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

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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>() {
  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?

Java 8 Lambdas - A Peek Under the Hood, This article sheds light on how Java 8 lambda expressions and method You can examine the bytecode generated for any class file using the A reference to the newly created object is pushed on the stack at the same time. 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).

When an instance representing the lambda is created sensitively depends on the exact contents of your lambda's body. Namely, the key factor is what the lambda captures from the lexical environment. If it doesn't capture any state which is variable from creation to creation, then an instance will not be created each time the for-each loop is entered. Instead a synthetic method will be generated at compile time and the lambda use site will just receive a singleton object that delegates to that method.

Further note that this aspect is implementation-dependent and you can expect future refinements and advancements on HotSpot towards greater efficiency. There are general plans to e.g. make a lightweight object without a full corresponding class, which has just enough information to forward to a single method.

Here is a good, accessible in-depth article on the topic:

Lambda Expressions in Java, In those objects, the lambda implementation is considered the said method's implementation. A lambda function can take any number of parameters, including zero, so you could have something like this: Unsubscribe at any time. In the spirit of making the syntax short and sweet, you don't actually have to type out the  However, the opposite is not true; lambda expressions can be converted to expression trees which allows for a lot of the magic like LINQ to SQL. The following is an example of a LINQ to Objects expression using anonymous delegates then lambda expressions to show how much easier on the eye they are: // anonymous delegate var evens = Enumerable.

You are passing a new instance to the forEach method. Every time you do that you create a new object but not one for every loop iteration. Iteration is done inside forEach method using the same 'callback' object instance until it is done with the loop.

So the memory used by the loop does not depend on the size of the collection.

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

Yes. It has slight differences at a very low level but I don't think you should care about them. Lamba expressions use the invokedynamic feature instead of anonymous classes.

Lambda Expressions in Java 8, A function that can be created without belonging to any class. A lambda expression can be passed around as if it was an object and executed on demand​. Apparently, JVM is smart enough to not create multiple objects for stateless lambdas. – MFIhsan Oct 28 '15 at 12:36. They mostly code like anonymous inner classes, but that does not mean that they simply are syntactic sugar. You should avoid making that assumption the future, there are other subtle differences too.

Method References (The Java™ Tutorials > Learning the Java , You use lambda expressions to create anonymous methods. Sometimes, however, a lambda expression does nothing but call an existing method. Reference to an instance method of an arbitrary object of a particular type, ContainingType::  To implement a lambda (which is a sort of a local function), the compiler has to create a delegate. Obviously, each time a lambda is called, a delegate is created as well. This means that if the lambda stays on a hot path (is called frequently), it will generate huge memory traffic.

Anonymous Classes (The Java™ Tutorials > Learning the Java , They enable you to declare and instantiate a class at the same time. Declaring Anonymous Classes; Syntax of Anonymous Classes; Accessing Local Extra methods (even if they do not implement any methods of the supertype) one method, you can use a lambda expression instead of an anonymous class expression. At run time, evaluation of a lambda expression is similar to evaluation of a class instance creation expression, insofar as normal completion produces a reference to an object. Evaluation of a lambda expression is distinct from execution of the lambda body.

Closure (computer programming), In programming languages, a closure, also lexical closure or function closure, is a technique for When the lambda expression is evaluated, Scheme creates a closure has a reference to threshold , it can use that variable each time filter calls it. These are analogous to private variables in object-oriented programming,  Now we create a list of the Person objects in which we have to perform several operations like finding a person on certain conditions, removing a person's record etc. These types of operations can be easily performed using a "Lambda Expression". We create the list and populate them in the following way: List<Person> listPersonsInCity = new List

  • Short answer: no. For stateless lambdas (those that do not capture anything from their lexical context), only one instance will ever be created (lazily), and cached at the capture site. (This is how the implementation works; the spec was carefully written to allow, but not require, this approach.)
  • Do you have any documentation specifying this? It's quite the interesting optimization.
  • Thanks, but what if I have a collection of collections of collections, for example when doing a depth-first search on a tree datastructure?
  • @A.Rama Sorry, I don't see the optimization. It is the same with or without lambdas and with or without forEach loop.
  • It's not really the same, but still at most one object per nesting level will be needed at any one time, which is negligible. Each new iteration of an inner loop will create a new object, most probably capturing the current item from the outer loop. This creates some GC pressure, but still nothing to really worry about.
  • @aalku: "Every time you do that you create a new object": Not according to to this answer by Holger and this comment by Brian Goetz.