Exercises on using Java API

Is there a tutorial / book / website that teaches you how to use the Java API? So, let say I am already in this website: http://download.oracle.com/docs/cd/E17476_01/javase/1.4.2/docs/api/

What are these packages / classes for? Or if I want to manipulate Strings... how do I know to use java.lang.String?

The开发者_开发问答n if I am in java.lang.String, I see there is the Interface, class and exception summary. I am sure somewhere there is a documentation that explains all of this. Also, I see a field, constructor, method summary... what are these?

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What you want sounds like the Java Tutorials.

Also, I see a field, constructor, method summary... what are these?

Learning the Java Language.

You also want to use a more recent reference than 1.4 since lots of things have changed since then: Java SE 6. (The documentation for SE 7 is still a draft for the moment)