How to get arithmetical operators at runtime?
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12-06-2021 - |
Question
How I can get arithmetical operators at run-time in Java? Suppose if I have values
ADD
it should add the numberMUL
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
}
La solution
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.
Autres conseils
public class Calculator {
public static enum Operation {ADD, MUL, SUB, DIV};
private int x; // store value from previous operations
public void calculate(int x, Operation operation) {
switch(operation) {
case ADD:
this.x += x;
break;
case MUL:
this.x *= x;
break;
case SUB:
this.x -= x;
break;
case DIV:
this.x /= x;
break;
}
}
public int getResult() {
return this.x;
}
}
To use it elsewhere in your code:
public static void main(String[] args) {
Calculator c = new Calculator();
c.calculate(4, Calculator.Operation.ADD);
// Other operations
c.getResult(); // get final result
}
Assuming you are trying to just add and multiply x
, just do the following:
public int calculate(int x, String str) {
// while(true) is gonna get you into some trouble
if( str.equals("ADD")) {
return x + x;
}
else if( str.equals("MUL")) {
return x * x;
}
else
return x; // not sure what you want to do in this case
}