10.4. Local Variables in Lambda Expressions

A lambda expression does not define a new scope, it inherits its enclosing one.

We can access local variables within our lambda expressions.

public static void main(String[] args) {
  int multiplier = 2;
  List.of(1, 2, 3, 4).forEach(x -> System.out.println(x * multiplier));
}

But we cannot modify their value.

int sum = 0;
// This will not compile!
List.of(1, 2, 3, 4).forEach(x -> sum += x);

We also cannot redeclare a variable.

public static void main(String[] args) {
  int value = 2;
  List.of(1, 2, 3, 4).forEach(x -> {
    int value = x;
    System.out.println(x);
  });
}

The local variables we use within our lambda bodies should be either

final, that is, variables whose values cannot be altered, or
effectively final, that is, variables whose values are not altered after initialization.

Note that the object referenced by a variable may be modified within a lambda expression.

class Accumulator {
  int value;

  public Accumulator(int value) {
    this.value = value;
  }

  void add(int value) {
    this.value += value;
  }
}

public class ModifyingObjects {
  public static void main(String[] args) {
    Accumulator acc = new Accumulator(0);
    List.of(1, 2, 3, 4).forEach(x -> acc.add(x));
  }
}