Java - Null Pointer Exception

In Java 14, a new feature was introduced as helpful NullPointerException. It was a preview feature then and later became the standard part of the JDK. The motivation behind this enhancement was to enrich the NullPointerException with more details as a traditional NullPointerException only gave details of filename, method, and the line number where a NullPointerException occurred.

Traditional Null Pointer Exception in Java

Before Java 14 NullPointerException enhancement, following code will show a NullPointerException message as shown below:

// Create an employee without department
Employee emp = new Employee(1,"Robert",null);

// get the name of the dept
// as name is null, this code will throw a null pointer exception
String dept = emp.getDept().getName();

If this code snippet is used to run, this will produce similar result −

Exception in thread "main" java.lang.NullPointerException
	at com.tutorialspoint.Tester.main(Tester.java:10)

New Informative Null Pointer Exception in Java

During debugging, such a error message is not useful. And with multiple nesting level, it becomes more difficult to check which field being null is causing the issue. Java 14 catered to this need. It provided an option to return a more helpful message like below in such case:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.tutorialspoint.Department.getName()" because the return value of "com.tutorialspoint.Employee.getDept()" is null
	at com.tutorialspoint.Tester.main(Tester.java:10)

Here we can see, that a more information message is getting printed to assist in debugging the issue that getDept() is null. In order to enable this feature in Java 14, we've to run the java code using following flag.

-XX:+ShowCodeDetailsInExceptionMessages

Now with Java 20, this flag is not needed anymore.

Helpful NullPointerException for Null Objects

In this example, we're showcasing the above learnt concept. It is a very useful feature to debug the issues caused due to null references. Here we've created two classes Employee and Department. In main method, we've created the object of Employee class but department is passed as null. Then we've tried to get the department name of the employee which results in a NullPointerException. In output we can see the informative null pointer exception raised by the JVM.

Example

package com.tutorialspoint;

import java.util.List;

public class Tester {

   public static void main(String[] args) {
      // declare an employee without department
      Employee emp = new Employee(1,"Robert",null);

      // get department name of null department will
      // throw a NullPointerException
      String dept = emp.getDept().getName();
      System.out.println(dept);
   }  
}

class Employee {
   int id;
   String name;
   Department dept;

   Employee (int id, String name, Department dept){
      this.id = id;
      this.name = name;
      this.dept = dept;
   }
   public int getId() {
      return id;
   }
   public void setId(int id) {
      this.id = id;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
   public Department getDept() {
      return dept;
   }
   public void setDept(Department dept) {
      this.dept = dept;
   }
}

class Department {
   int id;
   String name;
   List<Employee> employees;
   public int getId() {
      return id;
   }
   public void setId(int id) {
      this.id = id;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
   public List<Employee> getEmployees() {
      return employees;
   }
   public void setEmployees(List<Employee> employees) {
      this.employees = employees;
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.tutorialspoint.Department.getName()" because the return value of "com.tutorialspoint.Employee.getDept()" is null
	at com.tutorialspoint.Tester.main(Tester.java:10)

Informative NullPointerException - Local Null Variable

Similar to object reference, local variable can be null. In Java 14, if a null local variable is used, then JVM prints the name of variable as <local> as shown below:

String name = null;   
System.out.println("Length: " + name.length() );

If this code snippet is used to run, this will produce similar result −

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "<local1>" is null
	at com.tutorialspoint.Tester.main(Tester.java:8)

In order to print the name of the local variable, we had to use following flag while running the program.

-g

Using this flag results in following output.

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "name" is null
	at com.tutorialspoint.Tester.main(Tester.java:8)

In Java 20 onwards, this flag is not needed anymore. See the complete example below:

Helpful NullPointerException for Null Local Variable

In this example, we're showcasing the above learnt concept to debug null local references. Here we've created a local variable name and we're trying to the get the length of null string variable which will cause the NullPointerException. In output we can see the informative null pointer exception raised by the JVM with the actual name of the null variable.

Example

package com.tutorialspoint;

public class Tester {

   public static void main(String[] args) {	   
	   String name = null;
       // accessing property of null local variable
       // will cause a NullPointerException
	   System.out.println("Length: " + name.length() );
   }  
}

Output

Let us compile and run the above program, this will produce the following result −

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "name" is null
	at com.tutorialspoint.Tester.main(Tester.java:8)