In this article, we will discuss top exception and error in java with their description & examples

Top Exception in Java:

1. Null Pointer Exception

Exception Name :	NullPointerException
Fully qualified name :	java.lang.NullPointerException
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.RuntimeException
JVM raises NullPointerException, whenever operation is performed on null reference
For example, if we have an un-initialized String variable then trying to check or get its length using length() method throws/raises NullPointerException
See below example for reference

StringOperationForCheckingLength.java

package in.bench.resources.top.exception.in.java;

public class StringOperationForCheckingLength {

	public static void main(String[] args) {

		// string variable
		String str = null;

		// checking length
		int len = str.length();

		// printing to console
		System.out.println("Length of str is : " + len);
	}
}

Output:

Exception in thread "main" java.lang.NullPointerException
	at in.bench.resources.top.exception.in.java
	.StringOperationForCheckingLength.main(
			StringOperationForCheckingLength.java:11)

2. Array Index Out Of Bounds Exception

Exception Name :	ArrayIndexOutOfBoundsException
Fully qualified name :	java.lang.ArrayIndexOutOfBoundsException
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.IndexOutOfBoundsException, which is sub-class of java.lang.RuntimeException
JVM raises ArrayIndexOutOfBoundsException, whenever we are trying to perform operation like accessing or assigning an array element with out-of-range of index
That’s index is greater than length of an array or index with negative value
For example, if original length of an array is 5 then trying to assign/access a value at 7^th index-position, raises/throws ArrayIndexOutOfBoundsException
See below example for reference

ArrayOperationOnInvalidIndex.java

package in.bench.resources.top.exception.in.java;

public class ArrayOperationOnInvalidIndex {

	public static void main(String[] args) {

		// character array of length 5
		char[] ch = new char[5];

		// assigning value at 7th index-psotion
		ch[7] = 'B';

		// printing to console
		System.out.println("The value at 7th index-position is : " + ch[7]);
	}
}

Output:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 7
	at in.bench.resources.top.exception.in.java
	.ArrayOperationOnInvalidIndex.main(
			ArrayOperationOnInvalidIndex.java:11)

3. Arithmetic Exception

Exception Name :	ArithmeticException
Fully qualified name :	java.lang.ArithmeticException
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.RuntimeException
JVM raises ArithmeticException, whenever we perform invalid arithmetic operation like divide by zero, which throws ArithmeticException with “divide by zero” as exception description
For example, performing a division with divisor as zero, throws/raises ArithmeticException as stated above
See below example for reference

DivideByZeroExample.java

package in.bench.resources.top.exception.in.java;

public class DivideByZeroExample {

	public static void main(String[] args) {

		// performing division
		int quotient = 19/0;

		// printing to console
		System.out.println("The quotient after division is : " + quotient);
	}
}

Output:

Exception in thread "main" java.lang.ArithmeticException: / by zero
	at in.bench.resources.top.exception.in.java.DivideByZeroExample
	.main(DivideByZeroExample.java:8)

4. Class Cast Exception

Exception Name :	ClassCastException
Fully qualified name :	java.lang.ClassCastException
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.RuntimeException
JVM raises ClassCastException, whenever we are trying to do invalid type-casting
That’s type-casting from super-class type to sub-class type, throws/raises ClassCastException at runtime (although compilation succeeds)
In simple words, when RHS Object doesn’t satisfy “IS-A” relationship with LHS, then this exception is raised
For example, assigning Object-type to String-type results in type-casting exception at runtime
See below example for reference

ClassCastExceptionExample.java

package in.bench.resources.top.exception.in.java;

public class ClassCastExceptionExample {

	public static void main(String[] args) {

		// Object-type
		Object object = new Object();

		// assigning Object-type to String-type
		String str = (String) object;

		// printing to console
		System.out.println("String is : " + str);
	}
}

Output:

Exception in thread "main" java.lang.ClassCastException:
java.lang.Object cannot be cast to java.lang.String
	at in.bench.resources.top.exception.in.java.ClassCastExceptionExample
	.main(ClassCastExceptionExample.java:11)

