java generic addition

Question

I\'m attempting implement the add method mentioned in the Generic sparse matrix addition question

class Matrix
{
  private T add(T left,          


        

Answer 1

Java's type system is simply not capable of expressing this. Here is a work around.

Create an interface Numeric that provides the numeric operations you are interested in, and write its implementations for the data types you are interested in.

interface Numeric {
  public N add(N n1, N n2);
  public N subtract(N n1, N n2);
  // etc.
}

class IntNumeric extends Numeric {
  public static final Numeric INSTANCE = new IntNumeric();

  private IntNumeric() {
  }

  public Integer add(Integer a, Integer b) {
    return a + b;  
  }

  public Integer subtract(Integer a, Integer b) {
    return a - b;  
  }

  // etc.
}

And rewrite your Matrix class constructor to accept this implementation.

class Matrix {
  private final Numeric num;
  private final List> contents;

  public Matrix(Numeric num) {
    this.num = num;
    this.contents = /* Initialization code */;
  }

  public Matrix add(Matrix that) {
    Matrix out = new Matrix(num);
    for( ... ) {
      for( ... ) {
        out.contents.get(i).set(j,
          num.add(
            this.contents.get(i).get(j),
            that.contents.get(i).get(j),
          )
        );
      }
    }
    return out;
  }
}

// Use site
Matrix m = new Matrix(IntNumeric.INSTANCE);

Hope that helps.