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- Question
What will be the output of the program?
public class WrapTest { public static void main(String [] args) { int result = 0; short s = 42; Long x = new Long("42"); Long y = new Long(42); Short z = new Short("42"); Short x2 = new Short(s); Integer y2 = new Integer("42"); Integer z2 = new Integer(42); if (x == y) /* Line 13 */ result = 1; if (x.equals(y) ) /* Line 15 */ result = result + 10; if (x.equals(z) ) /* Line 17 */ result = result + 100; if (x.equals(x2) ) /* Line 19 */ result = result + 1000; if (x.equals(z2) ) /* Line 21 */ result = result + 10000; System.out.println("result = " + result); } }
Options- A. result = 1
- B. result = 10
- C. result = 11
- D. result = 11010
- Correct Answer
- result = 10
ExplanationLine 13 fails because == compares reference values, not object values. Line 15 succeeds because both String and primitive wrapper constructors resolve to the same value (except for the Character wrapper). Lines 17, 19, and 21 fail because the equals() method fails if the object classes being compared are different and not in the same tree hierarchy.
More questions
- 1. What will be the output of the program?
public class X { public static void main(String [] args) { try { badMethod(); System.out.print("A"); } catch (Exception ex) { System.out.print("B"); } finally { System.out.print("C"); } System.out.print("D"); } public static void badMethod() { throw new Error(); /* Line 22 */ } }
Options- A. ABCD
- B. Compilation fails.
- C. C is printed before exiting with an error message.
- D. BC is printed before exiting with an error message. Discuss
Correct Answer: C is printed before exiting with an error message.
Explanation:
Error is thrown but not recognised line(22) because the only catch attempts to catch an Exception and Exception is not a superclass of Error. Therefore only the code in the finally statement can be run before exiting with a runtime error (Exception in thread "main" java.lang.Error).- 2. What will be the output of the program?
class BoolArray { boolean [] b = new boolean[3]; int count = 0; void set(boolean [] x, int i) { x[i] = true; ++count; } public static void main(String [] args) { BoolArray ba = new BoolArray(); ba.set(ba.b, 0); ba.set(ba.b, 2); ba.test(); } void test() { if ( b[0] && b[1] | b[2] ) count++; if ( b[1] && b[(++count - 2)] ) count += 7; System.out.println("count = " + count); } }
Options- A. count = 0
- B. count = 2
- C. count = 3
- D. count = 4 Discuss
Correct Answer: count = 3
Explanation:
The reference variables b and x both refer to the same boolean array. count is incremented for each call to the set() method, and once again when the first if test is true. Because of the && short circuit operator, count is not incremented during the second if test.- 3. What will be the output of the program?
public class X { public static void main(String [] args) { try { badMethod(); /* Line 7 */ System.out.print("A"); } catch (Exception ex) /* Line 10 */ { System.out.print("B"); /* Line 12 */ } finally /* Line 14 */ { System.out.print("C"); /* Line 16 */ } System.out.print("D"); /* Line 18 */ } public static void badMethod() { throw new RuntimeException(); } }
Options- A. AB
- B. BC
- C. ABC
- D. BCD Discuss
Correct Answer: BCD
Explanation:
(1) A RuntimeException is thrown, this is a subclass of exception.(2) The exception causes the try to complete abruptly (line 7) therefore line 8 is never executed.
(3) The exception is caught (line 10) and "B" is output (line 12)
(4) The finally block (line 14) is always executed and "C" is output (line 16).
(5) The exception was caught, so the program continues with line 18 and outputs "D".
- 4. What will be the output of the program?
public class MyProgram { public static void main(String args[]) { try { System.out.print("Hello world "); } finally { System.out.println("Finally executing "); } } }
Options- A. Nothing. The program will not compile because no exceptions are specified.
- B. Nothing. The program will not compile because no catch clauses are specified.
- C. Hello world.
- D. Hello world Finally executing Discuss
Correct Answer: Hello world Finally executing
Explanation:
Finally clauses are always executed. The program will first execute the try block, printing Hello world, and will then execute the finally block, printing Finally executing.Option A, B, and C are incorrect based on the program logic described above. Remember that either a catch or a finally statement must follow a try. Since the finally is present, the catch is not required.
- 5. Which two statements are true about comparing two instances of the same class, given that the equals() and hashCode() methods have been properly overridden?
