Simulating Haskell typeclass with Java generic interface.

typeclass.java

// Simulating Haskell typeclass with Java generic interface.

public class Main {
  public static void main(String[] args) {
    MyInt i1 = new MyInt(1);
    MyInt i2 = new MyInt(2);
    MyInt i3 = i1.add(i2);
    System.out.println(i3); // "3"

    MyString s1 = new MyString("1");
    MyString s2 = new MyString("2");
    MyString s3 = s1.add(s2);
    System.out.println(s3); // "12"

    i3 = Adder.add(i1, i2);
    System.out.println(i3); // "3"
    s3 = Adder.add(s1, s2);
    System.out.println(s3); // "12"
  }
}

interface Addable<T extends Addable<T>> {
  T add(T another);  
}

class Adder {
  public static <T extends Addable<T>> T add(T t1, T t2) {
    return t1.add(t2);
  }
}

class MyInt implements Addable<MyInt> {
  private int value;

  public MyInt(int value) {
    this.value = value;
  }

  @Override
  public MyInt add(MyInt another) {
    return new MyInt(this.value + another.value);
  }

  @Override
  public String toString() {
    return Integer.toString(value);
  }
}

class MyString implements Addable<MyString> {

  private String value;

  public MyString(String value) {
    this.value = value;
  }

  @Override
  public MyString add(MyString another) {
    return new MyString(this.value + another.value);
  }

  @Override
  public String toString() {
    return value;
  }
}