ts1 = 3
si2 = 8, si1 = 9, ti2 = 10, ti1 = 11
counter = 12
static final int ss1;
static final int ss2 = counter ++;
static { ss1 = counter ++; }
被改为:
static final int ss1;
static { ss1 = counter ++; }
static final int ss2 = counter ++;
ss2 = 1, ss1 = 0, ts2 = 2, ts1 = 3
si2 = 4, si1 = 5, ti2 = 6, ti1 = 7
counter = 8
ss2
和ss1
的值将会改变。
原因在于这种行为被规定在Java语言规范中。