Java syntax
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If a class does not specify its superclass, it implicitly inherits from java.lang.Object class. Thus all classes in Java are subclasses of Object class.
If the superclass does not have a constructor without parameters the subclass must specify in its constructors what constructor of the superclass to use. For example:
class Foo {
public Foo(int n) {
// Do something with n
}
}
class Foobar extends Foo {
private int number;
// Superclass does not have constructor without parameters
// so we have to specify what constructor of our superclass to use and how
public Foobar(int number) {
super(number);
this.number = number;
}
}
Overriding methods
Unlike C++, all non-final methods in Java are virtual and can be overridden by the inheriting classes.
class Operation {
public int doSomething() {
return 0;
}
}
class NewOperation extends Operation {
@Override
public int doSomething() {
return 1;
}
}
Abstract classes
An Abstract Class is a class that is incomplete, or is to be considered incomplete, so cannot be instantiated.
A class C has abstract methods if any of the following is true:
• C explicitly contains a declaration of an abstract method.
• Any of C's superclasses has an abstract method and C neither declares nor inherits a method that implements it.
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