Real Default Methods in Java Collections API

Direct Answer

Java 8 added forEach() to Iterable; removeIf(), stream(), and parallelStream() to Collection; replaceAll() and sort() to List; and methods such as computeIfAbsent() and merge() to Map. Default implementations allowed existing types to gain these APIs with broad source and binary compatibility, although new default-method conflicts or behavior assumptions can still require attention.

Code Example

var orders = new ArrayList<Order>();
orders.add(new Order(1L, "Laptop", 1200.0, false));
orders.add(new Order(2L, "Mouse", 25.0, true));
orders.add(new Order(3L, "Keyboard", 80.0, false));
orders.add(new Order(4L, "Monitor", 300.0, true));

// forEach() - default method from Iterable
orders.forEach(order -> System.out.println(order.item()));

// removeIf() - default method from Collection
orders.removeIf(order -> order.shipped());

// stream() - default method from Collection
var total = orders.stream()
    .map(Order::price)
    .reduce(0.0, Double::sum);

// sort() - default method from List
orders.sort(Comparator.comparing(Order::price));

// replaceAll() - default method from List
orders.replaceAll(order -> 
    new Order(order.id(), order.item().toUpperCase(), 
              order.price(), order.shipped()));

record Order(Long id, String item, double price, boolean shipped) {}

Common Mistakes & Pitfalls

• Naming the wrong declaring interface: forEach() was added to Iterable, while stream() and removeIf() are on Collection; Map is a separate branch.
• Assuming all collections are mutable: removeIf(), sort(), and replaceAll() can throw UnsupportedOperationException when the implementation does not support the change.
• Assuming every computeIfAbsent() is an atomic cache loader: concurrency and mapping-function guarantees depend on the Map implementation; read its contract.
• Using forEach() for external accumulation: a collector or ordinary loop often makes ownership and concurrency clearer.