NullPointerException in Java

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Ask any Java developer about the most common exception they've encountered, and the NullPointerException (often abbreviated as NPE) is likely to top the list. In this article, we will learn what is NullPointerException, Why does it occur, practical example, and how to avoid and handle NullPointerException.

What is NullPointerException? 

A NullPointerException is a runtime exception thrown by the JVM when the application code tries to access or modify a member (e.g., call a method or access a property) of a null object. 

Why does it occur? 

Uninitialized objects: If an object is declared but not initialized, it defaults to null. Accessing its members will lead to NPE. 

Default values: In data structures (like arrays), uninitialized members default to null for reference types. 

Return values: Sometimes, methods might inadvertently return null, which, if not handled, can propagate and lead to NPEs in the calling code. 

External data sources: Data fetched from databases, APIs, or files might contain unexpected null values. 

NullPointerException Class Diagram

Java NullPointerException Example

In the below example, the person object is null and we are invoking its fields on null object leads to NullPointerException.

package com.javaguides.corejava;

public class NullPointerExceptionExample {

    public static void main(String[] args) {

        Person personObj = null;
        try {
            String name = personObj.personName; // Accessing the field of a null object
            personObj.personName = "Ramesh Fadatare"; // Modifying the field of a null object
        } catch (NullPointerException e) {
            System.err.println("NullPointerException caught!");
            e.printStackTrace();
        }

    }
}

class Person {

    public String personName;

    public String getPersonName() {
        return personName;
    }

    public void setPersonName(String personName) {
        this.personName = personName;
    }
}

Output:

NullPointerException caught!
java.lang.NullPointerException
 at com.javaguides.corejava.NullPointerExceptionExample.main(NullPointerExceptionExample.java:9)

How to Avoid and Handle NullPointerException 

Use Optional: Introduced in Java 8, the Optional class allows you to represent a value that might be absent without resorting to null. 

Null Checks: Before accessing members, ensure the object isn't null. 

Objects utility class: Java 7 introduced Objects.requireNonNull() which throws an NPE with a custom message, aiding in faster debugging. 

Initialize wisely: Ensure objects are initialized to meaningful defaults. 

Best Practices 

• Always validate external data for null values. 
• Favor returning empty collections (like Collections.emptyList()) or optional over null. 
• Make use of Java's enhanced switch statement (from Java 14 onwards) which can handle null values.
• Clearly document if your methods return null or expect null parameters. 

Conclusion 

In this article, we have learned what is NullPointerException, Why does it occur, practical example, and how to avoid and handle NullPointerException. Happy Coding!

Related Exceptions Posts

Java built-in checked exceptions:

• FileNotFoundException
• EOFException
• ClassNotFoundException
• SQLException
• NoSuchMethodException
• InterruptedException

Java built-in unchecked exceptions:

• NullPointerException
• ArrayIndexOutOfBoundsException
• StringIndexOutOfBoundsException
• ArithmeticException
• IllegalArgumentException
• NumberFormatException
• IllegalStateException
• ClassCastException

Java built-in errors:

• OutOfMemoryError
• StackOverflowError
• NoClassDefFoundError

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