What is an Interface in Java?

An Interface in Java is like a Class, with methods and variables declared but with no definition.
They are the blueprint of a Class. An interface can tell a class on what it must do.
Supposedly a class implements and Interface, all the methods declared in the interface have to be defined in the class implementing it.

For example:

Consider the following Interface Player

Interface Player {
int id;
String name;
void enter (String name);
}

Then the class implementing the interface would be.

Class Sport implements Player {
int id;
String name;
Void enter (String name) {
this.name = name;
}
}

An Interface is already defined for you as PortfolioManger.java for completing your task.

Note: All the functions declared in the Interface Portfolio Manager have to be declared in the class implementing it, in this case, PortfolioManagerImpl