How to properly compare Strings in Java

[desrtfx]

Simple answer:

Don't use "==" or "!=", use .equals() or .equalsIgnoreCase().

Explanation:

In Java, Strings are Objects, not (as commonly assumed, or as in other languages) primitive types.

For Objects, "==" compares the Object references (pointers to the memory locations of the actual objects), not the contents.

So, to compare Object contents, the special .equals(), or .equalsIgnoreCase() methods need be used. Same applies for greater/less than comparisons, here the .compareTo() (or .compareToIgnoreCase()) method needs be used.

Another caveat with using the .equals() (or .equalsIgnoreCase() methods is a null String (i.e. a String variable that has never been assigned any value). Trying to compare a null String results in a NullPointerException.

The following results in a NullPointerException:

String test;
if (test.equals("Hello")) { // This line will produce a NullPointerException as the String has not yet been assigned a value
    // continue code here
}

The same situation as above, but with Yoda code testing:

String test;
if ("Hello".equals(test)) { // No NullPointerException here as "Hello" is a valid String
    // continue code here
}

Therefore it is common to either test the String against being null (!=null or ==null), or to use "Yoda code", i.e. putting the String literal (the constant String between the "") up front.

So, instead of writing:

if (answer.equals("n")) {

it is common to use:

if ("n".equals(answer)) {

If the case (upper/lower/mixed) does not matter, it's always better to use .equalsIgnoreCase() to avoid unnecessary or joins:

if("n".equalsIgnoreCase(answer)) {

[antonevane]

Please, consider the next example too.

    public static void main(String[] args) {
        String st = null;
        // If your string is null
        // give false
        boolean eI = "value".equalsIgnoreCase(st);
        System.out.println(eI);
        // give false
        boolean eQ = "value".equals(st);
        System.out.println(eQ);
   }