5. Illegal Argument Exception

Exception Name :	IllegalArgumentException
Fully qualified name :	java.lang.IllegalArgumentException
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.RuntimeException
IllegalArgumentException is raised because of passing wrong/invalid argument to a method
For example, setting Thread priority to more than integer value 10 or negative value results in this type of exception
Because valid range for Thread priority is 1 to 10
See below example for reference

InvalidArgumentForThreadPriority.java

package in.bench.resources.top.exception.in.java;

public class InvalidArgumentForThreadPriority {

	public static void main(String[] args) {

		// create a new Thread
		Thread t = new Thread();

		// set wrong/invalid priority
		t.setPriority(11);
	}
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException
	at java.lang.Thread.setPriority(Unknown Source)
	at in.bench.resources.top.exception.in.java
	.InvalidArgumentForThreadPriority
	.main(InvalidArgumentForThreadPriority.java:11)

6. Number format Exception

Exception Name :	NumberFormatException
Fully qualified name :	java.lang.NumberFormatException
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.IllegalArgumentException, which is child-class of java.lang.RuntimeException
NumberFormatException is raised when we try to convert string to numeric value like integer, float, double, etc and string passed isn’t properly formatted
For example, if we pass numeric 10 as string to parseInt() method of Integer class, then it convert into numeric value 10
Whereas trying to convert string value of “ben” to integer, results in NumberFormatException
See below example for reference

ParseStringToInteger.java

package in.bench.resources.top.exception.in.java;

public class ParseStringToInteger {

	public static void main(String[] args) {

		// VALID - trying to convert string into integer value
		int resultValid = Integer.parseInt("10");
		System.out.println("The Value is : " + resultValid);

		// INVALID - trying to convert string into integer value
		int resultInvalid = Integer.parseInt("ben");
		System.out.println("The Value is : " + resultInvalid);
	}
}

Output:

The Value is : 10
Exception in thread "main" java.lang.NumberFormatException:
	For input string: "ben"
	at java.lang.NumberFormatException.forInputString(Unknown Source)
	at java.lang.Integer.parseInt(Unknown Source)
	at java.lang.Integer.parseInt(Unknown Source)
	at in.bench.resources.top.exception.in.java.ParseStringToInteger
	.main(ParseStringToInteger.java:12)

7. Illegal Thread State Exception

Exception Name :	IllegalThreadStateException
Fully qualified name :	java.lang.IllegalThreadStateException
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.IllegalArgumentException, which is child-class of java.lang.RuntimeException
IllegalThreadStateException is raised when we try to start the Thread again, when it is already in started state
For example, invoking start() method of Thread 2^nd time results in IllegalThreadStateException
See below example for reference

StartingThreadAgain.java

package in.bench.resources.top.exception.in.java;

public class StartingThreadAgain {

	public static void main(String[] args) {

		// create a new Thread
		Thread t = new Thread();

		// 1st time - starting Thread
		t.start();

		// 2nd time - starting Thread AGAIN
		t.start();
	}
}

Output:

Exception in thread "main" java.lang.IllegalThreadStateException
	at java.lang.Thread.start(Unknown Source)
	at in.bench.resources.top.exception.in.java.StartingThreadAgain
	.main(StartingThreadAgain.java:14)

8. Interrupted Exception

Exception Name :	InterruptedException
Fully qualified name :	java.lang.InterruptedException
Exception Type :	Checked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.Exception
Generally, this exception-type is encountered whenever we are working with threading or multi-threading programming
Being checked exception, it need to be handled either by try-catch block or declare exception using throws clause
Sleep() & join() method of Thread class & wait() method of Object class throw this exception
For example, invoking interrupt on a Thread which is in sleeping/waiting state results in throwing this exception-type
See below example for reference

InterruptingThread.java

package in.bench.resources.top.exception.in.java;

public class InterruptingThread {

	public static void main(String[] args) {

		// create a new Thread
		Thread t = new Thread(){

			public void run() {

				try {
					Thread.sleep(5000);
				}
				catch (InterruptedException iex) {
					iex.printStackTrace();
				}
			}
		};