- If the equals() method returns true, the hashCode() comparison == must return true.
- If the equals() method returns false, the hashCode() comparison != must return true.
- If the hashCode() comparison == returns true, the equals() method must return true.
- If the hashCode() comparison == returns true, the equals() method might return true.
Options- A. 1 and 4
- B. 2 and 3
- C. 3 and 4
- D. 1 and 3 Discuss
Correct Answer: 1 and 4
Explanation:
(1) is a restatement of the equals() and hashCode() contract. (4) is true because if the hashCode() comparison returns ==, the two objects might or might not be equal.(2) and (3) are incorrect because the hashCode() method is very flexible in its return values, and often two dissimilar objects can return the same hash code value.
- 6. Which three are methods of the Object class?
- notify();
- notifyAll();
- isInterrupted();
- synchronized();
- interrupt();
- wait(long msecs);
- sleep(long msecs);
- yield();
Options- A. 1, 2, 4
- B. 2, 4, 5
- C. 1, 2, 6
- D. 2, 3, 4 Discuss
Correct Answer: 1, 2, 6
Explanation:
(1), (2), and (6) are correct. They are all related to the list of threads waiting on the specified object.(3), (5), (7), and (8) are incorrect answers. The methods isInterrupted() and interrupt() are instance methods of Thread.
The methods sleep() and yield() are static methods of Thread.
D is incorrect because synchronized is a keyword and the synchronized() construct is part of the Java language.
- 7. Which two can be used to create a new Thread?
- Extend java.lang.Thread and override the run() method.
- Extend java.lang.Runnable and override the start() method.
- Implement java.lang.Thread and implement the run() method.
- Implement java.lang.Runnable and implement the run() method.
- Implement java.lang.Thread and implement the start() method.
Options- A. 1 and 2
- B. 2 and 3
- C. 1 and 4
- D. 3 and 4 Discuss
Correct Answer: 1 and 4
Explanation:
There are two ways of creating a thread; extend (sub-class) the Thread class and implement the Runnable interface. For both of these ways you must implement (override and not overload) the public void run() method.(1) is correct - Extending the Thread class and overriding its run method is a valid procedure.
(4) is correct - You must implement interfaces, and runnable is an interface and you must also include the run method.
(2) is wrong - Runnable is an interface which implements not Extends. Gives the error: (No interface expected here)
(3) is wrong - You cannot implement java.lang.Thread (This is a Class). (Implements Thread, gives the error: Interface expected). Implements expects an interface.
(5) is wrong - You cannot implement java.lang.Thread (This is a class). You Extend classes, and Implement interfaces. (Implements Thread, gives the error: Interface expected)
- 8. What is the numerical range of a char?
Options- A. -128 to 127
- B. -(215) to (215) - 1
- C. 0 to 32767
- D. 0 to 65535 Discuss
Correct Answer: 0 to 65535
Explanation:
A char is really a 16-bit integer behind the scenes, so it supports 2 16 (from 0 to 65535) values.- 9. Which four can be thrown using the throw statement?
- Error
- Event
- Object
- Throwable
- Exception
- RuntimeException
Options- A. 1, 2, 3 and 4
- B. 2, 3, 4 and 5
- C. 1, 4, 5 and 6
- D. 2, 4, 5 and 6 Discuss
Correct Answer: 1, 4, 5 and 6
Explanation:
The (1), (4), (5) and (6) are the only four that can be thrown.An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch.
The Throwable class is the superclass of all errors and exceptions in the Java language.
The class Exception and its subclasses are a form of Throwable that indicates conditions that a reasonable application might want to catch (checked exceptions)
RuntimeException is the superclass of those exceptions that can be thrown during the normal operation of the Java Virtual Machine.
- 10. Which three are valid method signatures in an interface?
- private int getArea();
- public float getVol(float x);
- public void main(String [] args);
- public static void main(String [] args);
- boolean setFlag(Boolean [] test);
Options- A. 1 and 2
- B. 2, 3 and 5
- C. 3, 4, and 5
- D. 2 and 4 Discuss
Correct Answer: 2, 3 and 5
Explanation:
(2), (3), and (5). These are all valid interface method signatures.(1), is incorrect because an interface method must be public; if it is not explicitly declared public it will be made public implicitly. (4) is incorrect because interface methods cannot be static.
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- 1. What will be the output of the program?