How to compare two hash sets in java?

Question

How can I compare two hash sets in java ? My first hash sets looks like below.

static Set<String> nounPhrases = new HashSet<>();

Above hash set contains elements like this.

List of Noun Parse : [java, jsp, book]

2nd hash set

static Set<String> nounPhrases2 = new HashSet<>();

List of Noun Parse : [web, php, java,book]

Note - I need to check if there are equal nouns in both sets. and if they have similar nouns then I need to do another task

Answer 1

This is a wheel already invented.

Set#equals() compares sets in the way you would expect:

set1.equals(set2)

If you want two Set variables that are both null to be "equal", then use:

Objects.equals(set1, set2)

Answer 2

So you mean like this?

public static void main(String[] args) {

    final Set<String> nounPhrases = new HashSet<>();
    nounPhrases.add("java");
    nounPhrases.add("jsp");
    nounPhrases.add("book");

    final Set<String> nounPhrases2 = new HashSet<>();
    nounPhrases2.add("web");
    nounPhrases2.add("php");
    nounPhrases2.add("java");
    nounPhrases2.add("book");

    // Checking for every element in first set
    for (final String element : nounPhrases) {

        // if second set has the current element
        if (nounPhrases2.contains(element)) {
            System.out.println("They have " + element);
        }
    }
}

My output:

They have java
They have book

Edit: Based on your comment, if i understand correctly, if you want to get the common elements in both sets, just store the values and return them:

public static void main(String[] args) {

    final Set<String> nounPhrases = new HashSet<>();
    nounPhrases.add("java");
    nounPhrases.add("jsp");
    nounPhrases.add("book");

    final Set<String> nounPhrases2 = new HashSet<>();
    nounPhrases2.add("web");
    nounPhrases2.add("php");
    nounPhrases2.add("java");
    nounPhrases2.add("book");

    System.out.println(getCommon(nounPhrases, nounPhrases2));
}

public final static Set<String> getCommon(Set<String> setA, Set<String> setB) {

    final Set<String> result = new HashSet<>();
    for (final String element : setA) {
        if (setB.contains(element)) {
            result.add(element);
        }
    }
    return result;
}

You could use generics to make the method work for other elements than strings:

public final static <T> Set<T> getCommon(Set<T> setA, Set<T> setB) {

    final Set<T> result = new HashSet<>();
    for (final T element : setA) {
        if (setB.contains(element)) {
            result.add(element);
        }
    }
    return result;
}

If performance is important, you should check sizes first and only iterate over the elements of the smaller set. If you have one set with 1 elements, and one set with 100, starting with the smaller will leve you with one iteration whereas starting with the bigger will leave you with 100 checks where only 1 could have been in both sets.

Answer 3

If u want to find the common element then use collect(Collectors.toList()) instead of count , If u want simply to find how many set has common element using java 8

long count = nounPhrases.stream().filter(tempstring -> {
            return nounPhrases2.stream().anyMatch(tempstring2 -> {
                return tempstring.equals(tempstring2);
            });
        }).count();
        if (count > 0)
            System.out.println("has common elements-"+count);
        else
            System.out.println("not common");

Answer 4

By the use of Java apache.commons.collections package we can implement

package com.StackoverFlow;

import java.util.Collection;
import java.util.HashSet;
import java.util.Set;
import org.apache.commons.collections.CollectionUtils;
public class MainClass {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub


        Set hs_1 = new HashSet();
        hs_1.add("A");
        hs_1.add("B");
        hs_1.add("C");
        hs_1.add("D");

        Set hs_2 = new HashSet();
        hs_2.add("A");
        hs_2.add("B");
        hs_2.add("C");
        hs_2.add("D");

        Collection result = CollectionUtils.subtract(hs_1, hs_2);
        System.out.println(result);
        if(result.isEmpty()){
            System.out.println("perform Task-->>Value maches  ");

        }else{
            System.out.println("perform Task-->>Value not maches  ");
        }

    }

}