Question

which if block is better?

private static void checkStr(String str) {

    if (str.equals("exit")) {
        System.out.println("It is equal");
    }
    if ("exit".equals(str)) {
        System.out.println("It is equal");
    }
}

Answer 1

With the first approach you are giving margin to NullPointerExceptions. So from my point of view the second one is better.

For the case of constants you should prefer always the second approach over the first one.

"exit".equals(str)

You can find a detailed explanation here

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I personally prefer another approach. Apache Commons – commons lang have ObjectUtils.equals(Object, Object). With this approach you are always safe even with null values.

From documentation:

Compares two objects for equality, where either one or both objects
may be null.

 ObjectUtils.equals(null, null)                  = true
 ObjectUtils.equals(null, "")                    = false
 ObjectUtils.equals("", null)                    = false
 ObjectUtils.equals("", "")                      = true
 ObjectUtils.equals(Boolean.TRUE, null)          = false
 ObjectUtils.equals(Boolean.TRUE, "true")        = false
 ObjectUtils.equals(Boolean.TRUE, Boolean.TRUE)  = true
 ObjectUtils.equals(Boolean.TRUE, Boolean.FALSE) = false