Java NullPointerException: Understanding and Identifying Causes

Any code fragment that attempts to call a method or access a property of the variable 's' will cause a NullPointerException to be thrown because 's' has not been initialized to reference any object. For example:

1. 's'.length();
2. int length = 's'.length();
3. if ('s'.equals('hello')) { // do something }
4. int hashCode = 's'.hashCode();