java注意的一些细节问题

public static void main(String[] args) {
    {
        int x;
        {
            int x;//编译错误：Duplicate local variable x
        }
    }
}

public static void main(String[] args) {
    if(true && false) {}
    if(true & false) {}
    System.out.println(true & false);
    System.out.println(true ^ false);
    System.out.println(true | false);
}

false
true
true

public static void main(String[] args) {
    label:
    for(int i=0; i<2; ++i){
        System.out.println("i=" + i);
        for(int j=0; j<3; ++j){
            if(j == 1){
                //continue;
                //break;
                //continue label;
                break label;
            }
            System.out.println("   j=" + j);
        }
    }
}

public static void main(String[] args) {
    label:
    for(int k=0; k<2; ++k){
        System.out.println("k=" + k);
        for(int i=0; i<2; ++i){
            System.out.println("    i=" + i);
            for(int j=0; j<3; ++j){
                if(j == 1){
                    //break;
                    continue label;
                }
                System.out.println("        j=" + j);
            }
        }
    }
}

class A{
    public void fun(int x){
        System.out.println("int 重载");
    }
    
    public void fun(Integer x){
        System.out.println("Integer 重载");
    }
}


public class Main{
    
    public static void main(String[] args) {
         A a = new A();
         int x = 1;
         Integer ix = 1;
         a.fun(x);
         a.fun(ix);
    }
}

int 重载
Integer 重载

public class Main{
    
    public static void test2(){
        throw new NullPointerException("空指针异常!");
    }
    
    public static void test() throws Exception{
        try{
            test2();
        } catch (Exception e){
            //throw new Exception("自定义异常!"); //(1)
            throw new Exception("自定义异常!", e);//(2)
        }
    }
    
    public static void main(String[] args) {
        try{
          test();
        } catch(Exception e) {
            StringWriter sw = new StringWriter();
            PrintWriter pw = new PrintWriter(sw);
            e.printStackTrace(pw);
            System.out.println(sw.toString());
        }
    }
}

(1)java.lang.Exception: 自定义异常!
    at com.hjzgg.Main.test(Main.java:28)
    at com.hjzgg.Main.main(Main.java:34)


(2)java.lang.Exception: 自定义异常!
    at com.hjzgg.Main.test(Main.java:29)
    at com.hjzgg.Main.main(Main.java:35)
Caused by: java.lang.NullPointerException: 空指针异常!
    at com.hjzgg.Main.test2(Main.java:21)
    at com.hjzgg.Main.test(Main.java:26)
    ... 1 more

//如果外部类返回是  private类型的内部类，非此外部类的其他方法不能访问这个内部类
class A{
    private class B{
        public void fun(){}
    }
    public B getBInstance(){
        return new B();
    }
    
    public void invokeBMethod(B b){
        b.fun();
    }
    
    private int x;
    public int getX(){
        return x;
    }
}


//内嵌接口， 如果你不想将你的借口暴露给外部，写成如下方式，否则将内嵌接口定义成 public，或者将接口写成非内嵌接口的形式
class C{
    interface E{
        void gun();
    }
    private interface D {
        void fun();
    }
    private class DImpl implements D{
        @Override
        public void fun() {}
    }
    public D getDImpl(){
        return new DImpl();
    }
    
    public void invokeDMethod(D d){
        d.fun();
    }
}

//内部链接提供给外部
interface H{
    void fun();
}
class I{
    private class G implements H{
        @Override
        public void fun() {}
    }
    
    public H getHImpl(){
        return new G();
    }
}

class F implements C.E{
    @Override
    public void gun() {}
}

public class Main{
    public static void main(String[] args) {
         A a = new A();
         int x = a.getX();
         //error, The type A.B is not visible
         //a.getBInstance().fun();
         a.invokeBMethod(a.getBInstance());
         
         C c = new C();
         //error, The type A.B is not visible
//         c.getDImpl().fun();
         c.invokeDMethod(c.getDImpl());
         
         H h = new I().getHImpl();
         h.fun();
    }
}