forEach with lambda

Learn to use lambdas inside forEach.

forEach with a lambda

The forEach method runs an action for each element of a collection. Its parameter is a Consumer<T> functional interface with a single accept(T) method. With a lambda you write names.forEach(s -> System.out.println(s));.

forEach is the functional counterpart of the for-each loop. You cannot modify elements inside it; only read or produce a side effect (print, write to a file). On ordered collections it walks in insertion order.

forEach vs for-each loop

Both forms walk a collection but differ. The classic for-each: for (String s : names) { ... } — you can exit with break or continue. forEach: names.forEach(s -> ...) — no break/continue from inside the lambda.

forEach is shorter and fits functional style; but for early exit a classic for-each is preferred. For side effects (printing, summing) forEach is more concise.

Exercise Sum prices with forEach

Declare List<Integer> prices = new ArrayList<>();; add 10, 20, 30. Use int[] total = {0}; to hold the sum (the lambda needs effectively final). Add each price with prices.forEach(p -> total[0] += p);. After the loop print the sum with System.out.println(total[0]);.

Show example solution

import java.util.List;
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        List<Integer> prices = new ArrayList<>();
        prices.add(10);
        prices.add(20);
        prices.add(30);
        int[] total = {0};
        prices.forEach(p -> total[0] += p);
        System.out.println(total[0]);
    }
}

Test yourself

Which functional interface does forEach expect?
- Consumer
- Comparator
- Supplier
- Runnable
Show answer

Consumer — forEach expects a Consumer<T> with a single accept(T) method.
Match the forEach concepts

forEach(...)

Runs an action
Consumer<T>

Single-arg interface
s -> ...

Lambda parameter
break

Not in forEach

Complete the forEach call

names.forEach(____ -> System.out.println(s));

Show answer

names.forEach(s -> System.out.println(s));

Next lesson: Comparator with lambda

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