Almindelige faldgruber

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<Integer> nums = Arrays.asList(1, 2, 3);
    Stream<Integer> s = nums.stream();
    s.forEach(System.out::println); // terminal
    try {
      s.forEach(System.out::println); // reuse -> IllegalStateException
    } catch (IllegalStateException e) {
      System.out.println("Oops: " + e.getMessage());
    }
  }
}

import java.util.*;import java.util.stream.*;public class Main {
  public static void main(String[] args) {
    List<Integer> nums = Arrays.asList(1,2,3);
    // Pitfall: mutating external list from map/forEach
    List<Integer> out = new ArrayList<>();
    nums.stream().map(n -> { out.add(n*n); return n*n; }).forEach(x -> {});
    System.out.println("With side effects: " + out);

    // Better: use collect to build the result
    List<Integer> squares = nums.stream().map(n -> n*n).collect(Collectors.toList());
    System.out.println("With collect: " + squares);
  }
}

import java.util.*;public class Main {
  public static void main(String[] args) {
    List<Integer> nums = java.util.Arrays.asList(1,2,3,4,5);
    // Pitfall: adding to a normal ArrayList from parallel pipeline (unsafe)
    List<Integer> unsafe = new ArrayList<>();
    nums.parallelStream().forEach(n -> unsafe.add(n)); // may be unsafe
    System.out.println("Parallel add to ArrayList (unsafe demo): " + unsafe);

    // Safer: use built-in collectors
    List<Integer> safe = nums.parallelStream().map(n -> n).collect(java.util.stream.Collectors.toList());
    System.out.println("Collected safely: " + safe);
  }
}