java merge two sets - Brave Search

Is there a better way to combine two string sets in java?

stackoverflow.com › questions › 9062574 › is-there-a-better-way-to-combine-two-string-sets-in-java

You can do it using this one-liner

Set<String> combined = Stream.concat(newStringSet.stream(), oldStringSet.stream())
        .collect(Collectors.toSet());

With a static import it looks even nicer

Set<String> combined = concat(newStringSet.stream(), oldStringSet.stream())
        .collect(toSet());

Another way is to use flatMap method:

Set<String> combined = Stream.of(newStringSet, oldStringSet).flatMap(Set::stream)
        .collect(toSet());

Also any collection could easily be combined with a single element

Set<String> combined = concat(newStringSet.stream(), Stream.of(singleValue))
        .collect(toSet());

Answer from ytterrr on Stack Overflow

geeksforgeeks.org › java › merge-two-sets-in-java

Merge Two Sets in Java - GeeksforGeeks

July 11, 2025 - // Java Program to Demonstrate ... Method 1 // To merge two sets // using DoubleBrace Initialisation public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) { // Adding all elements of respective Sets // using addAll() method ...

stackoverflow.com › questions › 9062574 › is-there-a-better-way-to-combine-two-string-sets-in-java

Is there a better way to combine two string sets in java? - Stack Overflow

You can do it using this one-liner

Set<String> combined = Stream.concat(newStringSet.stream(), oldStringSet.stream())
        .collect(Collectors.toSet());

With a static import it looks even nicer

Set<String> combined = concat(newStringSet.stream(), oldStringSet.stream())
        .collect(toSet());

Another way is to use flatMap method:

Set<String> combined = Stream.of(newStringSet, oldStringSet).flatMap(Set::stream)
        .collect(toSet());

Also any collection could easily be combined with a single element

Set<String> combined = concat(newStringSet.stream(), Stream.of(singleValue))
        .collect(toSet());

Since a Set does not contain duplicate entries, you can therefore combine the two by:

newStringSet.addAll(oldStringSet);

It does not matter if you add things twice, the set will only contain the element once... e.g it's no need to check using contains method.

how to combine or merge two strings in java?

java - Merging 2 strings

Merging String[] in Java

Java String utility/method to union two strings

collections - How to merge multiple (arbitrary number of) Set<String> into one in Java?

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techiedelight.com › home › java › merge two sets in java

Merge two sets in Java | Techie Delight

July 7, 2026 - The idea is to obtain a stream of elements from both sets by using the static factory method Stream.of() and accumulate all elements into a new set using a Collector, as shown below: We can avoid using a Collector by accumulating all elements using forEach() instead, as shown below: The Java Stream provides concat() that takes two streams as input and creates a lazily concatenated stream whose elements are all the elements of the first stream, followed by all the elements of the second stream.

tutorialspoint.com › merge-two-sets-in-java

Merge two sets in Java

import java.util.stream.*; import java.util.*; import java.io.*; public class Demo{ public static <T> Set<T> set_merge(Set<T> set_1, Set<T> set_2){ Set<T> my_set = set_1.stream().collect(Collectors.toSet()); my_set.addAll(set_2); return my_set; } public static void main(String[] args){ Set<Integer> my_set_1 = new HashSet<Integer>(); my_set_1.addAll(Arrays.asList(new Integer[] { 34, 67, 89, 102 })); Set<Integer> my_set_2 = new HashSet<Integer>(); my_set_2.addAll(Arrays.asList(new Integer[] { 77, 11, 0 , -33})); System.out.println("The first set contains " + my_set_1); System.out.println("The second set contains " + my_set_2); System.out.println("The two sets are merged " + set_merge(my_set_1, my_set_2)); } }

baeldung.com › home › java › java collections › java – combine multiple collections

