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You need to read the API doc for the Rectangle class and see what methods it has.
The compiler is not finding the method: getDiagonal

You must use methods that are defined for the class, you can...

What is that? A package or a class? Where are you seeing that in the documentation?

Note: Java class names start with Uppercase letters. The correct spelling for the Object class starts with a...

Did you look in the API doc for the class Rectangle? Does it have a getDiagonal() method defined?

See the API doc: Java Platform SE 7

Please explain why you think there is something wrong.
If you get error messages, copy the full text and paste it here.
If wrong output, post the output and explain what is wrong with the...