How to deal with NullPointerException in Java with Examples - java.lang.NullPointerException Guide

The java.lang.NullPointerException or NullPointerException in Java is probably the first error or exception you will face in Java. It is a true nightmare for beginners in Java but pretty easy to solve once you get familiar with Exception handling in Java. What makes NullPointerException a little bit tricky to solve is its name which has a pointer in itself and Java does not support pointers like C++ or C, just like you don't have multiple inheritances in Java. As a Java developer with 20 years of experience, I have solved countless amount of NullPointerException and now I have developed a coding and debugging sense through which I can spot what causing a particular NullPointerException and how to fix it. 

In this article, I am sharing that experience with you as you will learn everything about NullPointerException in Java right from what is NullPointerException in Java, How to solve Exceptions in thread "main" java.lang.NullPointerException, finding the possible cause of Java NullPointerException and how to troubleshoot NPE in Java.

Based on my experience once you know a little bit about NullPointerException it's pretty easy to solve. By the way, as said, "prevention is better than cure", you can avoid != null check and NullPointerException by following some best practices.

What is NullPointerException in Java?

NullPointerException in Java is nothing but an error or precisely an exception that occurs if we tried to perform any operation on an object which is null. 

In Java reference variable points to object created in heap but when you create a reference variable of type object by default its points to "null" and when you try to call any method on null, try to access any variable on null you will get this null pointer exception.

No matter by what reason your reference variable is pointing to null be it not initialized or set to null if you perform any operation it will throw Exception in thread main : java.lang.NullPointerException

When does NullPointerException occurs in Java?

Javadoc of java.lang.NullPointerException has outlined scenario when it could occur:

1) When you call the instance method on a null object. you won't get a null pointer exception if you call a static method or class method on the null object because the static method doesn't require an instance to call any method.

2) While accessing or changing any variable or field on a null object.

3) Throwing null when an Exception is expected to throw.

4) When getting the length of an array when the array is null.

5) Accessing or changing slots of null just like an array.

6) When you try to synchronize on a null object or using null inside the synchronized block in Java.

Now, we will see examples of NullPointerException for each of the above scenarios to get it right and understand it better. You can also see these free Java programming courses to learn more about NullPointerException in Java. 

Common Reasons of NullPointerException in Java as Example

Based upon my experience java.lang.NullPointerException repeats itself in various formats, I have collected the most common cause of java.lang.NullPointerException in java code and explained them here, we will use the following Trade class for example :

public class Trade {
   private String symbol;
   private int price;
   public static String market;

   public Trade(String symbol, int price) {
       this.symbol = symbol;
       this.price = price;
   }

   public int getPrice() {
       return price;
   }

   public void setPrice(int price) {
       this.price = price;
   }

   public String getSymbol() {
       return symbol;
   }

   public void setSymbol(String symbol) {
       this.symbol = symbol;
   }

}

1. Java  NullPointerException while calling instance method on null object

This is probably the most common cause of this error, you call method on some object and found that the reference is null, always perform null check if you see possibility of null before calling any method on object.

Trade pennyStock = null;
pennyStock.getPrice(); //this will throw NullPointerException

Exception in thread "main" java.lang.NullPointerException
at test.NullPointerExceptionTest.main(NullPointerExceptionTest.java:23)

2. NullPointerException in Java while accessing field on a null reference.

Trade fxtrade = null;
int price = fxtrade.price; //here fxtrade is null, you can’t access field here

Exception in thread "main" java.lang.NullPointerException
at test.NullPointerExceptionTest.main(NullPointerExceptionTest.java:64)

3. java.lang.NullPointerException when throwing null as an exception.

If you throw an Exception object and if that is null you will get a null pointer exception as shown in the below example

RuntimeException nullException = null;
throw nullException;

Exception in thread "main" java.lang.NullPointerException
at test.NullPointerExceptionTest.main(NullPointerExceptionTest.java:74)

4. example of NullPointerException is when getting the length of an array that is null.

Trade[] bluechips = null;
int length = bluechips.length;  //array is null here

Exception in thread "main" java.lang.NullPointerException
at test.NullPointerExceptionTest.main(NullPointerExceptionTest.java:85)

5. Example of NPE when accessing an element of a null array.

Trade[] bluechips = null;
Trade motorola = bluechips[0]; //array is null here

Exception in thread "main" java.lang.NullPointerException
at test.NullPointerExceptionTest.main(NullPointerExceptionTest.java:94)

6. You will also get NullPointerException in Java if you try to synchronize on a null object or try to use the null object inside the synchronized block in Java.

Trade highbetaTrade = null;
synchronized(highbetaTrade){
System.out.print("This statement is synchronized on null");
}

Exception in thread "main" java.lang.NullPointerException
at test.NullPointerExceptionTest.main(NullPointerExceptionTest.java:104)

How to solve NullPointerException in Java

To solve a NullPointerException in Java first we need to find the root cause, which is very easy just look at the stack-trace of NullPointerException and it will show the exact line number where NPE has occurred. 

Now go to that line and look for possible object operations like accessing the field, calling method or throwing exception etc, that will give you an idea of which object is null. Now once you found that which object is null job is half done, now find out why that object is null and solve the java.lang.NullPointerException. 

This second part always very sometimes you get null object from the factory or sometimes some other thread might have set it null, though using Assertion in early phase of development you can minimize the chances of java.lang.NullPointerException but as I said its little bit related to the environment and can come on production even if tested fine in test environment.

It's best to avoid NullPointerException by applying careful or defensive coding techniques and null-safe API methods.

When in Java Code NullPointerException doesn't come

1) When you access any static method or static variable with null reference.

If you are dealing with static variables or static methods then you won't get a null pointer exception even if you have your reference variable pointing to null because static variables and method calls are bonded during compile time based on the class name and not associated with an object. 

For example below code will run fine and not throw NullPointerException because "market" is an static variable inside Trade Class.

Trade lowBetaTrade = null;
String market = lowBetaTrade.market; //no NullPointerException market is static variable

Important points on NullPointerException in Java

1) NullPointerException is an unchecked exception because it extends RuntimeException and it doesn’t mandate try-catch block to handle it.

2) When you get NullPointerException to look at the line number to find out which object is null, it may be an object which is calling any method.

3) Modern IDE like NetBeans and Eclipse gives you the hyperlink of the line where NullPointerException occurs

4) You can set an Exception break point in Eclipse to suspend execution when NullPointerException occurs read 10 tips on java debugging in Eclipse for more details.

5) Don't forget to see the name of Thread on which NullPointerException occurs. in multi-threading, NPE can be a little tricky if some random thread is setting a reference to null.

6) It's best to avoid NullPointerException while coding by following some coding best practices or putting a null check on the database as a constraint.

That’s all on What is java.lang.NullPointerException, When it comes, and how to solve it. In the next part of this tutorial, we will look at some best java coding practices to avoid NullPointerException in Java.

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How to write production quality code in Java

Thanks for reading this article so far. If you like my tips and explanation then please share this article in social media. If you have any question or feedback or struggling with NullPointerException then please paste your error in comments and I will try to help you out.