Java - Combine Multiple Collections | Baeldung

May 11, 2024 - The Guava library from Google provides several convenience methods for operating on collections and can be used with Java 6 or later. The Iterables.concat() method is one of the Guava convenient methods that is used for merging collections:

codespeedy.com › home › merge two sets in java

Merge two sets in Java - CodeSpeedy

January 6, 2023 - So, here we take two different input sets, and then we get a new merged set which is also a sorted set which you can see. Now we will look into the code. import java.util.*; public class main { public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) { return new HashSet<T>() {{ addAll(a); addAll(b); } }; } public static void main(String[] args) { Set<Integer> a = new HashSet<Integer>(); a.addAll(Arrays.asList(new Integer[] { 8,4, 3, 7, 9 })); Set<Integer> b = new HashSet<Integer>(); b.addAll(Arrays.asList(new Integer[] { 0, 5, 1, 2, 6 })); System.out.println(a); System.out.println(b); System.out.println(mergeSet(a, b)); } }

techiedelight.com › home › java › merge multiple sets in java

Merge multiple sets in Java | Techie Delight

July 7, 2026 - We can use streams in Java 8 and above to merge multiple sets by obtaining a stream consisting of all elements from every set using the static factory method Stream.of() and accumulating all elements into a new set using a Collector. We can also accumulate all elements using forEach(), as shown below: Stream also has a concat() method that takes two streams as input and creates a lazily merged stream out of them.

docs.vultr.com › java › examples › calculate union of two sets

Java Program to Calculate union of two sets

December 19, 2024 - The union of setA and setB includes three different fruit names. As before, duplicates are removed, yielding the output ["apple", "banana", "cherry"]. Utilize Java 8's Stream API for a more modern approach. Merge two sets and collect the results into a new set.

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digitalocean.com › community › tutorials › merge-two-lists-in-java

How to Merge Two Lists in Java? | DigitalOcean

Technical tutorials, Q&A, events — This is an inclusive place where developers can find or lend support and discover new ways to contribute to the community.

baeldung.com › home › java › java collections › combining different types of collections in java

Combining Different Types of Collections in Java | Baeldung

April 4, 2025 - The Guava library provides us with straightforward Sets.union() method to combine Sets in Java: ... We can make use of the Map interface which itself provides us with the putAll() method which copies all of the mappings from the supplied argument ...

coderanch.com › t › 682461 › java › Merging-sorted-sets-set-treeset

Merging two sorted sets into one set without using treeset as the result set (Beginning Java forum at Coderanch)

July 23, 2017 - Well, if you want the result to be a LinkedHashSet, you can just do the following: If the original sets shouldn't be modified, you can do: Or: ... I moved your first line out of the code tags because it wasn't code. Look at line 68; there is no need for empty space there. That method for combining the two sets looks very complicated, particularly the while loop.

medium.com › @AlexanderObregon › merging-two-arrays-into-one-without-duplicates-in-java-6931f8f9121e

Merging Two Arrays into One Without Duplicates in Java

August 7, 2025 - HashSet is a common type of Set. It handles basic types like integers and strings by hashing them. The hash gives Java a quick way to store and look up values. When you use add, the item is turned into a hash value, and Java uses that hash to check whether the value has already been added. Here’s a simple way to merge two arrays using a set and avoid duplicates:

stackoverflow.com › questions › 27263604 › how-to-merge-multiple-arbitrary-number-of-setstring-into-one-in-java › 27263722

collections - How to merge multiple (arbitrary number of) Set<String> into one in Java? - Stack Overflow

Recommend com.google.common.collect.Sets#union(set1, set2) to get the merge instead of Set.addAll under hood, since Guava is already in your dependencies.

Reason: it's view which is memory effective, and also unmodifiable.

plus: I should have post it as a comment, sorry.

How about creating a collection of your input sets, then using flatMap?

