How to get arithmetical operators at runtime?

Question

How I can get arithmetical operators at run-time in Java? Suppose if I have values

• ADD it should add the number
• MUL then it should multiply the number

For Example

  public calculate(int x, String str){
   while(str.equals("some value")){
     If( str.equals("ADD"))
        // it should return me something like x+
     if( str.equals("MUL"))
        it return me something like x*
    }
    if( str.equals("FINAL"))
        it should return me x+x*x

  }

Answer 1

What you need is not runtime metaprogramming, but first class functions.

The following represent first class functions, with arity 1 and 2 respectively.

abstract class UnaryFunction<A, B> {
  public abstract B apply(A a);
}

abstract class BinaryFunction<A, B, C> {
  public abstract C apply(A a, B b);
}

For the sake of simplicity, let's use specialized versions of above classes.

abstract class UnaryOperation {
  public abstract int apply(int a);
}

abstract class BinaryOperation {
  public abstract int apply(int a, int b);
}

Now construct a dictionary of the required arithmetic operations.

Map<String, BinaryOperation> ops = new HashMap<String, BinaryOperation>();
ops.put("ADD", new BinaryOperation() {
  public int apply(int a, int b) {
    return a + b;
  }
});
ops.put("MUL", new BinaryOperation() {
  public int apply(int a, int b) {
    return a * b;
  }
});
// etc.

Add a method that partially applies BinaryOperation on one parameter.

abstract class BinaryOperation {
  public abstract int apply(int a, int b);

  public UnaryOperation partial(final int a) {
    return new UnaryOperation() {
      public int apply(int b) {
        return BinaryOperation.this.apply(a, b);
      }
    };
  }
}

Now we can write your calculate method.

public UnaryOperation calculate(int x, String opString) {
  BinaryOperation op = ops.get(opString);
  if(op == null)
    throw new RuntimeException("Operation not found.");
  else
    return op.partial(x);
}

Use:

UnaryOperation f = calculate(3, "ADD");
f.apply(5); // returns 8

UnaryOperation g = calculate(9, "MUL");
f.apply(11); // returns 99

The abstractions used in the above solution, namely first class function interfaces and partial application, are both available in this library.