Java代码查错题

Java代码查错题:

1.

abstract class Name {
   private String name;
   public abstract boolean isStupidName(String name) {}
  }

abstract class Name {
   private String name;
   public abstract boolean isStupidName(String name) {}
  }

答案: 错。abstract method必须以分号结尾，且不带花括号。

2.

public class Something {
   void doSomething () {
       private String s = "";
       int l = s.length();
   }
}

public class Something {
   void doSomething () {
       private String s = "";
       int l = s.length();
   }
}

有错吗?
答案: 错。局部变量前不能放置任何访问修饰符 (private，public，和protected)。final可以用来修饰局部变量
(final如同abstract和strictfp，都是非访问修饰符，strictfp只能修饰class和method而非variable)。

3.

abstract class Something {
   private abstract String doSomething ();
}

abstract class Something {
   private abstract String doSomething ();
}

答案: 错。abstract的methods不能以private修饰。abstract的methods就是让子类implement(实现)具体细节的，怎么可以用private把abstract
method封锁起来呢?(同理，abstractmethod前不能加final)。

4.

publicclass Something {
   public int addOne(final int x) {
       return ++x;
   }
}

publicclass Something {
   public int addOne(final int x) {
       return ++x;
   }
}

答案: 错。int x被修饰成final，意味着x不能在addOne method中被修改。

5.

public class Something {
   public static void main(String[] args) {
       Other o = new Other();
       new Something().addOne(o);
   }
   public void addOne(final Other o) {
       o.i++;
   }
}
class Other {
   public int i;
}

public class Something {
   public static void main(String[] args) {
       Other o = new Other();
       new Something().addOne(o);
   }
   public void addOne(final Other o) {
       o.i++;
   }
}
class Other {
   public int i;
}

和上面的很相似，都是关于final的问题，这有错吗?
答案: 正确。在addOne method中，参数o被修饰成final。如果在addOne method里我们修改了o的reference
(比如: o= new Other();)，那么如同上例这题也是错的。但这里修改的是o的member vairable
(成员变量)，而o的reference并没有改变。

6.

class Something {
    int i;
    public void doSomething() {
        System.out.println("i = "+ i);
    }
}

class Something {
    int i;
    public void doSomething() {
        System.out.println("i = "+ i);
    }
} 

答案: 正确。输出的是"i = 0"。int i属於instant variable (实例变量，或叫成员变量)。instant variable有default value。int的default value是0。

7.

class Something {
    final int i;
    public void doSomething() {
        System.out.println("i = "+ i);
    }
}

class Something {
    final int i;
    public void doSomething() {
        System.out.println("i = "+ i);
    }
}

答案: 错。final int i是个final的instant variable (实例变量，或叫成员变量)。final的instant variable没有default value，必须在constructor (构造器)结束之前被赋予一个明确的值。可以修改为"final int i = 0;"。

8.

public class Something {
     public static void main(String[] args) {
        Something s = new Something();
       System.out.println("s.doSomething() returns " + doSomething());
    }
    public String doSomething() {
        return "Do something ...";
    }
}

public class Something {
     public static void main(String[] args) {
        Something s = new Something();
       System.out.println("s.doSomething() returns " + doSomething());
    }
    public String doSomething() {
        return "Do something ...";
    }
}

答案: 错。看上去在main里call doSomething没有什么问题，毕竟两个methods都在同一个class里。但仔细看，main是static的。static method不能直接call non-static methods。可改成"System.out.println("s.doSomething()returns " + s.doSomething());"。同理，static method不能访问non-static instant variable。

9.此处，Something类的文件名叫OtherThing.java

class Something {
    private static void main(String[] something_to_do){
        System.out.println("Dosomething ...");
    }
}

class Something {
    private static void main(String[] something_to_do){        
        System.out.println("Dosomething ...");
    }
}

答: 正确。从来没有人说过Java的Class名字必须和其文件名相同。但public class的名字必须和文件名相同。

10．

interface A{
   int x = 0;
}
class B{
   int x =1;
}
class C extends B implements A {
   public void pX(){
      System.out.println(x);
   }
   public static void main(String[] args) {
      new C().pX();
   }
}

interface A{
   int x = 0;
}
class B{
   int x =1;
}
class C extends B implements A {
   public void pX(){
      System.out.println(x);
   }
   public static void main(String[] args) {
      new C().pX();
   }
}

答案：错误。在编译时会发生错误(错误描述不同的JVM有不同的信息，意思就是未明确的x调用，两个x都匹配（就象在同时import java.util和java.sql两个包时直接声明Date一样）。对于父类的变量,可以用super.x来明确，而接口的属性默认隐含为 public static final.所以可以通过A.x来明确。

11.

interface Playable {
    void play();
}
interface Bounceable {
    void play();
}
interface Rollable extends Playable, Bounceable {
    Ball ball = new Ball("PingPang");
}
class Ball implements Rollable {
    private String name;
    public String getName() {
        return name;
    }
    public Ball(String name) {
        this.name =name;
    }
   public void play() {
        ball = newBall("Football");
        System.out.println(ball.getName());
    }
}

interface Playable {
    void play();
}
interface Bounceable {
    void play();
}
interface Rollable extends Playable, Bounceable {
    Ball ball = new Ball("PingPang");
}
class Ball implements Rollable {
    private String name;
    public String getName() {
        return name;
    }
    public Ball(String name) {
        this.name =name;        
    }
   public void play() {
        ball = newBall("Football");
        System.out.println(ball.getName());
    }
}

这个错误不容易发现。
答案: 错。"interface Rollable extendsPlayable, Bounceable"没有问题。interface可继承多个interfaces，所以这里没错。问题出在interface Rollable里的"Ball ball = newBall("PingPang");"。任何在interface里声明的interface variable (接口变量，也可称成员变量)，默认为public static final。也就是说"Ball ball = new Ball("PingPang");"实际上是"public static final Ballball = new Ball("PingPang");"。在Ball类的Play()方法中，"ball = newBall("Football");"改变了ball的reference，而这里的ball来自Rollable interface，Rollable interface里的ball是public static final的，final的object是不能被改变reference的。因此编译器将在"ball = newBall("Football");"这里显示有错。