Understanding when to use stream() vs forEach() in Java

| 2 minutes read

In Java, choosing between Stream() and ForEach() depends on what you want to do with your data. While both are used to iterate over collections, each has its own strengths.

stream()

Using stream() is powerful for complex data transformations, allowing operations like filtering, mapping, and reducing. It’s ideal for:

  • Transformations and filters: Use Stream() to apply operations like filtering, mapping, grouping, or reducing data.
  • Chained processing: Apply multiple operations in a single chain.
  • Parallel processing: For large datasets, use parallelStream() to leverage multi-core processors.
  • Immutability: Streams avoid side effects, allowing new collections to be generated without changing the original.

Example:

List<String> names = Arrays.asList("Maria", "José", "João");
List<String> filteredNames = names.stream()
.filter(name -> name.startsWith("J"))
.map(String::toUpperCase)
.collect(Collectors.toList());

The results will be a new list only with the names started with J in uppercase.

forEach()

Using forEach() is perfect for simple tasks, applying an action to each item without returning anything. Use for:

  • Simple operations: Direct tasks without transformations or chaining.
  • Side effects: Modify elements in an existing collection.
  • Readability: Simple and direct code.

Example:

List<String> names = Arrays.asList("Maria", "José", "João");
names.forEach(name -> System.out.println(name));

This code snippet will only print all items on the list.

Final Thoughts

  • Use Stream() for complex transformations, parallel processing, or immutability.
  • Use ForEach() for simple, direct operations.

These two options complement each other, enabling Java developers to work with collections efficiently.

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