Do you want to use lambda expressions already today, but you are forced to use Java and a stable JRE in production? Now that's possible with Retrolambda, which will take bytecode compiled with Java 8 and convert it to run on Java 7, 6 and 5 runtimes, letting you use lambda expressions and method references on those platforms. It won't give you the improved Java 8 Collections API, but fortunately there are multiple alternative libraries which will benefit from lambda expressions.
Behind the Scenes
The original plan of copying the classes from OpenJDK didn't work (
LambdaMetafactory depends on some package-private classes and would have required modifications), but I figured out a better way to do it without additional runtime dependencies.
Retrolambda uses a Java agent to find out what bytecode
LambdaMetafactory generates dynamically, and saves it as class files, after which it replaces the
invokedynamic instructions to instantiate those classes directly. It also changes some private synthetic methods to be package-private, so that normal bytecode can access them without method handles.
After the conversion you'll have just a bunch of normal .class files - but with less typing.
P.S. If you hear about experiences of using Retrolambda for Android development, please leave a comment.