SCJP真题库更新15

QUESTION 71

Given:

A programmer is developing a class Key, that will be used as a key in a standard java.util.HashMap.

Which two methods should be overridden to assure that key works correctly as a key? (choose two) 

A. public int hashCode ( )  

B. public Boolean equals (Key k)

C. public int compareTo (Object o)

D. public boolean equals (Object o) 

 

 

Answer: ( A, D )

HashMap中的key不能重复，因此作为key的对象类必须实现hashCode和equals方法。

 

QUESTION 72  ---

Given the exhibit:

Which two, inserted at line 2 will allow the code to compile? (Choose Two) 

A. public class MinMax< ? > {

B. public class MinMax < ? extends Number> {

C. public class MinMax <N extends Object> {

D. public class MinMax <N extends Number > {    

E. public class MinMax < ? extends Object > {

F. public class MinMax < N extends Integer > {

 

 

Answer: ( D, F )

所有数字系的原始数据封装类都继承或者实现了它们的公共父类java.lang.Number类（抽象类）中定义的如下的6个数字取值方法：

 

    public abstract int intValue();

 

    public abstract long longValue();

 

    public abstract float floatValue();

 

    public abstract double doubleValue();

 

    public byte byteValue() {

   return (byte)intValue();

    }

 

 

    public short shortValue() {

   return (short)intValue();

    }

 

 

N是泛型类别变量，它相当于一个类型。

A      在类定义的时候声明泛型是不能用？这个万用字符的。

B      同上

C      Object没有doubleValue()方法。

D      OK.

E      同A。

F      OK.

 

 

QUESTION 73

Given the exhibit:

enum Example { ONE, TWO, THREE }

Which statement is true? 

A. The expressions (ONE = = ONE) and ONE.equals (ONE) are both guaranteed to be true.

B. The expression (ONE < TWO ) is guaranteed to be true and ONE.compareTo (TWO) is guaranteed to be less than one.

C. The Example values cannot be used in a raw java.util.HashMap; instead, the programmer must use a java.util.EnumMap.

D. The Example values can be used in a java.util.SortedSet, but the set will NOT be sorted because enumerated Type do NOT IMPLEMENT java.lang.Comparable.

 

 

Answer: ( A )

A 正确

B 不能用大于号和小于号比较枚举。

C 错误  枚举类型的元素可以用在HASHMAP里作为键或者值。

HashMap<Integer, Example>  h = new HashMap<Integer, Example>();

h.put(1, Example.ONE);

D 错误，枚举类型的元素值是可以放入SortedSet里的例如TreeSet. 把枚举类型的元素转成字符串在放入其中，字符串就已经实现comparable接口。可以自然排序。

SortedSet<Example> s = new TreeSet<Example>();

s.add(Example.TWO);

s.add(Example.ONE);

System.out.println(s);

打印出结果[ONE, TWO]

 

QUESTION 74

Given the exhibit:

 

Which line of code marks the earliest point that an object referenced by intObj becomes a candidate for garbage collection?

A. Line 16

B. Line 17

C. Line 18

D. Line 19

E. The object is NOT a candidate for garbage collection.

 

 

Answer: ( D )

intObj一直会被numbers引用，直到19行之后

 

QUESTION 75

Given:

And the command line invocation:

Java Yippee2 a b c

What is the result?

A. a b

B. b c

C. a b c

D. Compilation fails.

E. An exception is thrown at runtime.

 

 

Answer: ( B )