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What is Collectors.toList() in Java?

abhilash

What is the Collectors class?

Collectors is a utility class that provides various implementations of reduction operations such as grouping elements, collecting elements to different collections, summarizing elements according to various criteria, etc. The different functions in the Collectors class are usually used as the final operations on streams.

What is toList()?

toList() is a static method of the Collectors class that is used to collect/accumulate all the elements to a new List. The type, mutability, serializability, and thread safety of the List returned are not guaranteed.

The toList method is defined in the Collectors class. The Collectors class is defined in the java.util.stream package. To import the Collectors class, check the following import statement.

import java.util.stream.Collectors;

Syntax


public static <T> Collector<T, ?, List<T>> toList()

Parameters

The method has no parameters.

Return value

This method returns a collector that collects all the input elements in the encountered order.

Code

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {

    public static void main(String[] args)
    {
        List<String> stringList = Arrays.asList("educative", "io", "edpresso");

        System.out.println("Stream before modification - " + stringList);

        Stream<String> stringStream = stringList.stream();

        List<String> uppercaseList = stringStream.map(String::toUpperCase).collect(Collectors.toList());

        System.out.println("Resulting list after modification - " + uppercaseList);
    }
}
Implementation of the toList() method

Explanation

  • Lines 1-4: We import the relevant packages.
  • Line 10: We define a list of strings called stringList.
  • Line 12: We print the stringList.
  • Line 14: We create a stream out of the stringList.
  • Line 16: We convert the elements of the stream to uppercase using the map method and collect the resulting elements to a new list using the toList method.
  • Line 18: We print the new list of uppercase elements.

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