NullPointerException is a RuntimeException. In Java, a special null value can be assigned to an object reference. NullPointerException is thrown when program attempts to use an object reference that has the null value.
These can be:
Invoking a method from a null object.
Accessing or modifying a null object’s field.
Taking the length of null, as if it were an array.
Accessing or modifying the slots of null object, as if it were an array.
Throwing null, as if it were a Throwable value.
When you try to synchronize over a null object.
Why do we need the null value?
Null is a special value used in Java. It is mainly used to indicate that no value is assigned to a reference variable. One application of null is in implementing data structures like linked list and tree. Other applications include Null Object pattern (See this for details) and Singleton pattern. The Singleton pattern ensures that only one instance of a class is created and also, aims for providing a global point of access to the object.
A sample way to create at most one instance of a class is to declare all its constructors as private and then, create a public method that returns the unique instance of the class:
// To use randomUUID function.
import java.util.UUID;
import java.io.*;
class Singleton
{
// Initializing values of single and ID to null.
private static Singleton single = null;
private String ID = null;
private Singleton()
{
/* Make it private, in order to prevent the
creation of new instances of the Singleton
class. */
// Create a random ID
ID = UUID.randomUUID().toString();
}
public static Singleton getInstance()
{
if (single == null)
single = new Singleton();
return single;
}
public String getID()
{
return this.ID;
}
}
// Driver Code
public class TestSingleton
{
public static void main(String[] args)
{
Singleton s = Singleton.getInstance();
System.out.println(s.getID());
}
}
Output:
10099197-8c2d-4638-9371-e88c820a9af2
In above example, a static instance of the singleton class. That instance is initialized at most once inside the Singleton getInstance method.
How to avoid the NullPointerException?
To avoid the NullPointerException, we must ensure that all the objects are initialized properly, before you use them. When we declare a reference variable, we must verify that object is not null, before we request a method or a field from the objects.
Following are the common problems with the solution to overcome that problem.
Case 1 : String comparison with literals
A very common case problem involves the comparison between a String variable and a literal. The literal may be a String or an element of an Enum. Instead of invoking the method from the null object, consider invoking it from the literal.
// A Java program to demonstrate that invoking a method
// on null causes NullPointerException
import java.io.*;
class P_AI
{
public static void main (String[] args)
{
// Initializing String variable with null value
String ptr = null;
// Checking if ptr.equals null or works fine.
try
{
// This line of code throws NullPointerException
// because ptr is null
if (ptr.equals("prutor"))
System.out.print("Same");
else
System.out.print("Not Same");
}
catch(NullPointerException e)
{
System.out.print("NullPointerException Caught");
}
}
}
Output:
NullPointerException Caught
We can avoid NullPointerException by calling equals on literal rather than object.
// A Java program to demonstrate that we can avoid
// NullPointerException
import java.io.*;
class P_AI
{
public static void main (String[] args)
{
// Initialing String variable with null value
String ptr = null;
// Checking if ptr is null using try catch.
try
{
if ("prutor".equals(ptr))
System.out.print("Same");
else
System.out.print("Not Same");
}
catch(NullPointerException e)
{
System.out.print("Caught NullPointerException");
}
}
}
Output:
Not Same
Case 2 : Keeping a Check on the arguments of a method
Before executing the body of your new method, we should first check its arguments for null values and continue with execution of the method, only when the arguments are properly checked. Otherwise, it will throw an IllegalArgumentException and notify the calling method that something is wrong with the passed arguments.
// A Java program to demonstrate that we should
// check if parameters are null or not before
// using them.
import java.io.*;
class P_AI
{
public static void main (String[] args)
{
// String s set an empty string and calling getLength()
String s = "";
try
{
System.out.println(getLength(s));
}
catch(IllegalArgumentException e)
{
System.out.println("IllegalArgumentException caught");
}
// String s set to a value and calling getLength()
s = "PrutordotAi";
try
{
System.out.println(getLength(s));
}
catch(IllegalArgumentException e)
{
System.out.println("IllegalArgumentException caught");
}
// Setting s as null and calling getLength()
s = null;
try
{
System.out.println(getLength(s));
}
catch(IllegalArgumentException e)
{
System.out.println("IllegalArgumentException caught");
}
}
// Function to return length of string s. It throws
// IllegalArgumentException if s is null.
public static int getLength(String s)
{
if (s == null)
throw new IllegalArgumentException("The argument cannot be null");
return s.length();
}
}
Output:
0
13
IllegalArgumentException caught
Case 3 : Use of Ternary Operator
The ternary operator can be used to avoid NullPointerException. First, the Boolean expression is evaluated. If the expression is true then, the value1 is returned, otherwise, the value2 is returned. We can use the ternary operator for handling null pointers:
// A Java program to demonstrate that we can use
// ternary operator to avoid NullPointerException.
import java.io.*;
class P_AI
{
public static void main (String[] args)
{
// Initializing String variable with null value
String str = null;
String message = (str == null) ? "" :
str.substring(0,5);
System.out.println(message);
// Initializing String variable with null value
str = "PrutordotAi";
message = (str == null) ? "" : str.substring(0,5);
System.out.println(message);
}
}
Output:
Prutor