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Java applet is a small program which is used to build interactive user interface in java. It is more secure and robust.
Search online there are may sites where you can practice problems.
1)Meritcampus (Various Topics+ Online Compiler+ practice Tests + Expert Training)
For Difficult Programs:
For more Details Refer following link:
http://java.meritcampus.com/core-java-topics/java-programming-language in detail about java programming language