java - Does a lambda expression create an object on the heap every time it's executed?

Question

When I iterate over a collection using the new syntactic sugar of Java 8, such as

myStream.forEach(item -> {
  // do something useful
});

Isn't this equivalent to the 'old syntax' snippet below?

myStream.forEach(new Consumer<Item>() {
  @Override
  public void accept(Item item) {
    // do something useful
  }
});

Does this mean a new anonymous Consumer object is created on the heap every time I iterate over a collection? How much heap space does this take? What performance implications does it have? Does it mean I should rather use the old style for loops when iterating over large multi-level data structures?

Answer 1

It is equivalent but not identical. Simply said, if a lambda expression does not capture values, it will be a singleton that is re-used on every invocation.

The behavior is not exactly specified. The JVM is given big freedom on how to implement it. Currently, Oracle’s JVM creates (at least) one instance per lambda expression (i.e. doesn’t share instance between different identical expressions) but creates singletons for all expressions which don’t capture values.

You may read this answer for more details. There, I not only gave a more detailed description but also testing code to observe the current behavior.

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This is covered by The Java? Language Specification, chapter “15.27.4. Run-time Evaluation of Lambda Expressions”

Summarized:

These rules are meant to offer flexibility to implementations of the Java programming language, in that:

• A new object need not be allocated on every evaluation.

• Objects produced by different lambda expressions need not belong to different classes (if the bodies are identical, for example).

• Every object produced by evaluation need not belong to the same class (captured local variables might be inlined, for example).

• If an "existing instance" is available, it need not have been created at a previous lambda evaluation (it might have been allocated during the enclosing class's initialization, for example).