Technical Interview Questions & Answers: 2015

Collection Objective Questions

Which of these are core interfaces in the collections framework?

Select the three correct answers.

Set

Bag

LinkedList

Collection

Map

11.2 Which of these implementations are provided by the java.util package?

Select the two correct answers.

HashList

HashMap

ArraySet

ArrayMap

TreeMap

11.3 What is the name of the interface used to represent collections that maintain non-unique elements in order?

Select the one correct answer.

Collection

Set

SortedSet

List

Sequence

Example A Simple Class for Version Number

public class SimpleVNO {
    // Does not override equals() or hashCode().

    private int release;
    private int revision;
    private int patch;

    public SimpleVNO(int release, int revision, int patch) {
        this.release  = release;
        this.revision = revision;
        this.patch    = patch;
    }

    public String toString() {
        return "(" + release + "." + revision + "." + patch + ")";
    }
}

Example Natural Order and Total Order

import java.util.*;

public class ComparatorUsage {
    public static void main(String[] args) {

        // Choice of comparator.
    //  Set strSet = new TreeSet();                              // (1a)
    //  Set strSet = new TreeSet(String.CASE_INSENSITIVE_ORDER); // (1b)
        Set strSet = new TreeSet(new RhymingStringComparator()); // (1c)

        // Add each command line argument to the set.
        for (int i=0; i < args.length; i++) {                    // (2)
            strSet.add(args[i]);
        }
        System.out.println(strSet);                              // (3)
    }
}

class RhymingStringComparator implements Comparator {
    public int compare(Object obj1, Object obj2) {               // (4)

        // (5) Create reversed versions of the strings.
        String reverseStr1 = new StringBuffer((String) obj1).reverse().toString();
        String reverseStr2 = new StringBuffer((String) obj2).reverse().toString();

        // Compare the reversed strings lexicographically.
        return reverseStr1.compareTo(reverseStr2);               // (6)
    }
}

The program is run with the following program arguments on the command line:

>java ComparatorUsage court Stuart report Resort assort support transport distort

Output from the program using the natural order (1a):

[Resort, Stuart, assort, court, distort, report, support, transport]

Output from the program using the case insensitive order (1b):

[assort, court, distort, report, Resort, Stuart, support, transport]

Output from the program using the rhyming order (1c):

[Stuart, report, support, transport, Resort, assort, distort, court]

Example Using Maps

import java.util.*;

public class WeightGroups {
    public static void main(String[] args) {

        // Create a map to store the frequency for each group.
        Map groupFreqData = new HashMap();

        int numArgs = args.length;
        for (int i=0; i

Running the program with the following arguments:
>java WeightGroups 74 75 93 75 93 82 61 92 10 185


gives the following output:
10:     *
60:     *
75:     ***
80:     *
90:     *
95:     **
185:    *

Example Using Lists

import java.util.*;

public class TakeAGuess {
    final static int NUM_DIGITS = 5;

    public static void main(String[] args) {

        // Sanity check on the given data.
        if (args.length != NUM_DIGITS) {
            System.err.println("Guess " + NUM_DIGITS + " digits.");
            return;
        }
        /* Initialize the solution list. This program has a fixed solution. */
        List secretSolution = new ArrayList();                      // (1)
        secretSolution.add("5");
        secretSolution.add("3");
        secretSolution.add("2");
        secretSolution.add("7");
        secretSolution.add("2");

        // Convert the user's guess from string array to list.         (2)
        List guess = new ArrayList();
        for (int i=0; i

Running the program with the following arguments:
java TakeAGuess 3 2 2 2 7


gives the following output:
4 digit(s) correctly included.
1 digit(s) correctly placed.

Example Using Sets

import java.util.*;

public class CharacterSets {
    public static void main(String[] args) {
        int numArgs = args.length;

        // A set keeping track of all characters previously encountered.
        Set encountered = new HashSet();                              // (1)
        // For each program argument in the command line ...
        for (int i=0; i

Running the program with the following arguments:
java CharacterSets i said i am maids


results in the following output:
[i] and [] are disjunct.
[d, a, s, i] is a superset of [i]
[i] is a subset of [d, a, s, i]
[a, m] and [d, a, s, i] have [a] in common.
[d, a, s, m, i] is equivalent to [d, a, s, m, i]

Example Using an Iterator

import java.util.*;

public class IteratorUsage {
    public static void main(String[] args) {

        // (1) Create a list of Integers.
        Collection intList = new ArrayList();
        int[] values = { 9, 11, -4, 1, 13, 99, 1, 0 };
        for (int i = 0; i < values.length; i++)
             intList.add(new Integer(values[i]));

        System.out.println("Before: " + intList);         // (2)

        Iterator interator = intList.iterator();          // (3) Get an iterator.
        while (interator.hasNext()) {                     // (4) Loop
            Integer element = (Integer) interator.next(); // (5) The next element
            int value = element.intValue();
            if (value < 1 || value > 10) // (6) Remove the element if
                interator.remove();      //     its value is not between 1 and 10.
        }

        System.out.println("After:  " + intList);    // (7)
    }
}

Output from the program:

Before: [9, 11, -4, 1, 13, 99, 1, 0]
After:  [9, 1, 1]