Inherent math operation in Java - java-8

Question

I'm writing essentially a small calaculator where you input two numbers, an operand like "+, -, *, /" etc and it'll preform that function.
My initial thoughts have been to just have the variables get entered by the user, than just have them computed as imputed. I would like to do it this way, as to avoid writing several if else statements and keeping things a bit cleaner. But I can't find a place explaining how to inherently compute the values and imputed characters.
package hw2p1;
import java.util.Scanner;
import javax.script.*;
public class Calculator {
public static void PrintCalculator(){
Scanner input = new Scanner(System.in);
double num1; //1st entered number
double num2; //2nd entered number
String val1; //Math operator like + - * /.
double math1; //the results.
String equation = "";
System.out.print("Enter the first number:");
num1 = input.nextDouble();
System.out.print("Enter the second number:");
num2 = input.nextDouble();
System.out.print("Enter an operator:");
val1 = input.next();
I know this is incomplete, but I don't know/can't find the logic on how to string the three inputted values together and compute them.

Answer 1

The simplest solution would be
public static double calculate(double a, String op, double b) {
switch(op) {
case "+": return a+b;
case "-": return a-b;
case "*": return a*b;
case "/": return a/b;
case "%": return a%b;
case "^": return Math.pow(a, b);
default: throw new IllegalArgumentException("no such operator '"+op+"'");
}
}
which is semantically equivalent to a sequence of if-else statements, but clearly showing the intention and potentially more efficient, though there’s not much worth in speculating about performance here.
It can be invoked like
double math1 = calculate(num1, val1, num2); //the results.
but you really should retink your variable naming scheme. If you have to append a comment telling the purpose behind every declaration, you’re clearly doing something wrong. Why not naming the variables, e.g. firstInputNumber, secondInputNumber, operator and result in the first place? Then, you don’t need the comments telling the variable’s purposes.

Answer 2

I could not remove if-else but may be you can try something like this
public static void main(String[] args) {
int result = operate("+").applyAsInt(10, 20);
System.out.println(result);
}
private static IntBinaryOperator operate(String op){
switch(op){
case "+": return Math::addExact;
case "-": return Math::subtractExact;
//other cases
}
throw new RuntimeException("incorrect operator");
}
One issue though, Math do not have methods for double

Answer 3

tl;dr
You have to explicitly state that "+" means addition.
Long Anwser
When the user types something when your program runs, all of it is just characters. It's up to your program to interpret those characters to do something meaningful. Scanner can be helpful with this because it knows how to interpret the characters 1234 as an int. However, beyond data for the built-in types, Scanner is completely dumb.
In building your own calculator program, you are basically writing a simplified version of the Java compiler. The compiler is a program which reads characters from a file and converts them into something that can execute on a Java Virtual Machine. You will do much of the same thing in your own calculator program. This means that you have to explicitly write the rules that interpret characters that represent mathematical operators into the operations that they actually perform.
If you are interested in learning more about this topic, you can do some research on compilers and interpreters. They are generally made of two pieces: the lexical analyzer, or lexer, and the parser. This is all especially necessary if you want to allow the user to type in a complete mathematical expression like 2 + 3 rather than prompting them for each piece of the expression individually.
I hope this gives you some terminology that you can use in a Google search.