Do Java Lambda Expressions Cause Significant Heap Allocation and Performance Problems?

Lambda Expressions: Heap Allocation and Performance Implications

In Java 8, lambda expressions provide a succinct and functional syntax for writing code. However, concerns have been raised about whether lambda expressions create new objects on the heap every time they are executed, especially when iterating over collections.

Lambda Expressions and Object Creation

The equivalent syntax for lambda expressions using the traditional anonymous class approach is:

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

In both cases, a lambda expression or anonymous class instance is created to represent the action performed on each element of the collection. However, the key difference lies in how they are implemented.

Behavior Specified by Java Language Specification

The Java Language Specification states that a lambda expression needs not always create a new object for every evaluation. The JVM has flexibility in how it handles lambda expressions, including the reuse of objects and optimization based on captured variables.

Specifically, the specification allows the following scenarios:

• The same instance may be reused for multiple lambda expressions with identical bodies.
• Different lambda expressions need not necessarily belong to different classes.
• Every lambda evaluation object may not necessarily belong to the same class.
• The "existing instance" used could have been created during the enclosing class's initialization rather than a previous lambda evaluation.

Current Oracle JVM Implementation

Currently, Oracle's JVM adopts a conservative approach, creating one instance per lambda expression. However, singletons are used for lambda expressions that do not capture values. This means that in most cases, no new objects are allocated on the heap when iterating over collections using lambda expressions.

Performance Implications

The heap space allocated for lambda objects is negligible, especially for small collections. The performance implications are primarily related to the overhead of creating new objects in the case of non-singleton lambda expressions, which is often insignificant.

Conclusion

While lambda expressions provide a convenient and expressive way to write code, they do not typically lead to significant heap allocation or performance issues, especially when optimizing compilers are used. Developers can generally use lambda expressions without concerns about excessive object creation or heap consumption.

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