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:
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.
We can avoid NullPointerException by calling equals on literal rather than object.
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.
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:
The message variable will be empty if str’s reference is null as in case 1. Otherwise, if str point to actual data, the message will retrieve the first 6 characters of it as in case 2.
Related Article – Interesting facts about Null in Java
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