SCJP真题库更新6

QUESTION 18

Exhibit:

 

What is the result? 

A. The code will deadlock

B. The code may run with no output

C. An exception is thrown at runtime

D. The code may run with output " 0 6 "

E. The code may run with output "2 0 6 4"

F. The code may run with output "0 2 4 6"   

 

 

Answer: ( F )

情况1 run方法运行以后运行 再调用getResult();

情况2先调用getResult()，挂起，等待run()方法修改isComplete的值

线程0, 线程1, 线程2, 线程3, main 共有5个线程,中间线程执行的顺序如上面所说的,但是最后的结果是main的for(Computation c: computations)决定的,所以结果总是0 2 4 6

参考大纲：多线程

 

QUESTION 19

Given the exhibit:

 

What is the result? 

A. Compilation fails

B. An exception is thrown at runtime

C. The code executes normally and prints "sleep"

D. The code executes normally, but nothing is printed.

 

 

Answer: ( C ) 执行2行的时候main被放到Blocked中,过3秒后才会回到Runnable pool中,继续执行输出”sleep”

参考大纲：多线程

 

QUESTION 20

Which two statements are true about has-a and is-a relationships? (choose two) 

A. Inheritance represents an is -a relationship

B. Inheritance represents a has-a relationship

C. Interfaces must be used when creating a has-a relationship

D. Instance variables can be used when creating a has-a relationship

 

 

Answer: ( A, D )

Is-a:继承      class A extends B{  }  is-a  ”是一个”,属于上下的关系,

Has-a:聚合  class A{ B b;}         has-a  “ 有一个”   属于聚合的关系,在java中用来表示类中的成员变量

参考大纲：面向对象

 

QUESTION 21

Given the exhibit:

     

What can directly access and change the value of the variable name? 

A. any class

B. only the Target class

C. any class in the Certkiller package

D. any class that extends Target

 

 

Answer: ( C )

要修改name，必须先取得Target的对象，Target的存取权限是default，因此同一个包才能访问。

参考大纲：面向对象 — package与import

 

QUESTION 22

Which three statements are true? (choose three) 

A. A final method in class X can be abstract if and only if X is abstract

B. A protected method in class X can be overridden by any subclass of  X.    

C. A private static method can be called only within other static methods in class X.

D. A non-static public final method in class X can be overridden in any subclass of X.

E. A public static method in class X can be called by a subclass of X without explicitly

referencing the class X.             

F. A method with the same signature as a private final method in class X can be implemented in a subclass of X. 

 

 

Answer: ( B，E，F )

A 错误 抽象方法必须被实现，因此是不能加final修饰符的

B 正确 子类可以合法的复写父类的protected权限的方法

C 错误 private static方法在X类中的方法都可以调用,static 和non-static都可以

D 错误 final方法不可以复写，除非是private方法.但是这种情况不叫改写,而是子类有和父类一样的private final方法,两个方法是各自独立的!!!!因为子类看不到父类中的private方法

E 正确 不用加上父类别的引用，子类就可以直接调用父类的static方法

F 正确 父类的private final方法和子类中同名private final方法是两个独立方法,这种情况就不会发生改写,而是各自拥有自己的方法!所以不会与final的定义相冲突。

参考大纲：面向对象

 

QUESTION 23

Place the Types in one of the Type columns, and the Relationships in the

Relationship column, to define appropriate has-a and is-a relationships.

Answer: (  )

Dog   is-a  Animal        是一类

Forest  has-a  Tree         有一个

Rectangle  has-a  Side     矩形有一个正方形

Java Book  is-a  Programming Book    是一个

参考大纲：面向对象

 

QUESTION 24

Replace two of the Modifiers that appear in the Single class to make the code compile.

Note: Three modifiers will not be used and four modifiers in the code will remain

unchanged.

Answer: (  )

 

 

public class Single{

        private static Single instance;

        public static Single getInstance(){

                if(instance==null) instatnce = create();

                return instance;

}

protected Single(){}  因为SingleSub继承了Single,所以Single的构造器必须可以让子类访问到!!!!

static Single Create(){ return new Single();   }  因为第4行…=create();说明这个方法必须是static的方法

}

参考大纲：static成员和继承时方法的权限控制