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What is a NullPointerException, and how do I fix it?

What is a NullPointerException, and how do I fix it?

Gareth D.

The ProblemJump To Solution

You run your Java code and see an error similar to the following.

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Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "myString" is null

How do you fix it and prevent similar errors from happening in future?

The Solution

A NullPointerException in Java is one of the most common errors. It means that you are trying to access a part of something that doesn’t exist.

For example, in the code below we call .length() on myString, which would usually return the length of the string. In this case, the string doesn’t exist (we set it to null), and so this throws a NullPointerException.

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public class Main { public static void main(String[] args) { String myString = null; int stringLength = myString.length(); } }

You can fix this by ensuring that your object is not null before calling the method. For example:

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public class Main { public static void main(String[] args) { String myString = null; if (myString != null) { int stringLength = myString.length(); } else { System.out.println("myString is null"); } } }

Here we first ensure that myString is not null before calling length().

If it is, we print “myString is null” instead.

Of course, in real-world code it’s not normally as simple to identify which variables might be null, as they might be passed in from other methods or classes, or be dependent on user input.

You can use some defensive programming techniques to avoid NullPointerExceptions such as always validating user input, and always checking if objects are null before calling their methods.

Sometimes it is also helpful to think about which variables might be null. If you are checking a user input variable against a constant, it is better to call the method from the constant instead of from the variable.

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public class Main { public static void main(String[] args) { String userInput = null; System.out.println(userInput.equals("MY_VERY_SECRET_PASSWORD")); } }

This code throws a NullPointerException, and if we’re not sure ahead of time what userInput will be, we need to handle the case where it is null. We could add a null check as in the previous example, but a simpler way is to restructure the code as follows.

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public class Main { public static void main(String[] args) { String userInput = null; System.out.println("MY_VERY_SECRET_PASSWORD".equals(userInput)); } }

Output:

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false

In this case, we know that “MY_VERY_SECRET_PASSWORD” is not going to be null, as it is hardcoded, but we are not sure about userInput. Because a.equals(b) is the same as b.equals(a), we can simply swap the variables and operate on the one we know exists.

Further reading

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