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- Question
What will be the output of the program?
String x = "xyz"; x.toUpperCase(); /* Line 2 */ String y = x.replace('Y', 'y'); y = y + "abc"; System.out.println(y);
Options- A. abcXyZ
- B. abcxyz
- C. xyzabc
- D. XyZabc
- Correct Answer
- xyzabc
ExplanationLine 2 creates a new String object with the value "XYZ", but this new object is immediately lost because there is no reference to it. Line 3 creates a new String object referenced by y. This new String object has the value "xyz" because there was no "Y" in the String object referred to by x. Line 4 creates a new String object, appends "abc" to the value "xyz", and refers y to the result.
More questions
- 1. What will be the output of the program?
public class Test { public static void main(String args[]) { class Foo { public int i = 3; } Object o = (Object)new Foo(); Foo foo = (Foo)o; System.out.println("i = " + foo.i); } }
Options- A. i = 3
- B. Compilation fails.
- C. i = 5
- D. A ClassCastException will occur. Discuss
Correct Answer: i = 3
- 2. What will be the output of the program?
class Test116 { static final StringBuffer sb1 = new StringBuffer(); static final StringBuffer sb2 = new StringBuffer(); public static void main(String args[]) { new Thread() { public void run() { synchronized(sb1) { sb1.append("A"); sb2.append("B"); } } }.start(); new Thread() { public void run() { synchronized(sb1) { sb1.append("C"); sb2.append("D"); } } }.start(); /* Line 28 */ System.out.println (sb1 + " " + sb2); } }
Options- A. main() will finish before starting threads.
- B. main() will finish in the middle of one thread.
- C. main() will finish after one thread.
- D. Cannot be determined. Discuss
Correct Answer: Cannot be determined.
Explanation:
Can you guarantee the order in which threads are going to run? No you can't. So how do you know what the output will be? The output cannot be determined.add this code after line 28:
try { Thread.sleep(5000); } catch(InterruptedException e) { }
and you have some chance of predicting the outcome.
- 3. 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).- 4. 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.- 5. 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".
- 6. 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.
- 7. 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.
- 8. 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.
- 9. 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)
- 10. 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.
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- 1. What will be the output of the program?