Documentation Contents

# Collections Framework Enhancements in Java SE 8

## Support for Lambda Expressions, Streams, and Aggregate Operations

The Java Collections Framework has been updated to support lambda expressions, streams, and aggregate operations. For more information on these topics, see the following pages:

- Enhancements in Java SE 8
- New and Enhanced APIs That Take Advantage of Lambda Expressions and Streams in Java SE 8
- Lambda Expressions (The Java Tutorials)
- Aggregate Operations (The Java Tutorials)
- Collections (The Java Tutorials)

## Performance Improvement for HashMaps with Key Collisions

As part of the work for JEP 180, there is a performance improvement for `HashMap` objects where there are lots of collisions in the keys.

The alternative `String` hash function added in 7u6 has been removed from JDK 8, along with the `jdk.map.althashing.threshold` system property. Instead, hash bins containing a large number of colliding keys improve performance by storing their entries in a balanced tree instead of a linked list. This JDK 8 change applies only to `HashMap`, `LinkedHashMap`, and `ConcurrentHashMap`.

In rare situations, this change could introduce a change to the iteration order of `HashMap` and `HashSet`. A particular iteration order is not specified for `HashMap` objects - any code that depends on iteration order should be fixed.

Note that, other than removing the feature introduced in 7u6, the `java.util.Hashtable` class is not affected. The features added in 7u6 applied to `WeakHashMap` and `Hashtable` (and by extension `Properties` and `Provider`) but in JDK 8 these have been removed.

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