		// start a Thread
		t.start();

		// interrupt a Thread
		t.interrupt();
	}
}

Output:

java.lang.InterruptedException: sleep interrupted
	at java.lang.Thread.sleep(Native Method)
	at in.bench.resources.top.exception.in.java.InterruptingThread$1
	.run(InterruptingThread.java:13)

9. File Not Found Exception

Exception Name :	FileNotFoundException
Fully qualified name :	java.io.FileNotFoundException
Exception Type :	Checked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.io.IOException
Whenever we are working with file, then it is must to handle FileNotFoundException
Because sometime it is possible that accessing file location isn’t available or else file itself not available at the specified location
For example, trying to access a file from specified location but file isn’t available at that location
See below example for reference

FileProcessing.java

package in.bench.resources.top.exception.in.java;

import java.io.FileNotFoundException;
import java.io.FileReader;

public class FileProcessing {

	public static void main(String[] args) throws FileNotFoundException {

		// performing IO operation
		// assumed that, we are trying to access file from remote location
		FileReader fileReader = new FileReader("D:/Folder/test.txt");
	}
}

Output:

Exception in thread "main" java.io.FileNotFoundException:
D:\Folder\test.txt (The system cannot find the path specified)
	at java.io.FileInputStream.open(Native Method)
	at java.io.FileInputStream.<init>(Unknown Source)
	at java.io.FileInputStream.<init>(Unknown Source)
	at java.io.FileReader.<init>(Unknown Source)
	at in.bench.resources.top.exception.in.java.FileProcessing
	.main(FileProcessing.java:12)

10. SQL Exception

Exception Name :	SQLException
Fully qualified name :	java.lang.SQLException
Exception Type :	Checked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.io.IOException
After loading driver file for respective database, whenever we perform any operation with database and supplied credentials or database table/columns is incorrect
Then this exception-type is raised
For example, passing wrong database URL for a non-existent database results in rising SQLException
See below example for reference

JdbcConnectionExample.java

package in.bench.resources.top.exception.in.java;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class JdbcConnectionExample {

	public static void main(String[] args) {

		// declare variables
		Connection connection = null;
		Statement statement = null;
		ResultSet resultSet = null;

		// Step 1: Loading or
		// registering Oracle JDBC driver class
		try {
			Class.forName("oracle.jdbc.driver.OracleDriver");
		}
		catch(ClassNotFoundException cnfex) {
			System.out.println(
					"Problem in loading Oracle JDBC driver");
			cnfex.printStackTrace();
		}

		// Step 2: Opening database connection
		try {

			// Step 2.A: Create and
			// get connection using DriverManager class
			connection = DriverManager.getConnection(
					"jdbc:oracle:thin:@localhost:1521:xe",
					"scott",
					"tiger"); 

			// Step 2.B: Creating JDBC Statement
			statement = connection.createStatement();

			// Step 2.C: Executing SQL and
			// retrieve data into ResultSet
		}
		catch(SQLException sqlex){
			sqlex.printStackTrace();
		}
		finally {

			// Step 3: Closing database connection
		}
	}
}

Output:

java.sql.SQLException: Io exception:
	The Network Adapter could not establish the connection
	at oracle.jdbc.driver.DatabaseError.throwSqlException(
			DatabaseError.java:112)
	at oracle.jdbc.driver.DatabaseError.throwSqlException(
			DatabaseError.java:146)
	at oracle.jdbc.driver.DatabaseError.throwSqlException(
			DatabaseError.java:255)
	at oracle.jdbc.driver.T4CConnection.logon(
			T4CConnection.java:387)
	at oracle.jdbc.driver.PhysicalConnection.<init>(
			PhysicalConnection.java:414)
	at oracle.jdbc.driver.T4CConnection.<init>(
			T4CConnection.java:165)
	at oracle.jdbc.driver.T4CDriverExtension.getConnection(
			T4CDriverExtension.java:35)
	at oracle.jdbc.driver.OracleDriver.connect(
			OracleDriver.java:801)
	at java.sql.DriverManager.getConnection(
			Unknown Source)
	at java.sql.DriverManager.getConnection(
			Unknown Source)
	at in.bench.resources.top.exception.in.java.JdbcConnectionExample
	.main(JdbcConnectionExample.java:31)

