interface SomeInterface
{
  public final int x = 32;
  public double y;

  public double addup( );
}

Is it correct?

A good answer might be:

No. Variables (such as y) cannot be put in an interface. Only constants and methods.

A class definition must always extend one parent, but it can implement zero or more interfaces:

class SomeClass extends Parent implements SomeInterface { ordinary class definition body }

The body of the class definition is the same as always. However, since it implements an interface the body must have a definition of each of the methods from the interface. The class definition can use access modifiers as usual. Here is a class definition that implements three interfaces:

public class BigClass extends Parent implements InterfaceA, InterfaceB, InterfaceC { ordinary class definition body }

Now BigClass must provide a method definition for every method in each of the interfaces. Any number of classes can implement the same interfaces. Here is another class definition:

public class SmallClass implements InterfaceA { ordinary class definition body }

QUESTION 4:

Is the above class definition correct?