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Objects And Collections Online Exam Quiz

Important questions about Objects And Collections. Objects And Collections MCQ questions with answers. Objects And Collections exam questions and answers for students and interviews.

1. Which collection class allows you to access its elements by associating a key with an element's value, and provides synchronization?

Options

A : java.util.SortedMap

B : java.util.TreeMap

C : java.util.TreeSet

D : java.util.Hashtable

2. Which is valid declaration of a float?

Options

A : float f = 1F;

B : float f = 1.0;

C : float f = "1";

D : float f = 1.0d;

3. What will be the output of the program? TreeSet map = new TreeSet(); map.add("one"); map.add("two"); map.add("three"); map.add("four"); map.add("one"); Iterator it = map.iterator(); while (it.hasNext() ) { System.out.print( it.next() + " " ); }

Options

A : one two three four

B : four three two one

C : four one three two

D : one two three four one

4. What will be the output of the program? public class Test { private static int[] x; public static void main(String[] args) { System.out.println(x[0]); } }

Options

A : 0

B : null

C : Compile Error

D : NullPointerException at runtime

5. What will be the output of the program? import java.util.*; class H { public static void main (String[] args) { Object x = new Vector().elements(); System.out.print((x instanceof Enumeration)+","); System.out.print((x instanceof Iterator)+","); System.out.print(x instanceof ListIterator); } }

Options

A : Prints: false,false,false

B : Prints: false,false,true

C : Prints: false,true,false

D : Prints: true,false,false

6. What will be the output of the program? package foo; import java.util.Vector; /* Line 2 */ private class MyVector extends Vector { int i = 1; /* Line 5 */ public MyVector() { i = 2; } } public class MyNewVector extends MyVector { public MyNewVector () { i = 4; /* Line 15 */ } public static void main (String args []) { MyVector v = new MyNewVector(); /* Line 19 */ } }

Options

A : Compilation will succeed.

B : Compilation will fail at line 3.

C : Compilation will fail at line 5.

D : Compilation will fail at line 15.

7. What will be the output of the program? public class Test { private static float[] f = new float[2]; public static void main (String[] args) { System.out.println("f[0] = " + f[0]); } }

Options

A : f[0] = 0

B : f[0] = 0.0

C : Compile Error

D : Runtime Exception

8. What will be the output of the program? import java.util.*; class I { public static void main (String[] args) { Object i = new ArrayList().iterator(); System.out.print((i instanceof List)+","); System.out.print((i instanceof Iterator)+","); System.out.print(i instanceof ListIterator); } }

Options

A : Prints: false, false, false

B : Prints: false, false, true

C : Prints: false, true, false

D : Prints: false, true, true

9. What will be the output of the program? public class Test { public static void main (String args[]) { String str = NULL; System.out.println(str); } }

Options

A : NULL

B : Compile Error

C : Code runs but no output

D : Runtime Exception

10. What will be the output of the program? public class Test { public static void main (String[] args) { String foo = args[1]; String bar = args[2]; String baz = args[3]; System.out.println("baz = " + baz); /* Line 8 */ } } And the command line invocation: > java Test red green blue

Options

A : baz =

B : baz = null

C : baz = blue

D : Runtime Exception

1. What will be the output of the program? public static void main(String[] args) { Object obj = new Object() { public int hashCode() { return 42; } }; System.out.println(obj.hashCode()); }

Options

A : 42

B : Runtime Exception

C : Compile Error at line 2

D : Compile Error at line 5

2. 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

3. Assuming that the equals() and hashCode() methods are properly implemented, if the output is " x = 1111 ", which of the following statements will always be true? x = 0; if (x1.hashCode() != x2.hashCode() ) x = x + 1; if (x3.equals(x4) ) x = x + 10; if (!x5.equals(x6) ) x = x + 100; if (x7.hashCode() == x8.hashCode() ) x = x + 1000; System.out.println("x = " + x);

Options

A : x2.equals(x1)

B : x3.hashCode() == x4.hashCode()

C : x5.hashCode() != x6.hashCode()

D : x8.equals(x7)

4. What two statements are true about properly overridden hashCode() and equals() methods? hashCode() doesn't have to be overridden if equals() is. equals() doesn't have to be overridden if hashCode() is. hashCode() can always return the same value, regardless of the object that invoked it. equals() can be true even if it's comparing different objects.

Options

A : 1 and 2

B : 2 and 3

C : 3 and 4

D : 1 and 3

5. Which statement is true for the class java.util.HashSet ?

Options

A : The elements in the collection are ordered.

B : The collection is guaranteed to be immutable.

C : The elements in the collection are guaranteed to be unique.

D : The elements in the collection are accessed using a unique key.

6. Which of the following are true statements? The Iterator interface declares only three methods: hasNext , next and remove . The ListIterator interface extends both the List and Iterator interfaces. The ListIterator interface provides forward and backward iteration capabilities. The ListIterator interface provides the ability to modify the List during iteration. The ListIterator interface provides the ability to determine its position in the List.

Options

A : 2, 3, 4 and 5

B : 1, 3, 4 and 5

C : 3, 4 and 5

D : 1, 2 and 3

7. Which statement is true for the class java.util.ArrayList ?

Options

A : The elements in the collection are ordered.

B : The collection is guaranteed to be immutable.

C : The elements in the collection are guaranteed to be unique.

D : The elements in the collection are accessed using a unique key.

8. Which of the following statements about the hashcode() method are incorrect? The value returned by hashcode() is used in some collection classes to help locate objects. The hashcode() method is required to return a positive int value. The hashcode() method in the String class is the one inherited from Object . Two new empty String objects will produce identical hashcodes.

Options

A : 1 and 2

B : 2 and 3

C : 3 and 4

D : 1 and 4

9. Which statement is true? class Test1 { public int value; public int hashCode() { return 42; } } class Test2 { public int value; public int hashcode() { return (int)(value^5); } }

Options

A : class Test1 will not compile.

B : The Test1 hashCode() method is more efficient than the Test2 hashCode() method.

C : The Test1 hashCode() method is less efficient than the Test2 hashCode() method.

D : class Test2 will not compile.

1. Which class does not override the equals() and hashCode() methods, inheriting them directly from class Object?

Options

A : java.lang.String

B : java.lang.Double

C : java.lang.StringBuffer

D : java.lang.Character

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