Java NullPointerException - Detect, Fix, and Best Practices

NullPointerException is a runtime exception, so we don’t need to catch it in the program. NullPointerException is raised in an application when we are trying to do some operation on null where an object is required. Some of the common reasons for NullPointerException in java programs are:

1. Invoking a method on an object instance but at runtime the object is null.
2. Accessing variables of an object instance that is null at runtime.
3. Throwing null in the program
4. Accessing index or modifying value of an index of an array that is null
5. Checking the length of an array that is null at runtime.

Let’s look at some examples of NullPointerException in java programs.

1. NullPointerException when calling an instance method

public class Temp {

	public static void main(String[] args) {

		Temp t = initT();
		
		t.foo("Hi");
		
	}

	private static Temp initT() {
		return null;
	}

	public void foo(String s) {
		System.out.println(s.toLowerCase());
	}
}

When we run the above program, it throws the following NullPointerException error message.

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:7)

We are getting NullPointerException in the statement t.foo("Hi"); because “t” is null here.

2. Java NullPointerException while accessing/modifying field of a null object

public class Temp {

	public int x = 10;
	
	public static void main(String[] args) {

		Temp t = initT();
		
		int i = t.x;
		
	}

	private static Temp initT() {
		return null;
	}

}

The above program throws the following NullPointerException stack trace.

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:9)

NullPointerException is being thrown in statement int i = t.x; because “t” is null here.

3. Java NullPointerException when null is passed in method argument

public class Temp {

	public static void main(String[] args) {

		foo(null);

	}

	public static void foo(String s) {
		System.out.println(s.toLowerCase());
	}
}

This is one of the most common occurrences of java.lang.NullPointerException because it’s the caller who is passing the null argument.

The below image shows the null pointer exception when the above program is executed in Eclipse IDE.

4. java.lang.NullPointerException when null is thrown

public class Temp {

	public static void main(String[] args) {

		throw null;
	}

}

Below is the exception stack trace of the above program, showing NullPointerException because of throw null; statement.

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:5)

5. NullPointerException when getting the length of null array

public class Temp {

	public static void main(String[] args) {

		int[] data = null;
		
		int len = data.length;
	}

}

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:7)

6. NullPointerException when accessing index value of null array

public class Temp {

	public static void main(String[] args) {

		int[] data = null;
		
		int len = data[2];
	}

}

Exception in thread "main" java.lang.NullPointerException
	at Temp.main(Temp.java:7)

7. NullPointerException when synchronized on a null object

public class Temp {

	public static String mutex = null;
	
	public static void main(String[] args) {

		synchronized(mutex) {
			System.out.println("synchronized block");
		}		
	}
}

The synchronized(mutex) will throw NullPointerException because the “mutex” object is null.

8. HTTP Status 500 java.lang.NullPointerException

Sometimes we get an error page response from a java web application with an error message as “HTTP Status 500 – Internal Server Error” and root cause as java.lang.NullPointerException.

For this, I edited the Spring MVC Example project and changed the HomeController method as below.

@RequestMapping(value = "/user", method = RequestMethod.POST)
	public String user(@Validated User user, Model model) {
		System.out.println("User Page Requested");
		System.out.println("User Name: "+user.getUserName().toLowerCase());
		System.out.println("User ID: "+user.getUserId().toLowerCase());
		model.addAttribute("userName", user.getUserName());
		return "user";
	}

The below image shows the error message thrown by the web application response.

Here is the exception stack trace:

HTTP Status 500 – Internal Server Error

Type Exception Report

Message Request processing failed; nested exception is java.lang.NullPointerException

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982)
	org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
	org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
Root Cause

java.lang.NullPointerException
	com.journaldev.spring.controller.HomeController.user(HomeController.java:38)
	sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	java.lang.reflect.Method.invoke(Method.java:498)

The root cause is NullPointerException in the statement user.getUserId().toLowerCase() because user.getUserId() is returning null.

Detecting NullPointerException is very easy, just look at the exception trace and it will show you the class name and line number of the exception. Then look at the code and see what could be null causing the NullPointerException. Just look at all the above examples, it’s very clear from stack trace what is causing null pointer exception.

java.lang.NullPointerException is an unchecked exception, so we don’t have to catch it. The null pointer exceptions can be prevented using null checks and preventive coding techniques. Look at below code examples showing how to avoid java.lang.NullPointerException.

if(mutex ==null) mutex =""; //preventive coding
		
synchronized(mutex) {
	System.out.println("synchronized block");
}

//using null checks
if(user!=null && user.getUserName() !=null) {
System.out.println("User Name: "+user.getUserName().toLowerCase());
}
if(user!=null && user.getUserName() !=null) {
	System.out.println("User ID: "+user.getUserId().toLowerCase());
}

1. Let’s consider the below function and look out for scenario causing null pointer exception.

public void foo(String s) {
    if(s.equals("Test")) {
	System.out.println("test");
    }
}

The NullPointerException can occur if the argument is being passed as null. The same method can be written as below to avoid NullPointerException.

public void foo(String s) {
	if ("Test".equals(s)) {
		System.out.println("test");
	}
}

2. We can also add null check for argument and throw IllegalArgumentException if required.

public int getArrayLength(Object[] array) {
	
	if(array == null) throw new IllegalArgumentException("array is null");
	
	return array.length;
}

3. We can use ternary operator as shown in the below example code.

String msg = (str == null) ? "" : str.substring(0, str.length()-1);

4. Use String.valueOf() rather than toString() method. For example check PrintStream println() method code is defined as below.

public void println(Object x) {
        String s = String.valueOf(x);
        synchronized (this) {
            print(s);
            newLine();
        }
    }

The below code snippet shows the example where the valueOf() method is used instead of toString().

Object mutex = null;

//prints null
System.out.println(String.valueOf(mutex));

//will throw java.lang.NullPointerException
System.out.println(mutex.toString());

5. Write methods returning empty objects rather than null wherever possible, for example, empty list, empty string, etc.

6. There are some methods defined in collection classes to avoid NullPointerException, you should use them. For example contains(), containsKey(), and containsValue().

Reference: API Document