Sunday, January 13, 2008

SCJP Mock Questions and Answers

1. public class MyThread implements Runnable {
2. private String holdA = "This is ";
3. private int[] holdB = {1,2,3,4,5,6,7,8,9,10};
5. public static void main(String args[]) {
6. MyThread z = new MyThread();
7. (new Thread(z)).start();
8. (new Thread(z)).start();
9. }
11. public synchronized void run() {
12. for(int w = 0;w <>
13. System.out.println(holdA + holdB[w] + ".");
14. }
15. }
16. }

What is the result?

(1) Compilation fails because of an error on line 6
(2) Compilation fails because of an error on line 11
(3) Compilation fails because of errors on lines 7 and 8
(4) Compilation succeeds and the program prints each value in the holdB array at the end of the "This is " line. Each value is printed two times before the program ends, and the values are not printed in sequential order
(5) Compilation succeeds & the prog. prints each val in the holdB array at the end of the "This is " line. Each val is printed in order from 1-10 & after the val 10 prints, it starts printing the vals 1-10 in order again

Answer : 5
Explanation :
Option 5 is correct because the Runnable interface is implemented by declaring a synchronized run() method. The method is declared as synchronized to signify that the object lock must be obtained

Options 1, 2, and 3 are incorrect because compilation succeeds. Option 4 is incorrect, but would be correct if the run() method were not declared as synchronized.

Ques 2 :Which statement about the Map interface is true?

(1) Entries are placed in a Map using the values() method
(2) Entries are placed in a Map using the entrySet() method
(3) A key/value association is added to a Map using the put() method
(4) A key/value association is added to a Map using the putAll() method

Answer : 3
Explanation :
Option 3 is correct because the put() method is used to add a key/value association to a Map.

Option 1 is incorrect because the values() method returns a Collection of all values in a Map.Option 2 is incorrect because the entrySet() method returns a Set of all mappings in a Map. Option 4 is incorrect because the pubAll() method copies all mappings from one Map to another.

Ques 3 :
Consider the following class definition:
1. public class Test extends Base {
2. public Test(int j) {
3. }
4. public Test(int j, int k) {
5. super(j, k);
6. }
7. }

Which of the following forms of constructor must exist explicitly in the definition of the Base class?

(1) Base() { }
(2) Base(int j) { }
(3) Base(int j, int k) { }
(4) Base(int j, int k, int l) { }

Answer : 1,3
Explanation :
1 and 3 are correct. In the constructor at lines 2 and 3, there is no explicit call to either this() or super(), which means that the compiler will generate a call to the zero argument superclass constructor, as in 1. The explicit call to super() at line 5 requires that the Base class must have a 7.constructor as in 3. This has two consequences. First, 3 must be one of the required constructors and therefore one of the answers.Second, the Base class must have at least that constructor defined explicitly, so the default constructor is not generated, but must be added explicitly. Therefore the constructor of 1 is also required and must be a correct answer.At no point in the Test class is there a call to either a superclass constructor with one or three arguments, so 2 and 4 need not explicitly exist.


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