How to convert string to operator in java

Question

I want to convert some String to an operator like this:

int value = 1;
int valueToCompare = 3;
String operation = \"<\";

if (value operation         


        

Answer 1

The simplest way would be a chain of if-else statements checking what the the operator is and then performing the mathematical expression.

Answer 2

Your approach will never work in Java. Instead you may define your own enum which perform the proper boolean operation like this:

public static void main(String[] args) {
    boolean result = Operator.parseOperator("==").apply(2, 2);
}

enum Operator {

    LESS("<") {
        @Override public boolean apply(int left, int right) {
            return left < right;
        }
    },
    EQUAL("==") {
        @Override public boolean apply(int left, int right) {
            return left == right;
        }
    };

    private final String operator;

    private Operator(String operator) {
        this.operator = operator;
    }

    public static Operator parseOperator(String operator) {
        for (Operator op : values()) {
            if (op.operator.equals(operator)) return op;
        }
        throw new NoSuchElementException(String.format("Unknown operator [%s]", operator));
    }

    public abstract boolean apply(int left, int right);
}

Answer 3

You can use ScriptEngine for that, works for any other JavaScript expressions also :

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

ScriptEngineManager factory = new ScriptEngineManager();
ScriptEngine engine = factory.getEngineByName("JavaScript");


int value = 1;
int valueToCompare = 3;
String operation = "<";

if ((Boolean)engine.eval("" + value + operation + valueToCompare)) {
    System.out.println("Here we are.");  
}