Find all occurrences of a value in a List in Java

This post will discuss how to find all occurrences of a value in a List in Java.

1. Using `IntStream`

To find an index of all occurrences of an item in a list, you can create an IntStream of all the indices and apply a filter over it to collect all matching indices with the given value. This is demonstrated below for a List of Integers:

import java.util.List;

import java.util.Objects;

import java.util.stream.Collectors;

import java.util.stream.IntStream;

class Main

{

public static void main(String[] args)

{

List<Integer> values = List.of(1, 3, 5, 2, 3, 4, 6);

int item = 3;

List<Integer> indices = IntStream.range(0, values.size())

.filter(i -> Objects.equals(values.get(i), item))

.boxed().collect(Collectors.toList());

System.out.println(indices);

}

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Output:

[1, 4]

Note that we’re iterating over the range of the list’s indices, and not the actual list itself. The above solution returns a List, you can use the following solution to return an integer array:

import java.util.Arrays;

import java.util.List;

import java.util.Objects;

import java.util.stream.IntStream;

class Main

{

public static void main(String[] args)

{

List<Integer> values = List.of(1, 3, 5, 2, 3, 4, 6);

int item = 3;

int[] indices = IntStream.range(0, values.size())

.filter(i -> Objects.equals(values.get(i), item))

.toArray();

System.out.println(Arrays.toString(indices));

}

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Output:

[1, 4]

2. Using for loop

Here’s a version without Java Stream API, using a for loop. It can be used with Java 7 or earlier.

import java.util.ArrayList;

import java.util.List;

import java.util.Objects;

class Main {

public static void main(String[] args) {

List<String> values = List.of("A", "B", "C", "B");

String item = "B";

List<Integer> indices = new ArrayList<>();

for (int i = 0; i < values.size(); i++) {

if (Objects.equals(values.get(i), item)) {

indices.add(i);

}

System.out.println(indices);

}

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Output:

[1, 3]

That’s all about finding all occurrences of a value in a List in Java.

Find all occurrences of a value in a List in Java

1. Using IntStream

2. Using for loop

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1. Using `IntStream`