Abstract Classes

An abstract class is a class that can't be instantiated on its own; it exists purely to be extended by other classes. Abstract classes are useful when you want to define shared structure and behavior for a group of related subclasses, while forcing each subclass to fill in some of the details itself.

Declaring an Abstract Class

An abstract class is declared using the abstract keyword.

Abstract Class Formula

abstract class [ClassName] {
    [regular fields and methods]
    abstract [return type] [method name]([parameters]);
}

abstract class Subsystem {
    boolean isEnabled;

    void enable() {
        isEnabled = true;
    }

    abstract void periodic(); // No body, just a method signature
}

Notice that periodic() has no body, just a signature ending in a semicolon. This is an abstract method, a method that the abstract class declares, but doesn't implement, leaving that responsibility to its subclasses.

Why You Can't Instantiate an Abstract Class

Subsystem subsystem = new Subsystem(); // Compiler error!

Java won't let you create a Subsystem object directly, since Subsystem has an abstract method (periodic()) with no actual implementation. If Java allowed this, and you called subsystem.periodic(), there would be no code to actually run. Instead, any concrete (non-abstract) subclass is required to provide an implementation for every abstract method it inherits.

class Flywheel extends Subsystem {
    double voltage;

    @Override
    void periodic() {
        System.out.println("Spinning at " + voltage + " volts");
    }
}

Subsystem shooterWheel = new Flywheel(); // This works, since Flywheel implements periodic()
shooterWheel.periodic();

Output:

Spinning at 0.0 volts