How to turn a string into an operator in Java? [duplicate]

Question

I'm trying to make a simple text based calculator in java my first program, EVER and I can't figure out how to turn an input String into a variable opOne. I would then attempt to operate numOne against numTwo using opOne as the operator. Code is as follows:

import java.io.*; import java.math.*; public class ReadString { public static void main (String[] args) { System.out.print("Enter the first number: "); BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); int numOne = 0 ; int numTwo = 0 ; String opOne = null; while(true){ try { numOne = Integer.valueOf(br.readLine()); break; } catch (IOException error) { System.out.println("error; try again."); System.exit(1); } catch (NumberFormatException nfe) { System.out.println("error;try again."); } } System.out.print("Enter the second number: "); while(true){ try { numTwo = Integer.valueOf(br.readLine()); break; } catch (IOException error2) { System.out.println("error"); System.exit(1); } catch (NumberFormatException nfe) { System.out.println("error;try again."); } } System.out.println("What would you like to do with " + numOne + " and " + numTwo + "?"); try { operator = br.readLine(); } catch (IOException ioe) { System.out.println("error"); System.exit(1); } catch (NumberFormatException nfe) { System.out.println("error"); } } }

"text based calculator" Use ScriptEngine instead. E.G. 1) in web page (applet) 2) non GUI 3) simple GUI 4) nice GUI.. — Andrew Thompson
– Andrew Thompson, Commented Mar 2, 2013 at 1:21

user3513254 · Accepted Answer · 2016-06-23 15:19:07Z

The simplest way of doing this would be a sequence of if-then-else statements:

if ("+".equals(opOne)) { res = numOne + numTwo; } else if ("-".equals(opOne)) { res = numOne - numTwo; } ...

An advanced way would be to define an interface for your operators, and putting the instances in a Map container:

interface Operation { int calculate(int a, int b); } static final Map<String,Operation> opByName = new HashMap<String,Operation>(); static { opByName.put("+", new Operation() { public int calculate(int a, int b) { return a+b; } }); opByName.put("-", new Operation() { public int calculate(int a, int b) { return a-b; } }); opByName.put("*", new Operation() { public int calculate(int a, int b) { return a*b; } }); opByName.put("/", new Operation() { public int calculate(int a, int b) { return a/b; } }); }

With a map initialized like this, you can perform calculations as follows:

int res = opByName.get(opOne).calculate(numOne, numTwo);

daOnlyBG · Accepted Answer · 2014-08-30 19:57:49Z

There's no way you can convert a Java string to an operator. It's not a Java data type.

 double Calculate(String opOne, double numOne, double numTwo) { if (opOne.equals("+")) return numOne + numTwo; else if (opOne.equals("-")) return numOne - numTwo; else if (opOne.equals("*")) return numOne * numTwo; else if (opOne.equals("/")) return numOne / numTwo; }

luiges90 · Accepted Answer · 2013-03-02 01:15:19Z

Computer, by itself, does not know what an operator (in human language) is, so you need to tell the computer to do that.

String opOne; try { opOne = br.readLine(); } catch (IOException ioe) { System.out.println("error"); System.exit(1); } switch (opOne) { //Note: String in switch is possible only from Java 7, check your version "+": System.out.println('Result: ' + (numOne + numTwo)); break; "-": System.out.println('Result: ' + (numOne - numTwo)); break; // and so on... }

I can't vote you up, since I am new, but I will say that a great big "THANK YOU"

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