Java Package defnition

Is there any website where we can come to know the information about

1. Technical implementation Possibilities
2. Where this package will be used extensively
3. why this is used?

For example TimerT开发者_运维技巧ask, this is used to scheduling a task to execute in a batch mode. Other than this do we have any other scenarios where we can use it?

Please share the information that will be really helpful.

I am sorry if this question appears vague, I want to learn the possibilities in java

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The offical java api: http://java.sun.com/javase/6/docs/api/index.html?overview-summary.html

And any good Java book or tutorial will tell you a lot of good examples and possibilities with Java.