Question 1

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

Accepted Answer

java.lang.StringBuffer is the only class in the list that uses the default methods provided by class Object.

Question 2

Which collection class allows you to associate its elements with key values, and allows you to retrieve objects in FIFO (first-in, first-out) sequence?

Accepted Answer

LinkedHashMap is the collection class used for caching purposes. FIFO is another way to indicate caching behavior. To retrieve LinkedHashMap elements in cached order, use the values() method and iterate over the resultant collection.

Question 3

Which interface does java.util.Hashtable implement?

Accepted Answer

Hash table based implementation of the Map interface.

Question 4

What is the numerical range of char?

Accepted Answer

The char type is integral but unsigned. The range of a variable of type char is from 0 to 216-1 or 0 to 65535. Java characters are Unicode, which is a 16-bit encoding capable of representing a wide range of international characters. If the most significant nine bits of a char are 0, then the encoding is the same as seven-bit ASCII.

Question 5

You need to store elements in a collection that guarantees that no duplicates are stored and all elements can be accessed in natural order. Which interface provides that capability?

Accepted Answer

Option B is correct. A set is a collection that contains no duplicate elements. The iterator returns the elements in no particular order (unless this set is an instance of some class that provides a guarantee). A map cannot contain duplicate keys but it may contain duplicate values. List and Collection allow duplicate elements. Option A is wrong. A map is an object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value. The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order (ascending key order); others, like the HashMap class, do not (does not guarantee that the order will remain constant over time). Option C is wrong. A list is an ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list. Unlike sets, lists typically allow duplicate elements. Option D is wrong. A collection is also known as a sequence. The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list. Unlike sets, lists typically allow duplicate elements.

Question 6

Suppose that you would like to create an instance of a new Map that has an iteration order that is the same as the iteration order of an existing instance of a Map. Which concrete implementation of the Map interface should be used for the new instan…

Accepted Answer

The iteration order of a Collection is the order in which an iterator moves through the elements of the Collection. The iteration order of a LinkedHashMap is determined by the order in which elements are inserted. When a new LinkedHashMap is created by passing a reference to an existing Collection to the constructor of a LinkedHashMap the Collection.addAll method will ultimately be invoked. The addAll method uses an iterator to the existing Collection to iterate through the elements of the existing Collection and add each to the instance of the new LinkedHashMap. Since the iteration order of the LinkedHashMap is determined by the order of insertion, the iteration order of the new LinkedHashMap must be the same as the interation order of the old Collection.

Question 7

Which collection class allows you to grow or shrink its size and provides indexed access to its elements, but whose methods are not synchronized?

Accepted Answer

All of the collection classes allow you to grow or shrink the size of your collection. ArrayList provides an index to its elements. The newer collection classes tend not to have synchronized methods. Vector is an older implementation of ArrayList functionality and has synchronized methods; it is slower than ArrayList.

Question 8

What line of code should replace the missing statement to make this program compile? /* Missing Statement? */
public class foo 
{
    public static void main(String[]args)throws Exception 
    {
        java.io.PrintWriter out = new java.io.Pri…

Accepted Answer

The usual method for using/importing the java packages/classes is by using an import statement at the top of your code. However it is possible to explicitly import the specific class that you want to use as you use it which is shown in the code above. The disadvantage of this however is that every time you create a new object you will have to use the class path in the case "java.io" then the class name in the long run leading to a lot more typing.

Question 9

Which interface provides the capability to store objects using a key-value pair?

Accepted Answer

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

Question 10

Which of the following are Java reserved words? run import default implement

Accepted Answer

(2) - This is a Java keyword (3) - This is a Java keyword (1) - Is incorrect because although it is a method of Thread/Runnable it is not a keyword (4) - This is not a Java keyword the keyword is implements

Question 11

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

Accepted Answer

Hashtable is the only class listed that provides synchronized methods. If you need synchronization great; otherwise, use HashMap, it's faster.

Question 12

Which is valid declaration of a float?

Accepted Answer

Option A is valid declaration of float. Option B is incorrect because any literal number with a decimal point u declare the computer will implicitly cast to double unless you include "F or f" Option C is incorrect because it is a String. Option D is incorrect because "d" tells the computer it is a double so therefore you are trying to put a double value into a float variable i.e there might be a loss of precision.

Question 13

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() + " " );
}

Accepted Answer

TreeSet assures no duplicate entries; also, when it is accessed it will return elements in natural order, which typically means alphabetical.

Question 14

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]); 
    } 
}

Accepted Answer

In the above code the array reference variable x has been declared but it has not been instantiated i.e. the new statement is missing, for example: private static int[]x = new int[5]; private static int[x] declares a static i.e. class level array. the "new" keyword is the word that actually creates said array. int[5] in association with the new sets the size of the array. so since the above code contains no new or size decalarations when you try and access x[0] you are trying to access a member of an array that has been declared but not intialized hence you get a NullPointerException at runtime.

Question 15

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.pr…

Accepted Answer

The Vector.elements method returns an Enumeration over the elements of the vector. Vector implements the List interface and extends AbstractList so it is also possible to get an Iterator over a Vector by invoking the iterator or listIterator method.

Question 16

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…

Accepted Answer

Option B is correct. The compiler complains with the error "modifier private not allowed here". The class is created private and is being used by another class on line 19.

Question 17

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]); 
    } 
}

Accepted Answer

The choices are between Option A and B, what this question is really testing is your knowledge of default values of an initialized array. This is an array type float i.e. it is a type that uses decimal point numbers therefore its initial value will be 0.0 and not 0

Question 18

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((…

Accepted Answer

The iterator() method returns an iterator over the elements in the list in proper sequence, it doesn't return a List or a ListIterator object. A ListIterator can be obtained by invoking the listIterator method.

Question 19

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

Accepted Answer

Option B is correct because to set the value of a String variable to null you must use "null" and not "NULL".

Question 20

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);…

Accepted Answer

When running the program you entered 3 arguments "red", "green" and "blue". When dealing with arrays in java you must remember ALL ARRAYS IN JAVA ARE ZERO BASED therefore args[0] becomes "red", args[1] becomes "green" and args[2] becomes "blue". When the program entcounters line 8 above at runtime it looks for args[3] which has never been created therefore you get an ArrayIndexOutOfBoundsException at runtime.

Objects and Collections Questions