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- Question
What will be the output of the program?
public class TestDogs { public static void main(String [] args) { Dog [][] theDogs = new Dog[3][]; System.out.println(theDogs[2][0].toString()); } } class Dog { }
Options- A. null
- B. theDogs
- C. Compilation fails
- D. An exception is thrown at runtime
- Correct Answer
- An exception is thrown at runtime
ExplanationThe second dimension of the array referenced by theDogs has not been initialized. Attempting to access an uninitialized object element (System.out.println(theDogs[2][0].toString());) raises a NullPointerException.
Language Fundamentals problems
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- 1. What will be the output of the program?
public class CommandArgsTwo { public static void main(String [] argh) { int x; x = argh.length; for (int y = 1; y <= x; y++) { System.out.print(" " + argh[y]); } } }and the command-line invocation is> java CommandArgsTwo 1 2 3
Options- A. 0 1 2
- B. 1 2 3
- C. 0 0 0
- D. An exception is thrown at runtime Discuss
Correct Answer: An exception is thrown at runtime
Explanation:
An exception is thrown because at some point in (System.out.print(" " + argh[y]);), the value of x will be equal to y, resulting in an attempt to access an index out of bounds for the array. Remember that you can access only as far as length - 1, so loop logical tests should use x < someArray.length as opposed to x < = someArray.length.- 2. What will be the output of the program, if this code is executed with the command line:
> java F0091 world
public class F0091 { public void main( String[] args ) { System.out.println( "Hello" + args[0] ); } }
Options- A. Hello
- B. Hello Foo91
- C. Hello world
- D. The code does not run. Discuss
Correct Answer: The code does not run.
Explanation:
Option D is correct. A runtime error will occur owning to the main method of the code fragment not being declared static:Exception in thread "main" java.lang.NoSuchMethodError: main
The Java Language Specification clearly states: "The main method must be declared public, static, and void. It must accept a single argument that is an array of strings."
- 3. What will be the output of the program?
public class X { public static void main(String [] args) { String names [] = new String[5]; for (int x=0; x < args.length; x++) names[x] = args[x]; System.out.println(names[2]); } }and the command line invocation is> java X a b
Options- A. names
- B. null
- C. Compilation fails
- D. An exception is thrown at runtime Discuss
Correct Answer: null
Explanation:
The names array is initialized with five null elements. Then elements 0 and 1 are assigned the String values "a" and "b" respectively (the command-line arguments passed to main). Elements of names array 2, 3, and 4 remain unassigned, so they have a value of null.- 4. What will be the output of the program?
public class CommandArgsThree { public static void main(String [] args) { String [][] argCopy = new String[2][2]; int x; argCopy[0] = args; x = argCopy[0].length; for (int y = 0; y < x; y++) { System.out.print(" " + argCopy[0][y]); } } }and the command-line invocation is> java CommandArgsThree 1 2 3
Options- A. 0 0
- B. 1 2
- C. 0 0 0
- D. 1 2 3 Discuss
Correct Answer: 1 2 3
Explanation:
In argCopy[0] = args;, the reference variable argCopy[0], which was referring to an array with two elements, is reassigned to an array ( args) with three elements.- 5. Which of these will create and start this thread?
public class MyRunnable implements Runnable { public void run() { // some code here } }
Options- A. new Runnable(MyRunnable).start();
- B. new Thread(MyRunnable).run();
- C. new Thread(new MyRunnable()).start();
- D. new MyRunnable().start(); Discuss
Correct Answer: new Thread(new MyRunnable()).start();
Explanation:
Because the class implements Runnable, an instance of it has to be passed to the Thread constructor, and then the instance of the Thread has to be started.A is incorrect. There is no constructor like this for Runnable because Runnable is an interface, and it is illegal to pass a class or interface name to any constructor.
B is incorrect for the same reason; you can't pass a class or interface name to any constructor.
D is incorrect because MyRunnable doesn't have a start() method, and the only start() method that can start a thread of execution is the start() in the Thread class.
