Q:

Simple calculator is not printing the answer? 

My program's not working.  What do you think is wrong? 
Scanner in = new Scanner(System.in);
    System.out.print("Enter first number: ");
    double num1 = in.nextDouble();
    System.out.print("Enter second number: ");
    double num2 = in.nextDouble();
    System.out.println("Enter operation to perform: ");
    String oper = in.next();

    if(oper == "add" || oper == "addition" || oper == "+") {
        double sum = num1 + num2;
        System.out.printf("The sum of the two numbers is %d", sum);
    }

When I type the operation(which is a String), program terminates.  Output:
Enter first number: 12
Enter second number: 8
Enter operation to perform: 
"add"

Process completed. 

I can't seem to find the error, please help? 

A:

Never compare strings with operator == - it is rough mistake.  Use equals instead:
if(oper.equals("add") || oper.equals("addition") || oper.equals("+")) {

