Advanced Audio Coding 

AAC is a part of MPEG-2 AND MPEG-4 specifications.

Open source codec implementation can be found in FFmpeg, FAAC and opencore-amr.

JAAD is a Java version of AAC decoder (decoding only).

aacdecoder-android is a library for Android to play AAC streams or files (decoding only).

Java AAC encoder is hard to find.