Convert List of Integer to List of String in Java

This post will discuss how to convert list of Integer to list of string in Java.

1. Using Java 8

We can use Java 8 Stream to convert List<Integer> to List<String>. Following are the complete steps:

Convert List<Integer> to Stream<Integer> using List.stream().
Convert Stream<Integer> to Stream<String> using Stream.map().
Accumulate Stream<String> into List<String> using Collectors.toList().

The following program demonstrates it:

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

List<Integer> list = Arrays.asList( 1, 2, 3, 4, 5 );

List<String> newList = list.stream()

.map(String::valueOf)

.collect(Collectors.toList());

System.out.println(newList); // [1, 2, 3, 4, 5]

}

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It is recommended to use method references for referring to an existing method instead of a lambda expression. For converting an integer to a string, we can pass String::valueOf to the map() method.

Using Generics:

Following is the generic version of the above program. It passes the method reference as a parameter to the generic function.

import java.util.Arrays;

import java.util.List;

import java.util.function.Function;

import java.util.stream.Collectors;

class Main

{

// Generic method to convert `List<Integer>` to `List<String>`

public static <T, U> List<U> transform(List<T> list, Function<T, U> function)

{

return list.stream()

.map(function)

.collect(Collectors.toList());

}

public static void main(String[] args)

{

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

List<String> newList = transform(list, String::valueOf);

System.out.println(newList); // [1, 2, 3, 4, 5]

}

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Here’s an equivalent version without using the Stream API.

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.function.Function;

class Main

{

// Generic method to convert `List<Integer>` to `List<String>`

public static <T, U> List<U> transform(List<T> list, Function<T, U> function)

{

List<U> result = new ArrayList<>();

for (T t : list) {

U u = function.apply(t);

result.add(u);

}

return result;

}

public static void main(String[] args)

{

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

List<String> newList = transform(list, String::valueOf);

System.out.println(newList); // [1, 2, 3, 4, 5]

}

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2. Using Guava Library

1. Guava’s Lists class provides the transform() method that returns a list that applies a specified method to each element of the specified list. This transformation happens in such a way that any changes to the original list will be reflected in the returned list, but no new items can be added to the returned list.

import com.google.common.collect.Lists;

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

List<String> newList = Lists.transform(list, String::valueOf);

System.out.println(newList); // [1, 2, 3, 4, 5]

}

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2. We can also transform a list using Guava’s Iterables.transform(). It returns a view containing the result of applying a method to each list element.

import com.google.common.collect.Iterables;

import java.util.Arrays;

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

Iterable<String> iterable = Iterables.transform(list, String::valueOf);

System.out.println(iterable); // [1, 2, 3, 4, 5]

}

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Here’s how we can call Iterables.transform() with Java 7 and before:

Iterables.transform(list, new Function<Integer, String>() {

@Override

public String apply(Integer integer) {

return String.valueOf(integer);

}

});

3. Similar to Iterables.transform(), we can also use Collections2.transform() that returns a collection, as shown below:

import com.google.common.collect.Collections2;

import java.util.Arrays;

import java.util.Collection;

import java.util.List;

class Main

{

public static void main(String[] args)

{

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

Collection<String> iterable = Collections2.transform(list, String::valueOf);

System.out.println(iterable); // [1, 2, 3, 4, 5]

}

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Here’s how we can call Collections2.transform() with Java 7 and before:

Collections2.transform(list, new Function<Integer, String>() {

@Override

public String apply(Integer integer) {

return String.valueOf(integer);

}

});

That’s all about converting Integer List to List of Java String.

Convert List of Integer to List of String in Java

1. Using Java 8

Using Generics:

2. Using Guava Library

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