Java Stream How to - Use Stateful lambda expressions

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Question

We would like to know how to use Stateful lambda expressions.

Answer

/*  ww  w  .j  a v a  2 s.c o  m*/
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;


public class Main {
    public static void main(String[] args) {
      Integer[] intArray = {1, 2, 3, 4, 5, 6, 7, 8 };
      List<Integer> listOfIntegers = new ArrayList<>(Arrays.asList(intArray));

      List<Integer> serialStorage = new ArrayList<>();
      
      System.out.println("Serial stream:");
      listOfIntegers
          .stream()
          
          // Don't do this! It uses a stateful lambda expression. 
          .map(e -> { serialStorage.add(e); return e; })
          
          .forEachOrdered(e -> System.out.print(e + " "));
      System.out.println("");
   
      serialStorage
          .stream()
          .forEachOrdered(e -> System.out.print(e + " "));
      System.out.println("");

      System.out.println("Parallel stream:");
      List<Integer> parallelStorage = Collections.synchronizedList(
          new ArrayList<>());
      listOfIntegers
          .parallelStream()
          
          // Don't do this! It uses a stateful lambda expression.
          .map(e -> { parallelStorage.add(e); return e; })
          
          .forEachOrdered(e -> System.out.print(e + " "));
      System.out.println("");
   
      parallelStorage
          .stream()
          .forEachOrdered(e -> System.out.print(e + " "));
      System.out.println("");

    }
}

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The code above generates the following result.

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