java复习题及答案.doc
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一、选择
1．下面哪些是java语言中的关键字？ A．sizeof B．abstract C．NULL D．Native
2．下面语句哪个是正确的？ A．char=abc; B．long l=oxfff; C．float f=0.23; D．double=0.7E-3;
3．以下程序测试String 类的各种构造方法，试选出其运行效果。
class STR{
public static void main(String args[]){
String s1=new String();
String s2=new String(String 2);
char chars[]={a, ,s,t,r,i,n,g};
String s3=new String(chars);
String s4=new String(chars,2,6);
byte bytes[]={0,1,2,3,4,5,6,7,8,9};
StringBuffer sb=new StringBuffer(s3);
String s5=new String(sb);
System.out.println(The String No.1 is +s1);
System.out.println(The String No.2 is +s2);
System.out.println(The String No.3 is +s3);
System.out.println(The String No.4 is +s4);
System.out.println(The String No.5 is +s5);
}
}A．The String No.1 is
The String No.2 is String 2
The String No.3 is a string
The String No.4 is string
The String No.5 is a string
B．The String No.1 is
The String No.2 is String 2
The String No.3 is a string
The String No.4 is tring
The String No.5 is a string
C．The String No.1 is
The String No.2 is String 2
The String No.3 is a string
The String No.4 is strin
The String No.5 is a string
D．以上都不对4．下面语句段的输出结果是什么？
int i = 9;
switch (i) {
default:
System.out.println(default);
case 0:
System.out.println(zero);
break;
case 1:
System.out.println(one);
case 2:
System.out.println(two); } A．default B．default, zero
C．error default clause not defined D．no output displayed
5．有关类Demo，哪句描述是正确的？
public class Demo extends Base{
private int count;
public Demo(){
System.out.println(A Demo object has been created);
}
protected void addOne() {count++; }
} A．当创建一个Demo类的实例对象时，count的值为0。B．当创建一个Demo类的实例对象时，count的值是不确定的。C．超类对象中可以包含改变count 值的方法。D．Demo的子类对象可以访问count。当编译和运行下列程序段时，会发生什么？
class Base {}
class Sub extends Base {}
class Sub2 extends Base {}
public class CEx{
public static void main(String argv[]){
Base b = new Base();
Sub s = (Sub) b;
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