11. Class Not Found Exception

Exception Name :	ClassNotFoundException
Fully qualified name :	java.lang.ClassNotFoundException
Exception Type :	Checked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.Exception
Generally this exception raised, when we try to execute a program and required .class files is missing from class-path
For example, trying to load driver file for respective databases but it isn’t available at runtime
Possible reason for this type of exception is, executing a JDBC program without updating class-path with required JAR files
See below example for reference

JdbcConnectionExample.java

package in.bench.resources.top.exception.in.java;

public class JdbcConnectionExample {

	public static void main(String[] args) {

		// declare variables

		// Step 1: Loading or registering Oracle JDBC driver class
		try {
			Class.forName("oracle.jdbc.driver.OracleDriver");
		}
		catch(ClassNotFoundException cnfex) {
			System.out.println("Problem in loading Oracle JDBC driver");
			cnfex.printStackTrace();
		}

		// Step 2: Opening database connection
	}
}

Output:

java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver
Problem in loading Oracle JDBC driver
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Unknown Source)
	at in.bench.resources.top.exception.in.java.JdbcConnectionExample
.main(JdbcConnectionExample.java:11)

Top Error in Java:

12. No Class Def Found Error

Exception Name :	NoClassDefFoundError
Fully qualified name :	java.lang.NoClassDefFoundError
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.LinkageError, which is child-class of java.lang.Error
JVM raises NoClassDefFoundError, whenever we try to execute a program and required .class files is missing from class-path
Possible reason for this type of exception is, required file is present during compilation but missing while executing the same program
For example, below program exhibits “HAS-A” relationship & compilation succeeds whereas during program execution JVM unable to find required .class file
See below example for reference

SimilarException.java

package in.bench.resources.top.exception.in.java;

public class SimilarException {

	// using below declared TestFile class
	static TestFile tf = new TestFile();

	public static void main(String[] args) {

		// invoke method
		tf.display();
	}
}

class TestFile {

	public void display() {
		System.out.println("Display message");
	}
}

Output:

java.lang.NoClassDefFoundError:
	in/bench/resources/top/exception/in/java/TestFile
	at in.bench.resources.top.exception.in.java.SimilarException.<clinit>(
			SimilarException.java:6)
Caused by: java.lang.ClassNotFoundException:
	in.bench.resources.top.exception.in.java.TestFile
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	... 1 more
Exception in thread "main"

Q) Difference between ClassNotFoundException and NoClassDefFoundError ?

ClassNotFoundException occurs, when we try to execute a program and required .class file isn’t available at runtime. Also, there won’t be compile-time error as we are trying to load class using Class.forName(“fully.qualified.name”);
NoClassDefFoundError occurs, when we try to execute a program and required .class file isn’t available at runtime. Because required file was available during program compilation but missing while executing (for some reasons)
Read ClassNotFoundException v/s NoClassDefFoundError for more details

13. Exception in Initializer Error

Exception Name :	ExceptionInInitializerError
Fully qualified name :	java.lang.ExceptionInInitializerError
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.LinkageError, which is child-class of java.lang.Error
JVM raises ExceptionInInitializerError, whenever we try perform some assignment operation in static variable or inside static blocks
For example, printing length of a string variable inside static block where string variable is referencing to null reference, results in throwing ExceptionInInitializerError
See below example for reference

ExceptionErrorDemo.java

package in.bench.resources.top.exception.in.java;

public class ExceptionErrorDemo {

	// static block
	static {

		String str = null;
		System.out.println("The length of string is : " + str.length());
	}

	// main() method - JVM execution start here
	public static void main(String[] args) {
	}
}

