GNU Trove - High performance collections for Java

The GNU Trove library has two objectives:

• Provide "free" (as in "free speech" and "free beer"), fast, lightweight implementations of the java.util Collections API. These implementations are designed to be pluggable replacements for their JDK equivalents.
• Whenever possible, provide the same collections support for primitive types. This gap in the JDK is often addressed by using the "wrapper" classes (java.lang.Integer, java.lang.Float, etc.) with Object-based collections. For most applications, however, collections which store primitives directly will require less space and yield significant performance gains.

• Commons Collections
• Primitives
• Mango
• fastutil: Fast & compact type-specific collections for Java
• Type-Specific Collections Library
• Primitive Collections for Java