- 6. What will be the output of the program?
public class Test { public static void main(String [] args) { signed int x = 10; for (int y=0; y<5; y++, x--) System.out.print(x + ", "); } }
Options- A. 10, 9, 8, 7, 6,
- B. 9, 8, 7, 6, 5,
- C. Compilation fails.
- D. An exception is thrown at runtime. Discuss
Correct Answer: Compilation fails.
Explanation:
The word "signed" is not a valid modifier keyword in the Java language. All number primitives in Java are signed. Hence the Compilation will fails.- 7. What will be the output of the program?
public class CommandArgs { public static void main(String [] args) { String s1 = args[1]; String s2 = args[2]; String s3 = args[3]; String s4 = args[4]; System.out.print(" args[2] = " + s2); } }and the command-line invocation is> java CommandArgs 1 2 3 4
Options- A. args[2] = 2
- B. args[2] = 3
- C. args[2] = null
- D. An exception is thrown at runtime. Discuss
Correct Answer: An exception is thrown at runtime.
Explanation:
An exception is thrown because in the code String s4 = args[4];, the array index (the fifth element) is out of bounds. The exception thrown is the cleverly named ArrayIndexOutOfBoundsException.- 8. In the given program, how many lines of output will be produced?
public class Test { public static void main(String [] args) { int [] [] [] x = new int [3] [] []; int i, j; x[0] = new int[4][]; x[1] = new int[2][]; x[2] = new int[5][]; for (i = 0; i < x.length; i++) { for (j = 0; j < x[i].length; j++) { x[i][j] = new int [i + j + 1]; System.out.println("size = " + x[i][j].length); } } } }
Options- A. 7
- B. 9
- C. 11
- D. 13
- E. Compilation fails Discuss
Correct Answer: 11
Explanation:
The loops use the array sizes (length).It produces 11 lines of output as given below.
D:\Java>javac Test.java D:\Java>java Test size = 1 size = 2 size = 3 size = 4 size = 2 size = 3 size = 3 size = 4 size = 5 size = 6 size = 7
Therefore, 11 is the answer.
- 9. Which three statements are true?
- f1 == f2
- f1 == f3
- f2 == f1[1]
- x == f1[0]
- f == f1[0]
import java.awt.Button; class CompareReference { public static void main(String [] args) { float f = 42.0f; float [] f1 = new float[2]; float [] f2 = new float[2]; float [] f3 = f1; long x = 42; f1[0] = 42.0f; } }
Options- A. 1, 2 and 3
- B. 2, 4 and 5
- C. 3, 4 and 5
- D. 1, 4 and 5 Discuss
Correct Answer: 2, 4 and 5
Explanation:
(2) is correct because the reference variables f1 and f3 refer to the same array object.(4) is correct because it is legal to compare integer and floating-point types.
(5) is correct because it is legal to compare a variable with an array element.
(3) is incorrect because f2 is an array object and f1[1] is an array element.
- 10. Which two of the following statements, inserted independently, could legally be inserted into missing section of this code?
- boolean test = (Component instanceof t);
- boolean test = (t instanceof Ticker);
- boolean test = t.instanceof(Ticker);
- boolean test = (t instanceof Component);
import java.awt.*; class Ticker extends Component { public static void main (String [] args) { Ticker t = new Ticker(); /* Missing Statements? */ } }
Options- A. 1 and 4
- B. 2 and 3
- C. 1 and 3
- D. 2 and 4 Discuss
Correct Answer: 2 and 4
Explanation:
(2) is correct because class type Ticker is part of the class hierarchy of t; therefore it is a legal use of the instanceof operator. (4) is also correct because Component is part of the hierarchy of t, because Ticker extends Component.(1) is incorrect because the syntax is wrong. A variable (or null) always appears before the instanceof operator, and a type appears after it. (3) is incorrect because the statement is used as a method (t.instanceof(Ticker);), which is illegal.
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- 1. What will be the output of the program?