scjp考题1（详尽答案）(8)

Given class parent method1 be public, while class child method1 be pivate, there will be a compile error, for method1 in class child tries to give a weak overridden; given class parent method1 be private, and main() in class child used p.method1(), there will be another compile error for method1 in class parent is not accessible; given class parent method1 be public and class child method1 be public, the end will be “prints: parent’s method2() child’s method1()”; given method1 in class child be private, there will be a compile error which is “mehod1 in class child is not accessible”.

29. What will happen when you attempt to compile and run the following code snippet?
String str = "Java";
StringBuffer buffer = new StringBuffer(str);
if(str.equals(buffer)){
System.out.println("Both are equal");
}else{
System.out.println("Both are not equal");
}
A. it will print – both are not equal
B. it will print – both are equal
C. compile time error
D. Runtime error
A is the correct choice. The equals method overridden in String class returns true if and only if the argument is not null and is a String object that represents the same sequence of characters as this String object. Hence, though the contents of both str and buffer contain "Java", the str.equals(buffer) call results in false.
The equals method of Object class is of form -public boolean equals(Object anObject). Hence, comparing objects of different classes will never result in compile time or runtime error.

30. What will happen when you attempt to compile and run the following code?
public class MyThread extends Thread{
String myName;
MyThread(String name){
myName = name;
}
public void run(){
for(int i=0; i<100;i ){
System.out.println(myName);
}
}
public static void main(String args[]){
try{
MyThread mt1 = new MyThread("mt1");
MyThread mt2 = new MyThread("mt2");
mt1.start();
// XXX
mt2.start();
}catch(InterruptedException ex){}
}
}
A. compile error
B. mt1.join();
C. mt1.sleep(100);
D. mt1.run()
E. nothing need
Choice A and B are correct. In its current condition, the above code will not compile as "InterruptedException" is never thrown in the try block. The compiler will give following exception: "Exception java.lang.InterruptedException is never thrown in the body of the corresponding try statement."
Note that calling start() method on a Thread doesn't start the Thread. It only makes a Thread ready to be called. Depending on the operating system and other running threads, the thread on which start is called will get executed. After making the above code to compile (by changing the InterruptedException to some other type like Exception), the output can't be predicted (the order in which mt1 and mt2 will be printed can't be guaranteed). In order to make the MyThread class prints "mt1" (100 times) followed by "mt2" (100 times), mt1.join() can be placed at //XXX position. The join() method waits for the Thread on which it is called to die. Thus on calling join() on mt1, it is assured that mt2 will not be executed before mt1 is completed. Also note that the join() method throws InterruptedException, which will cause the above program to compile successfully. Thus choice A and B are correct.