Question

CS 209 Data Structure

5. Consider the Pair class covered in class:

class Pair

{

    public A first;

    public B second;

    public Pair(A a, B b)

    {

        first = a;

        second = b;

    }

    public void setFirst(A a)

    {

        first = a;

    }

    public A getFirst()

    {

        return first;

    }

    public void setSecond(B b)

    {

        second = b;

    }

    public B getSecond()

    {

        return second;

    }

}

a. Create a toString() method inside the Pair class.

b. Create an equals() method inside the Pair class.

c. Create a hashCode() method inside the Pair class.

d. Create a HashSet of Pair and insert a few elements including duplicates.

Verify that the HashSet removes all the duplicates.

Answer 1

A.java:

import java.util.Objects;

public class A {
    String str;

    /**
     * constructor with single parameter
     * @param str
     */
    A(String str) {
        this.str = str;
    }

    /**
     * convert current object to string
     * @return
     */
    public String toString() {
        return str;
    }

    /**
     * function to equality check
     * @param o
     * @return
     */
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        A a = (A) o;
        return Objects.equals(str, a.str);
    }

    public int hashCode() {
        return Objects.hash(str);
    }
}

B.java:

import java.util.Objects;

public class B {
    String str;

    /**
     * constructor with single parameter
     * @param str
     */
    B(String str) {
        this.str = str;
    }

    /**
     * convert current object to string
     * @return
     */
    public String toString() {
        return str;
    }

    /**
     * function to equality check
     * @param o
     * @return
     */
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        B b = (B) o;
        return Objects.equals(str, b.str);
    }

    public int hashCode() {
        return Objects.hash(str);
    }
}

Pair.java:

import java.util.Objects;

/**
 * class pair
 */
class Pair

{
    public A first;
    public B second;

    /**
     * constructor with two parameter
     * @param a
     * @param b
     */
    public Pair(A a, B b)
    {
        first = a;
        second = b;
    }

    /**
     * setter method for first
     * @param a
     */
    public void setFirst(A a)
    {
        first = a;
    }

    /**
     * getter method for first
     * @return
     */
    public A getFirst()
    {
        return first;
    }

    /**
     * setter method for second
     * @param b
     */
    public void setSecond(B b)
    {
        second = b;
    }

    /**
     * getter method for second
     * @return
     */
    public B getSecond()
    {
        return second;
    }

    /**
     * function to get string
     * @return
     */
    public String toString() {
        return "Pair{" +
                "first=" + first +
                ", second=" + second +
                '}';
    }

    /**
     * check whether two pairs are equal or not
     * @param o
     * @return
     */
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Pair pair = (Pair) o;
        return Objects.equals(first, pair.first) &&
                Objects.equals(second, pair.second);
    }

    /**
     * function to get hash code of current object
     * @return
     */
    public int hashCode() {
        return Objects.hash(first, second);
    }
}

Driver.java:

import java.util.HashSet;

public class Driver {
    public static void main(String[] args) {
        // create new hashset of pair
        HashSet<Pair> hashSet = new HashSet<>();

        // add new pair
        hashSet.add(new Pair(new A("A0"), new B("B0")));

        // add new pair
        hashSet.add(new Pair(new A("A1"), new B("B1")));

        // add duplicate
        hashSet.add(new Pair(new A("A1"), new B("B1")));

        // add new pair
        hashSet.add(new Pair(new A("A2"), new B("B2")));

        // addd duplicate
        hashSet.add(new Pair(new A("A2"), new B("B2")));

        // print hash set
        for (Pair p: hashSet) {
            System.out.println(p);
        }
    }
}

Output: