Module 2 · Objects and collections Lesson 5 of 15
Inheritance, interfaces and why service companies ask
Inheritance shares behaviour from a parent class; an interface is a contract a class promises to keep. "Why is an interface better than an abstract class here?" is a standard first-round question.
The concept
Inheritance extends a class: class SavingsAccount extends Account inherits the fields and methods and can override behaviour. An interface declares what a class can do — interface Payable { double calculatePay(); } — and any class implementing it must supply the method, which is how unrelated types join a common contract.
In practice interfaces age better than inheritance. A class can implement many interfaces and extend only one, so a design built on interfaces stays changeable. The phrase to remember in an interview: prefer composition and interfaces, use inheritance for genuine is-a relationships.
interface Payable {
double amount();
}
abstract class Worker {
abstract String role();
}
class Intern extends Worker implements Payable {
public String role() { return "intern"; }
public double amount() { return 15000; }
}
// Java has no multiple inheritance of CLASSES; interfaces give many types.
// Prefer an interface when you only need a capability (Comparable, Runnable).Common mistakes
The mistake is a deep inheritance chain made to reuse three lines of code, which becomes impossible to reason about by the fourth level. Copy the three lines, or move them into a helper. Reuse is not a reason to couple two classes for ever.
Worked example
A Print interface with one print() method lets three unrelated report classes be stored in one list and printed in a loop. An abstract base class would force them to share a family tree they do not have.
Your practice task
Write one interface, implement it in three classes, store them in a List<Print> and print all three through the interface.
Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.
Key takeaway
Inheritance extends a class: class SavingsAccount extends Account inherits the fields and methods and can override behaviour.
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