Filter Set in Java

In this post, we’ll illustrate how to filter set in Java.

1. Java 7 and less

In Java 7 and less, we can iterate over the set using an advanced for-loop and filter elements using a conditional statement.

import java.util.HashSet;

import java.util.Arrays;

import java.util.Set;

class Main

{

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("C", "C++", "Java", "C#"));

// Java 7 and less:

// create an empty set

Set<String> filteredSet = new HashSet<>();

// iterate through the set

for (String s: set)

{

// filter languages that start with `C`

if (s.startsWith("C")) {

filteredSet.add(s);

}

System.out.println(filteredSet);

}

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Output:

[C#, C++, C]

The above solution creates a new set for filtered values. We can apply a filter to the same set using an iterator. The following code uses the remove() method provided by the Iterator class to filter elements from the same set.

import java.util.HashSet;

import java.util.Arrays;

import java.util.Iterator;

import java.util.Set;

class Main

{

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("C", "C++", "Java", "C#"));

// Java 7 and less:

// get an iterator to the set

Iterator<String> itr = set.iterator();

// iterate through the set

while (itr.hasNext())

{

String curr = itr.next();

// filter languages that start with `C`

if (!curr.startsWith("C")) {

itr.remove();

}

System.out.println(set);

}

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Output:

[C#, C++, C]

Please note that ConcurrentModificationException will be thrown if the remove() method of Set interface is used as it is not permitted to modify a set while iterating over it except by iterator’s own remove method.

2. Java 8 and above

In Java 8 and above, we can convert the set into a stream and filter it using the filter() method provided by Stream. Finally, we collect the filtered elements in a String.

import java.util.Arrays;

import java.util.HashSet;

import java.util.Set;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("C", "C++", "Java", "C#"));

// Java 8 and above:

String filteredSet = set.stream()

.filter(s -> s.startsWith("C"))

.collect(Collectors.joining(", ", "[", "]"));

System.out.println(filteredSet);

}

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Output:

[C#, C++, C]

We can also convert the filtered stream back to a set by using a set collector.

import java.util.Arrays;

import java.util.HashSet;

import java.util.Set;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("C", "C++", "Java", "C#"));

// Java 8 and above:

Set<String> filteredSet = set.stream()

.filter(s -> s.startsWith("C"))

.collect(Collectors.toSet());

System.out.println(filteredSet);

}

Download Run Code

Output:

[C#, C++, C]

This is equivalent to:

import java.util.Arrays;

import java.util.HashSet;

import java.util.Set;

import java.util.StringJoiner;

class Main

{

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("C", "C++", "Java", "C#"));

StringJoiner joiner = new StringJoiner(", ", "[", "]");

for (String s : set) {

if (s.startsWith("C")) {

joiner.add(s);

}

String filteredSet = joiner.toString();

System.out.println(filteredSet);

}

Download Run Code

Output:

[C#, C++, C]

That’s all about filtering Set in Java.

1. Java 7 and less

2. Java 8 and above

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