Abstract Classes

Abstract classes are classes which you cannot make an instance of directly.¹

abstract class Plant {}

void main() {
    // You cannot make an instance because 
    // it is an abstract class.
    var p = new Plant();
}

The use-case for these is making classes which are designed to be subclassed. An example of this that comes with Java is AbstractList.

AbstractList defines most of the methods required by the List interface and is intended to lower the effort required to make a custom List implementation.

import java.util.AbstractList;

// This subclass is a List containing the numbers 5 and 7
class FiveAndSeven 
    // Almost all of the implementation is inherited from AbstractList
    extends AbstractList<Integer> {

    @Override
    public Integer get(int index) {
        return switch (index) {
            case 0 -> 5;
            case 1 -> 7;
            default -> throw new IndexOutOfBoundsException();
        };
    }

    @Override
    public int size() {
        return 2;
    }
}

void main() {
    var l = new FiveAndSeven();
    IO.println(l);
}