if statement checks for null but still throws a NullPointerException

The edit shows exactly the difference between code that works and code that doesn't.

This check always evaluates both of the conditions, throwing an exception if str is null:

 if (str == null | str.length() == 0) {

Whereas this (using || instead of |) is short-circuiting - if the first condition evaluates to true, the second is not evaluated.

See section 15.24 of the JLS for a description of ||, and section 15.22.2 for binary |. The intro to section 15.24 is the important bit though:

The conditional-or operator || operator is like | (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is false.

You can use StringUtils:

import org.apache.commons.lang3.StringUtils;

if (StringUtils.isBlank(str)) {

System.out.println("String is empty");

} else { 

System.out.println("String is not empty");

}

Have a look here also: StringUtils.isBlank() vs String.isEmpty()

isBlank examples:

StringUtils.isBlank(null)      = true
StringUtils.isBlank("")        = true  
StringUtils.isBlank(" ")       = true  
StringUtils.isBlank("bob")     = false  
StringUtils.isBlank("  bob  ") = false