Convert Set of String to Set of Integer in Java 8 and above

This post will discuss how to convert Set of String to Set of Integer in Java 8 and above. If the value of the specified string is negative, the solution should preserve the sign in the resultant integer.

We can use Java 8 Stream to convert Set<String> to Set<Integer>. The following are the complete steps:

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

Note that map operation will throw a NumberFormatException if the string does not contain a parsable integer.

import java.util.Arrays;

import java.util.Set;

import java.util.HashSet;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("-1" , "2", "-3", "4"));

Set<Integer> ints = set.stream()

.map(s -> Integer.parseInt(s))

.collect(Collectors.toSet());

System.out.println(ints); // [-1, 2, -3, 4]

}

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The lambda expression used in the above program does nothing but calls an existing method. For converting a string to an Integer, we can pass any one of the following method references to the map() method:

Integer::parseInt
Integer::valueOf
Integer::decode
NumberUtils::toInt // (provided by Apache Commons Lang)

Following is a 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.Set;

import java.util.HashSet;

import java.util.function.Function;

import java.util.stream.Collectors;

class Main

{

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

public static <T, U> Set<U> transform(Set<T> set, Function<T, U> function)

{

return set.stream()

.map(function)

.collect(Collectors.toSet());

}

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("-1" , "2", "-3", "4"));

Set<Integer> ints = transform(set, Integer::parseInt);

System.out.println(ints); // [-1, 2, -3, 4]

}

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This is equivalent to:

import java.util.Arrays;

import java.util.HashSet;

import java.util.Set;

import java.util.function.Function;

class Main

{

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

public static <T, U> Set<U> transform(Set<T> set, Function<T, U> function)

{

Set<U> result = new HashSet<>();

for (T t : set) {

U u = function.apply(t);

result.add(u);

}

return result;

}

public static void main(String[] args)

{

Set<String> set = new HashSet<>(Arrays.asList("-1" , "2", "-3", "4"));

Set<Integer> ints = transform(set, Integer::parseInt);

System.out.println(ints); // [-1, 2, -3, 4]

}

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

Convert Set of String to Set of Integer in Java 8 and above

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