A NullPointerException is thrown in Java when you're trying to use a reference that points to no location in memory or simply has a value of null. Common causes include: 1. **Invoking methods on an object that is null**: If you call a method on an object that is currently null, you'll get a NullPointerException. Make sure your object is properly initialized before calling any methods on it. Example: ```java String str = null; int length = str.length(); // This will throw NullPointerException ``` 2. **Accessing or modifying the fields of a null object**: Similar to the previous point, if you try to access or modify fields of a null object, a NullPointerException is thrown. Example: ```java YourClass yourObj = null; yourObj.someField = 5; // This will throw NullPointerException ``` 3. **Throwing null yourself**: You can also get a NullPointerException if you throw null yourself in your code. Example: ```java throw null; // This will throw NullPointerException ``` 4. **Null array**: If you have a null array and you are trying to access its elements, you'll get a NullPointerException. Example: ```java int[] arr = null; int i = arr[0]; // This will throw NullPointerException ``` 5. **Calling `java.util.Objects.requireNonNull()`:** If you pass a null argument to `Objects.requireNonNull()`, it throws NullPointerException. Example: ```java Objects.requireNonNull(null); // This will throw NullPointerException ``` To fix it, you need to ensure that all objects are properly initialized before you use them. You should also add appropriate null checks in your code where you've identified possible null references. Remember, null-safety is a significant aspect of writing robust and error-free code. The best way to handle NullPointerException is to prevent them by using good coding practices and proper exception handling.