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rules for extending classes

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Are there any rules for extending classes?

I found out the hard way the superclass must have a default no arg constructor.

I suspect there are other rules, but I can't seem to find any.

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No, a superclass doesn't need a default constructor. It just needs an accessible constructor. The subclass then needs to call this constructor explicitly in the first line of its own constructors.

You can subclass any class, as long as 1) the class is accessible, 2) the class is non-final and 3) the class has an accessible constructor.

Here's an example of subclassing when there is no default constructor in the superclass:
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As "Stephan van Hulst" said Default Constructor existence is not a Compulsion for any Class.
If you don't have a Don't have a default Constructor in the Super class and you forget to insert a super(<with appropriate parameters>) inside every subclass you create then the compiler will Complain .

The following code will result in a Compiler Error :

"Implicit super Constructor A() undefined will be the error"
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