Not much. The latest jdk 1.6, if you study now, you'd better not watch jdk 1.4 again. You can choose a grammar book of jdk5 or 6 to learn. Many new functions have been added from 4 to 5 to 6, and the functions have been strengthened step by step. If you are just starting to learn java, you don't need to see what is added to each version. Only when you use it, you will naturally know something about java.