Java作业8及答案

一、判断程序段
1. class Something {
final int i;
public void doSomething() {
System.out.println("i = " + i);
}
}
7. 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) {
= name;
}
public void play() {
ball = new Ball("Football");
System.out.println(ball.getName());
}
}
二、下面代码编译和运行的结果是什么？
1. class MyThread extends Thread{
public void run(){
System.out.println("MyThread: run()");
}
public void start(){
System.out.println("MyThread: start()");
}
}
class MyRunnable implements Runnable{
public void run(){
System.out.println("MyRunnable: run()");
}
public void start(){
System.out.println("MyRunnable: start()");
}
}
public class MyTest {
public static void main(String args[]){
MyThread myThread = new MyThread();
MyRunnable myRunnable = new MyRunnable();
Thread thread = new Thread(myRunnable);
myThread.start();
thread.start();
}
}
A.打印MyThread: start() 后面跟MyRunnable: run()
B.打印MyThread: run() 后面跟MyRunnable: start()
C.打印MyThread: start() 后面跟MyRunnable: start()
D.打印MyThread: run() 后面跟MyRunnable: run()
2. // 文件名：SuperclassX.java
package packageX;
public class SuperclassX{
protected void superclassMethodX(){}
int superclassVarX;
}
//文件名：SubclassY.java
1.package packageX.packageY;
2.
3.public class SubclassY extends SuperclassX
4.{
5. SuperclassX objX = new SubclassY();
6. SubclassY objY = new SubclassY();
7. void subclassMethodY()
8. {
9. objY.superclassMethodX();
10. int i;
11. i = objY.superclassVarX;
12. }
13.}
A.第5行编译错
B.第9行编译错
C.第11行意外
D.都不是
3. 程序段如下：
1.class MyClass
2.{
3. void myMethod(int i) {System.out.println("int version");}
4. void myMethod(String s) {System.out.println("String version");}
5. public static void main(String args[])
6. {
7. MyClass obj = new MyClass();
8. char ch = 'c';
9. obj.myMethod(ch);
10. }
11.}
A.第四行编译出错
B.第九行抛出例外
C.输出结果：int version
D.输出结果：String version
4. public class ThrowsDemo {
static void throwMethod() {
System.out.println("Inside throwMethod.");
throw new IllegalAccessException("demo");
}
public static void main(String args[]) {
try {
throwMethod();
} catch (IllegalAccessException e) {
System.out.println("Caught " + e);
}
}
}
A.编译错
B.运行错
C.编译成功，但是没有输出结果
D.以上都不是
5. class X {
Y b = new Y();
X() {
System.out.print("X");
}
}
class Y {
Y() {
System.out.print("Y");
}
}
public class Z extends X {
Y y = new Y();
Z() {
System.out.print("Z");
}
public static void main(String[] args) {
new Z();
}
}
A.Z
B.YZ
C.XYZ
D.YXYZ
6. public class MyThread implements Runnable {
String myString = "Yes ";
public void run() {
this.myString = "No ";
}
public static void main(String[] args) {
MyThread t = new MyThread();
new Thread(t).start();
for (int i=0; i < 10; i++)
System.out.print(t.myString);
}
}
Ａ. 打印yes yes yes yes yes yes Ｂ. 打印no no no no no no no no
Ｃ. 打印yes no yes no ye no ye no Ｄ. 不确定
答案：
一、1. 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;"。

2. 错。

Ball ball = new Ball("PingPang");"实际上是"public static final Ball ball = new Ball("PingPang");"。

在Ball类的Play()方法中，"ball = new Ball("Football");"改变了ball的reference，所以编译时间错。

二、1.答案A
2.答案D
3.答案C
4.答案：A
5.答案D
6.答案Ｄ。