while (num2 == null) {
System.out.print("Enter another number: ");
num2 = getNumber(stratScan.next());
}
Context ctx = new Context(new OperationAdd());
System.out.println(num1 + " + " + num2 + " = " + ctx.executeStrategy(num1, num2));
ctx = new Context(new OperationSubtract());
System.out.println(num1 + " - " + num2 + " = " + ctx.executeStrategy(num1, num2));