Output:

java.lang.ExceptionInInitializerError
Caused by: java.lang.NullPointerException
	at in.bench.resources.top.exception.in.java.ExceptionErrorDemo.<clinit>(
			ExceptionErrorDemo.java:9)
Exception in thread "main"

14. Stack Overflow Error

Exception Name :	StackOverflowError
Fully qualified name :	java.lang.StackOverflowError
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.VirtualMachineError, which is child-class of java.lang.Error
JVM raises StackOverflowError, whenever we perform repeated recursive method call
See below example for reference

RecursiveDemo.java

package in.bench.resources.top.exception.in.java;

public class RecursiveDemo {

	// main() method - JVM execution starts here
	public static void main(String[] args) {

		// invoking methodA
		methodA();
	}

	// method A - recursively invoking method B
	public static void methodA() {

		// invoking methodB
		methodB();
	}

	// method B - recursively invoking method A
	public static void methodB() {

		// invoking methodA
		methodA();
	}
}

Output:

Exception in thread "main" java.lang.StackOverflowError
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodA(RecursiveDemo.java:16)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodB(RecursiveDemo.java:23)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodA(RecursiveDemo.java:16)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodB(RecursiveDemo.java:23)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodA(RecursiveDemo.java:16)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodB(RecursiveDemo.java:23)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodA(RecursiveDemo.java:16)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodB(RecursiveDemo.java:23)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodA(RecursiveDemo.java:16)
	at in.bench.resources.top.exception.in.java.RecursiveDemo
.methodB(RecursiveDemo.java:23)
........
........
........
........

15. Assertion Error

Exception Name :	AssertionError
Fully qualified name :	java.lang.AssertionError
Exception Type :	Unchecked Exception
Exception hierarchy :	See below figure

Description:

It is the direct sub-class of java.lang.Error
Whenever we want to test our assumption, then JVM raises AssertionError if our assumption goes wrong (basically the other way of our assumption)
This is mainly used in JUnit for testing purpose
For example, in the below assertion test when user enters integer value less than 18 then our assumpotion fails
And then accordingly JVM raises AssertionError
Note: AssertionError can be thrown explicitly using throw keyword
See below example for reference

AssertionTest.java

package in.bench.resources.top.exception.in.java;

import java.util.Scanner;

public class AssertionTest {

	public static void main(String[] args) {

		// for taking input from users
		Scanner scanner = new Scanner(System.in);
		System.out.print("Age for obtaning Driver License : ");
		int value = scanner.nextInt();  

		// assertion test for Age
		assert value>=18 : "Age should be more than 18 years old";
	}
}

Output:

Age for obtaning Driver License : 16
Exception in thread "main" java.lang.AssertionError:
Age should be more than 18 years old
	at in.bench.resources.top.exception.in.java.AssertionTest
.main(AssertionTest.java:15)

References:

Happy Coding !!
Happy Learning !!

Top Exception in Java:

1. Null Pointer Exception

Description:

StringOperationForCheckingLength.java

2. Array Index Out Of Bounds Exception

Description:

ArrayOperationOnInvalidIndex.java

3. Arithmetic Exception

Description:

DivideByZeroExample.java

4. Class Cast Exception

Description:

ClassCastExceptionExample.java

5. Illegal Argument Exception

Description:

InvalidArgumentForThreadPriority.java

6. Number format Exception

Description:

ParseStringToInteger.java

7. Illegal Thread State Exception

Description:

StartingThreadAgain.java

8. Interrupted Exception

Description:

InterruptingThread.java

9. File Not Found Exception

Description:

FileProcessing.java

10. SQL Exception

Description:

JdbcConnectionExample.java

11. Class Not Found Exception

Description:

JdbcConnectionExample.java

Top Error in Java:

12. No Class Def Found Error

Description:

SimilarException.java

Q) Difference between ClassNotFoundException and NoClassDefFoundError ?

13. Exception in Initializer Error

Description:

ExceptionErrorDemo.java

14. Stack Overflow Error

Description:

RecursiveDemo.java

15. Assertion Error

Description:

AssertionTest.java

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References:

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