java-for-comprehension.md

List<String> evens = someList.stream()
  .filter(i -> i % 2 == 0)
  .toList()

For-comprehension Version:

ArrayList<String> evens = for (int i : myArrayOrIterable) {
  if (i % 2 == 0) yield i;
}

Compiles to:

ArrayList<String> evens = new ArrayList<>();
for (int i : myArrayOrIterable) {
  if (i % 2 == 0) evens.add(i);
}

If the return-type of a for-expression is not specified, a Collection is returned.

One could make the return type HashSet and it will work as expected, because ArrayList, HashSet, and any container that wants to support for-expressions implements Collection, which requires a 0-arg constructor (Javadoc: "should provide two "standard" constructors: a void (no arguments) constructor..."), and an add() method. If interfaces someday allow us to require a 0-arg constructor (which would improve Spring and Hibernate) then the for-expression return type could be opened to any class implementing an interface.

There are many reasons why for-expressions are better than Stream for simple tasks like converting between collection types.

In complex cases, a regular for statement is better than both Stream or for-expressions.

AugustNagro/java-for-comprehension.md