Abstract classes vs interfaces

public interface INumber {
    public INumber add(INumber other);
}

public class RomanNumber implements INumber {
    public RomanNumber add(INumber other) {
        // when `other` is not a RomanNumber
        // convert other to RomanNumber
        // add `other` to `this`
    }
}

public class HexadecimalNumber implements INumber {
    public HexadecimalNumber add(INumber other) {
        // convert other to HexadecimalNumber and add...
    }
}

public abstract class Number {
    public abstract Integer toDecimal();
    public abstract Number fromDecimal(Integer);

    public Number add(Number other) {
        returh fromDecimal(this.toDecimal() + other.toDecimal());
    }
}

public class RomanNumber extends Number {
    public Integer toDecimal() {
        // convert `this` to Integer
    }

    public RomanNumber fromDecimal(Integer n) {
        // convert `n` to RomanNumber
    }
}

public class HexadecimalNumber extends Number {
    public Integer toDecimal() {
        // convert `this` to Integer
    }

    public HexadecimalNumber fromDecimal(Integer n) {
        // convert `n` to HexadecimalNumber
    }
}

Vector2d implements IVector { // THIS IS NOT A CONSTRUCTOR! @Override public void Vector() { } } ```

<p>and compare it to this:</p>

```java public abstract class BaseVector { public abstract BaseVector() { } } public class Vector2d extends
BaseVector { public Vector2d() { // now it DOES HAVE a default constructor super(); } } ```