Set<String> allElements = ImmutableSet.of(
        One.SET,
        Two.SET,
        Three.SET
).stream().flatMap(Collection::stream).collect(Collectors.toSet());

How to do in Java

howtodoinjava.com › home › collections framework › java arraylist › merge two arraylists in java, with or without duplicates

Merge Two ArrayLists in Java, With or Without Duplicates

January 19, 2023 - The usual way to merge two ArrayList objects is to copy the first list into a new ArrayList and call addAll() with the second, which appends every element of the second list and keeps the duplicates.

codevisionz.com › home › java code example: combine hashsets

Java combine HashSets | Code Example

February 18, 2023 - import java.util.HashSet; class Cube { public static void main(String[] args) { HashSet<Integer> set1 = new HashSet<Integer>(), set2 = new HashSet<Integer>(); set1.add(1); set1.add(2); set1.add(3); set2.add(4); set2.add(5); set2.add(6); // output elements using enhanced for loop for (Integer s : set1) { System.out.println(s); } System.out.println("Combined sets:"); // Adds all of the elements in the specified collection to this collection (optional operation).

stackoverflow.com › questions › 61003909 › how-to-merge-two-immutable-sets-in-java › 61003937

collections - How to Merge two Immutable Sets in Java.? - Stack Overflow

This is an example of how you can combine 2 or more ImmutableSet objects and create another ImmutableSet. This uses the Integer type for the parameterized type because I do not have access to your Fruit class.

        Set<Integer> first = ImmutableSet.of(1);

        Set<Integer> second = ImmutableSet.of(2);

        Set<Integer> third = ImmutableSet.<Integer>builder()
                .addAll(first)
                .addAll(second)
                .build();

stackoverflow.com › questions › 6301510 › combining-multiple-sets-into-a-single-one-and-remove-duplicate-ones

java - combining multiple sets into a single one and remove duplicate ones - Stack Overflow

See: javadoc of java.util.Set.addAll(Collection):

/**
 * Adds all of the elements in the specified collection to this set if
 * they're not already present (optional operation).  If the specified
 * collection is also a set, the <tt>addAll</tt> operation effectively
 * modifies this set so that its value is the <i>union</i> of the two
 * sets.  The behavior of this operation is undefined if the specified
 * collection is modified while the operation is in progress.

/**
 * Join multiple sets into one.
 */
@SafeVarargs
private final <T> Set<T> join(Set<T>... sets)
{
    Set<T> result = new HashSet<>();
    if (sets == null)
        return result;

    for (Set<T> set : sets)
    {
        if (set != null)
            result.addAll(set);
    }
    return result;
}

stackoverflow.com › questions › 63227389 › how-to-merge-two-unmodifiable-sets-in-java › 63227492

How to merge two unmodifiable Sets in Java? - Stack Overflow

Stream#concat is useful too

Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD  =  
         Stream.concat(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD.stream(),
                       INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD.stream())
                .collect(Collectors.toSet());

If you want to use streams:

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD =
    Stream.of(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD, INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
        .flatMap(Set::stream)
        .collect(Collectors.toSet());

Or, in your case, and as mentioned in comment, you can use Collectors.toUnmodifiableSet() :

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD =
    Stream.of(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD, INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
        .flatMap(Set::stream)
        .collect(Collectors.toUnmodifiableSet());

thelinuxcode.com › home › merge two sets in java: a deep, practical guide with modern workflows

Merge Two Sets in Java: A Deep, Practical Guide with Modern Workflows – TheLinuxCode

January 7, 2026 - You’ll get concrete code for each method, plus performance notes and memory guidance from a small benchmark run on Java 21. If you only need a merged set and you’re fine with HashSet, this is the baseline I recommend for 90% of cases: ... I recommend pre-sizing the HashSet. When I merge two 1,000,000-element sets, pre-sizing reduces rehash work.

geeksforgeeks.org › java › how-to-merge-two-linkedhashset-objects-in-java

How to Merge Two LinkedHashSet Objects in Java? - GeeksforGeeks

July 23, 2025 - import java.util.LinkedHashSet; ... * To merge two LinkedHashSet objects or * append LinkedHashSet object to another, use the * addAll method */ lhSetColors1.addAll(lhSetColors2); System.out.println("Merged LinkedHashSet: " + ...