JAVA 面试题及思考

public class Test {

    public static void main(String[] args) {
        String str = "123";
        changeStr(str);
        System.out.print(str);
    }
    
    public static void changeStr(String str){
        str = "abc";
    }

}

public class Test {
    static boolean foo(char c) {
        System.out.print(c);
        return true;
    }

    public static void main(String[] args) {
        int i = 0;
        for (foo(‘A‘); foo(‘B‘) && (i < 2); foo(‘C‘)) {
            i++;
            foo(‘D‘);
        }
    }
}

public class B extends A {  
 // here  
}  
  
class A {  
    protected int method1(int a, int b) {  
        return 0;  
    }  
}  
//Which two are valid in a class that extends class A? (Choose two)  
//A. public int method1(int a, int b) { return 0; }  
//B. private int method1(int a, int b) { return 0; }  
//C. private int method1(int a, long b) { return 0; }  
//D. public short method1(int a, int b) { return 0; }  
//E. static protected int method1(int a, int b) { return 0; } 

public class Outer {  
    public void someOuterMethod() {  
        // Line 3  
    }  
  
    public class Inner {  
    }  
  
    public static void main(String[] args) {  
        Outer o = new Outer();  
        // Line 8  
    }  
}  
  
// Which instantiates an instance of Inner?  
// A. new Inner(); // At line 3  
// B. new Inner(); // At line 8  
// C. new o.Inner(); // At line 8  
// D. new Outer.Inner(); // At line 8  
// E. new Outer().new Inner(); // At line 8  

CREATE TABLE zxg(a VARCHAR2(10),b VARCHAR2(10))  
  
INSERT INTO zxg VALUES(‘a‘,NULL);  
INSERT INTO zxg VALUES(‘b‘,‘234‘);  
INSERT INTO zxg(a,b) VALUES (‘c‘,‘‘);  
INSERT INTO zxg(a,b) VALUES (‘d‘,‘ ‘);  
  
SELECT * FROM zxg  
--1 a     
--2 b   234  
--3 c     
--4 d      
SELECT * FROM zxg WHERE b LIKE ‘%‘  
--1 b   234  
--2 d      

//final 是形容词最终的，final 类不可以被继承，final 字段不可以被改变，final方法不可以被重写  
//The type A cannot subclass the final class B  
//Cannot override the final method from B  
//The final field A.s cannot be assigned  
//finally 是副词 try{}catch(){}finally{}  
//finalize() 是方法，垃圾回收机制 

public class Test {

    public static void main(String[] args) {
        // 向上转型
        Father f = new Child();
        // f.mygirlfriendis(); 
        // The method mygirlfriendis() is undefined for the type Father
        System.out.println(f.name);
        f.iam();
        // 向下转型
        if (f instanceof Child) {
            Child c = (Child) f;
            System.out.println(c.name);
            c.iam();
            c.mygirlfriendis();
        }
    }

}

class Father {

    String name = "Father";

    public void iam() {
        System.out.println(this.name);
    }

}

class Child extends Father {

    protected String name = "Child";

    public void iam() {
        System.out.println(this.name);
    }

    public void mygirlfriendis() {
        System.out.print("Xuhua");
    }
}

Father
Child
Child
Child
Xuhua

public static void main(String[] args) {

        String s = null;
        System.out.println(String.valueOf(s));
        
        System.out.println(String.valueOf(null));
        
    }

class Test {
    Person person;

    public void f() {
//        Person xx;
//        System.out.println(xx); //The local variable xx may not have been initialized
        System.out.println(person);
    }
}

class Person {

}