各种问题收集

1.下面运行结果是什么?解释为什么

public class Base
{
    Base() {
        preProcess();
    }

    void preProcess() {}
}

public class Derived extends Base
{
    public String whenAmISet = "set when declared";

    @Override void preProcess()
    {
        whenAmISet = "set in preProcess()";
    }
}

ublic class Main
{
    public static void main(String[] args)
    {
        Derived d = new Derived();
        System.out.println( d.whenAmISet );
    }
}

运行结果:

set when declared

为什么是这个结果?

...

2.阿里笔试题, 下面运行结果是什么?解释为什么是这样?

public class Alibaba {
    public static int k = 0;
    public static Alibaba t1 = new Alibaba("t1");
    public static Alibaba t2 = new Alibaba("t2");
    public static int i = print("i");
    public static int n = 99;
    private int a = 0;
    public int j = print("j");
    {
        print("构造块");
    }
    static {
        print("静态块");
    }
    public Alibaba(String str) {
        System.out.println((++k) + ":" + str + "   i=" + i + "    n=" + n);
        ++i;
        ++n;
    }
    public static int print(String str) {
        System.out.println((++k) + ":" + str + "   i=" + i + "    n=" + n);
        ++n;
        return ++i;
    }
    public static void main(String args[]) {
        Alibaba t = new Alibaba("init");
    }
}

运行结果:

1:j   i=0    n=0
2:构造块   i=1    n=1
3:t1   i=2    n=2
4:j   i=3    n=3
5:构造块   i=4    n=4
6:t2   i=5    n=5
7:i   i=6    n=6
8:静态块   i=7    n=99
9:j   i=8    n=100
10:构造块   i=9    n=101
11:init   i=10    n=102

3.

public class Singleton {
    private static Singleton singleton = new Singleton();

    public static int counter1;
    public static int counter2 = 0;

    private Singleton() {
        counter1++;
        counter2++;
    }

    public static Singleton getInstance() {
        return singleton;
    }
}

public class Test {

    public static void main(String[] args){
        Singleton singleton = Singleton.getInstance();
        System.out.println(singleton.counter1);
        System.out.println(singleton.counter2);
    }

}

运行结果:

1
0

为什么呢?

明日解释为啥是上面的结果....