reading commands from console

[merik]

I'm trying to create a program which reads a command from console. If the command is "quit" then the program ends; otherwise, it does specific things.

The code I have is here:

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		boolean unsavedWork = false;

		while (true) {
			System.out.println("Command:");
			String userChoice = in.next();
			if (userChoice == "quit") {
				if (unsavedWork) {
					//
				}
				break;
			} else if (userChoice == "load") {
				//
			} else if (userChoice == "help") {
				System.out.println("Blah Blah");
			} else if (userChoice == "info") {
				//
			} else if (userChoice == "save") {
				//
			} else {
				System.out.println("Command " + userChoice + " is not supported.");
			}
		}

		System.out.print("Bye!");
	}

However, when I run the program and type any of the commands (like "quit"), it doesn't work.

Can you please help me?

[Fubarable]

Don't use == with Strings. Instead use the equals(...) or equalsIgnoreCase(...) method.

[merik]

Thank you! Can you explain why == doesn't work with strings in Java?

[JosAH]

Thank you! Can you explain why == doesn't work with strings in Java?

The == operator compares references (pointers) for objects. Only if two references point to the same String this operator yields true; it doesn't check the character content of the Strings.

kind regards,

Jos

[m00nchile]

Because the == operator uses memory locations to compare objects.

String s1 = new String("abc");
String s2 = new String("abc");

The above two strings are equal by content, but are stored at two memory locations, so the == operator would return false when comparing them.
EDIT: beaten by Jos

[venerik]

Don't use in.next() either if you want to read the whole command and not just the first character.

Have a look at the other next... methods of the Scanner class at Scanner (Java Platform SE 6)

Cheers,
Erik

[merik]

So if == operator only compares references, why does it work for integers and other primary types?

int a = 2;
int b = 2;
a == b is true!


@venerik: Sure. This was only sample code to show the problem.

[venerik]

Primary types do not have references.

[Fubarable]

So if == operator only compares references, why does it work for integers and other primary types?

int a = 2;
int b = 2;
a == b is true!

And the tricky thing is == will usually work for Strings as Strings are stored in something called the String pool, so Strings with the same character content often reference the same object, but this is not guaranteed!

[merik]

Thank you for the explanation.