I have a question. Bruce Eckel says in his book
"Thinking in Java":

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It is also possible that your object might contain
other objects that contain data you’d like to modify.
For this, you just keep “connecting the dots.” For
example:

myPlane.leftTank.capacity = 100;

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Now, how can I do this? I tried to do it by inner
classes but I failed. If someone could give me an
example program, I'd be very glad.