Abstract Class

• Cannot have multiple inheritance. Aka sub class can only extends from one parent. Eg, class Child extends Parent { … }
• The Child class must contain all the abstract methods declared. All the overridden methods in its Child class must be public. Can have method definition. Eg, toString()
• Can have method definition. Eg, toString()
• Can have properties / attributes
• Members can be private/ protected/ public etc.
• abstract keyword is needed in front of the class and methods. Eg, abstract class Hotel { abstract int roomNum { … } }
• Cannot have multiple inheritance. Aka sub class can only extends from one parent. Eg, class Child extends Parent { … }

Interface

• Allows for multiple inheritance. Can inherit ‘behaviors’ from multiple interfaces. Eg, class Child implements A, B, C { … }
• All the overridden methods in its Child class must be public.
• Cannot have method definition. Eg, toString()
• Cannot have properties / attributes
• Members are all public by default.
• There is no need for an abstract keyword after Java 8.
• Allows for multiple inheritance. Can inherit ‘behaviors’ from multiple interfaces. Eg, class Child implements A, B, C { … }

When to use which?

Scenario 1: You have multiple classes with the exact same method.

• An abstract class will be more appropriate.
• It removes code repetitions.

Scenario 2: You have multiple classes with same method name but different implementation of code.

• An interface would be more appropriate.
• Override the method from interface.

Scenario 3: If you want to inherit multiple ‘behaviors’ from multiple parents.

• An interface would be more appropriate.
• Allows multiple inheritance of ‘behavior’.

All in all,

• Use abstract class if you have common methods between multiple classes.
• Use interface if you want more flexibility.