A `NullPointerException` typically occurs when you are attempting to access a method or field of an object which has not been initialized (i.e. it is null). In Java, objects have default value of null if they are not explicitly initialized. Let's go through several common causes of this exception: 1. Calling a method on a null reference. ``` String str = null; int length = str.length(); // NullPointerException ``` 2. Accessing or modifying fields of a null object. ``` class MyClass { int value; } MyClass obj = null; obj.value = 10; // NullPointerException ``` 3. Trying to access an array element at a null array reference. ``` int[] array = null; int element = array[0]; // NullPointerException ``` 4. Passing null as an argument to a method that expects an object. ``` public void myMethod(Object obj) { obj.hashCode(); // NullPointerException if obj == null } ``` To solve the problem, make sure you never dereference a null value. Always check if an object is not null before trying to call its methods or access its fields. For instance, in the first example, we could avoid the exception like: ``` if (str != null) { int length = str.length(); } ``` Also, a better practice is to handle null situations in your method body itself, for instance in the fourth example: ``` public void myMethod(Object obj) { if (obj != null) { obj.hashCode(); } else { // handle the situation when obj is null } } ``` Review your code around where the `NullPointerException` is thrown, find out what could be null and causing the issue, then apply proper null checking to prevent it. The stack trace of the exception can guide you to the exact line where the exception is thrown. If you post your code where the exception occurs, we can provide more specific help.