Abstract Classes

An abstract class can declare abstract methods, like interface methods, with no body — but unlike an interface, it can also hold real fields and fully-implemented methods, and a subclass only has to implement the abstract parts. Vehicle can't be instantiated directly with new; only a concrete subclass like Car, which supplies describe(), can be.

public class Main {
    static abstract class Vehicle {
        abstract String describe();

        void printDescription() {
            System.out.println(describe());
        }
    }

    static class Car extends Vehicle {
        String describe() {
            return "A car with four wheels";
        }
    }

    public static void main(String[] args) {
        Vehicle v = new Car();
        v.printDescription();
    }
}

A car with four wheels

printDescription() is fully implemented in Vehicle itself and inherited by Car unchanged, while describe() has to come from whichever concrete subclass extends Vehicle — abstract classes let you share real code alongside the contract, which a plain interface can't do for you.

Next: exception handling — try, catch, checked exceptions, and what a real unreported-exception compile error looks like.