Can I map a String to a method in java?

Question

I'm writing an expression evaluator in Java. I would like the ability to add more operators (I currently have only (, ), +, -, *, /, and ^). Currently, my code looks like this:

case '+':  
return a+b;  
case '-':  
return a-b;  
case '*':  
return a*b;  
...

This works for my code because I have only a few operators. However, if I were to add more operators, the code would become cluttered. I am looking for a way to map an operator (represented by a String) to a method. For example, "ln" would be mapped to Math.log(), "^" would be mapped to Math.pow(), etc.

How would I go about doing this? If it's not feasible, what are some alternatives?

Answer 1

Not possible unless you want to use reflection. A solution without reflection could look like this:

public interface Operation {
  int apply(int... operands);
}

public abstract class BinaryOperation implements Operation {
  @Override
  public int apply(int... operands) {
    return apply(operands[0], operands[1]);
  }

  abstract int apply(int a, int b);
}

Map<String, Operation> operations = new HashMap<String, Operation>() {{
  put("+", new Operation() {
    @Override
    public int apply(int... operands) {
      return operands[0] + operands[1];
    }
  });
  put("-", new BinaryOperation() {
    @Override
    public int apply(int a, int b) {
      return a - b;
    }
  });
}};

Answer 2

You could use template methods.

public enum Functions {
    ADD() {
        @Override public int execute(int a, int b) {
            return a+b;
        }
    },
    SUB() {
        @Override public int execute(int a, int b) {
            return a-b;
        }
    };

   //Template method
   public abstract int execute(int a, int b);
}

Then map between string and enum with Map<String, Functions> functionMap So if you want to add you can do functionMap.put("+", Functions.ADD); Then call functionMap.get("+").execute(a,b);

I suppose you could also use varargs if different functions take different numbers of arguments. public abstract int execute (Integer... inputs);

This example is modified from Making the Most of Java 5.0: Enum Tricks and what @duffymo said.