Comparing strings

Module 6 showed that text is compared with equals. Here's the rule once more, with the reason behind it. == asks whether two variables refer to one and the same String object in memory, while equals compares the characters:

answer == "yes"         // wrong: are they the very same object?
answer.equals("yes")    // right: do they have the same characters?

What makes == tricky is that it sometimes seems to work. Java may reuse one String object for equal texts written in the code, and whether two strings end up as the same object depends on where they came from and on Java's own internals. So a program that compares text with == can pass a quick test and then fail on real input. With equals, the result depends only on the text. (For int and char values, which aren't objects, == is the right tool.)

Ignoring capitals

equals counts capitals: "Yes".equals("yes") is false. When capitals shouldn't matter, use equalsIgnoreCase:

Scanner scanner = new Scanner(System.in);
System.out.println("Play again? (yes/no)");
String answer = scanner.nextLine();
if (answer.equalsIgnoreCase("yes")) {
    System.out.println("New game!");
} else {
    System.out.println("Thanks for playing");
}

YES

Play again? (yes/no)
New game!

Two more tools go well with comparing:

• ! in front turns the result around: if (!answer.equals("no")) runs its block for every answer except no.
• People sometimes type a space before or after a word without noticing. answer.trim().equalsIgnoreCase("yes") trims first and then compares, so " Yes " counts too.

A method can return the result of a comparison as a boolean:

public class Main {
    public static void main(String[] args) {
        System.out.println(isWeekend("Sunday"));
        System.out.println(isWeekend("monday"));
        System.out.println(isWeekend("SATURDAY"));
    }

    public static boolean isWeekend(String day) {
        return day.equalsIgnoreCase("saturday") || day.equalsIgnoreCase("sunday");
    }
}

true
false
true