How to create a variable that contains an Operator in Java?

Question

I'm trying to write a program in 30 lines or less for my class (self imposed challenge).

The program asks the user a simple addition, division, multiplication, or subtraction question and the player answers, rinse and repeat 10 times, then the player is asked if they want to continue or end the program. The type of question (add, mult, etc.) should be selected randomly.

So that I don't need to use a huge switch case or if-else tree, I want to know if there is any way to contain an operator in a variable, and then use it later.

Example:

var operator = +;
int[] n = {1, 2};
System.out.println(n[0] operator n[1]);

And the output would be "3"

That's an example of what kinda thing I want to do. Is this possible?

Answer 1

It's not possible to assign an operator to a variable.

in 30 lines or less ... the program asks the user a simple addition, division, multiplication, or subtraction

If you want to implement it using as fewer lines as possible, build-in functional interfaces will be a good choice. For that, you need IntBinaryOperator that represents an operation done on two int arguments.

Functional interface can be implemented either by using a lambda expression or a method reference (also, you can do that with an anonymous inner class as well by it'll not be shorter). Addition operation can be represented like that:

IntBinaryOperator add = Integer::sum; // or (i1, i2) -> i1 + i2

The type of question (add, mult, etc.) should be selected randomly

For that, firstly, you need to define a Random object. In order to obtain a random integer in the given range, use nextInt() method that expects an int bound, and returns a value from 0 up to the bound (exclusive):

rand.nextInt(RANGE)

To avoid hard-coding, RANGE should be defined as a global constant.

Because your application has to interact with the user, every operation should be associated with a name that will be exposed to the user.

It can be done by declaring a record (a special kind class with final field and auto-generated constructor, getters, hashCode/equals, toString()). Syntax for declaring records is very concise:

public record Operation(String name, IntBinaryOperator operation) {}

Records representing arithmetical operations can be stored in a list. And you can pick an operation by generating a random index (from 0 up to list size).

operations.get(rand.nextInt(operations.size()))

Unlike the common getters, names of getters that will be generated by the compiler for the record will identical to names of its fields, i.e. name() and operation().

In order to use the function retrieved from the record, you need to invoke the method applyAsInt() on it, passing the two previously generated numbers.

That's how it might look like.

public class Operations {
    public static final int RANGE = 100;
    public static final Random rand = new Random();
    
    public record Operation(String name, IntBinaryOperator operation) {}
    public static final List<Operation> operations =
            List.of(new Operation("add", Integer::sum), new Operation("sub", (i1, i2) -> i1 - i2),
                    new Operation("mult", (i1, i2) -> i1 * i2), new Operation("div", (i1, i2) -> i1 / i2));

    public static void main(String[] args) {
        // you code (instansiate a scanner, enclose the code below with a while loop) 
        for (int i = 0; i < 10; i++) {
            Operation oper = operations.get(rand.nextInt(operations.size()));
            int operand1 = rand.nextInt(RANGE);
            int operand2 = rand.nextInt(RANGE);
            System.out.println(operand1 + " " + oper.name() + " " + operand2); // exposing generated data to the user
            int userInput = sc.nextInt();                                      // reading user's input
            int result = oper.operation().applyAsInt(operand1,operand2);       // exposing the result
            System.out.println(result + "\n__________________");
        }
        // termination condition of the while loop
    }
}

That's an example of the output the user will see:

38 add 67
105 // user input
105
_____________________
97 sub 15
...