Question

I am a beginner level of studying java and revising for my exams through answering the questions on previous past exam papers and there is one question that I am stuck on.

Consider the code fragment below that reads an input command then processes it.

String cmd = scanner.next();

if (cmd == "forward" )
    robot.forward(1);
else if (cmd == "turn" ) 
    robot.turn();
else
     System.out.println("Unknown command: " + cmd);

When testing the program the scanner reads in the String “forward” into cmd, but the program outputs “Unknown command: forward”.

a) Explain, in detail, why this occurs.

b) What changes should be made to the code to correct this error.

If someone can help me answer question a) and b) I would be grateful.

p.s. I understand that this isn’t a website to just look for answers (#noeasywayout) so I’m going to try my best not to act greedy here. My apologies for any inconvenience.

Answer 1

This old chestnut…

In java, == tests if the two operands are the exact same object, which obviously they are not (one of the objects is a String constant, the other was read from input).

Use String.equals() method to compare their value.

Try this:

if (cmd.equals("forward"))
    robot.forward(1);
else if (cmd.equals("turn")) 
    robot.turn();
else
    System.out.println("Unknown command: " + cmd);

BTW, with this pattern of code, beware calling .equals() on cmd if it is null – you’ll get a NPE. A common way to avoid this without adding any code is to use a “yoda test” (one with a “reversed” logic):

if ("forward".equals(cmd))
    robot.forward(1);
else if ("turn".equals(cmd)) 
    robot.turn();
else
    System.out.println("Unknown command: " + cmd);

This code won’t throw an NPE if cmd is null