Top 10 Common Exceptions in Java

Top 10 Common Exceptions in Java | Based on who raises the exception all exceptions are divided into 2 categories:-

• JVM Exceptions
• Programmatic Exceptions

JVM Exceptions:- The exceptions raised by JVM itself whenever a particular event occurs are called JVM exceptions. Examples:- ArithmeticException, NullPointerException, etc.

Programmatic Exception: A programmer raises explicitly to indicate that something goes wrong is called a programmatic exception. Example:- IllegalArgumentException, InsufficientFundsException, and e.t.c.

ArrayIndexOutOfBoundException

int x[] = new int[4]; 
System.out.println(x[0]); // valid
System.out.println(x[10]); // ArrayIndexOutOfBoundException
System.out.println(x[-4]); // ArrayIndexOutOfBoundException

It is the child class of RuntimeException & hence it is unchecked. It is raised by JVM whenever we try to access an array element out of range index.

NullPointerException

It is the child class of Runtime exception and hence it is an unchecked exception. It is raised by JVM whenever we are trying to perform any operation on null.

String s = null;
System.out.println(s.length());

ClassCastException

It is the child class of RuntimeException & hence it is an unchecked exception. It is raised by JVM whenever we are trying to typecast parent object to child type.

String s = new String("KnowProgram");
Object o = (Object)s; // valid

Object obj = new Object();
String str = (String)obj; // invalid; RE

Object obj = new String("KnowProgram");
String str = (String)obj; // valid

Internally “obj” object is a string type.

StackOverflowError

It is the child class of Error & hence the unchecked exception. It is raised by JVM whenever we are trying to perform a recursive method call and it goes into an infinite method call or a large number of stacks is created.

public class Test {
   public static void main(String[] args) {
      m1();
   }
   public static void m1() {
      m2();
   }
   public static void m2() {
      m1();
   }
}

——

——

m1()

m2()

m1()

main()

NoClassDefFoundError

It is the child class of Error & hence it is an unchecked exception. It is raised by JVM whenever JVM is unable to find the required class file.

> java Test 
NoClassDefFoundError

If the Test.class file is not available then we will get run time exception:- NoClassDefFoundError

ExceptionInInitializerError

It is the child class of Error & hence it is an unchecked exception. It is raised by JVM if any exception occurs while executing static variable assignments and static blocks.

public class Test {
   static int x = 9/0;
   public static void main(String[] args) {
   }
}

Exception in thread “main” java.lang.ExceptionInInitializerError
Caused by: java.lang.ArithmeticException: / by zero
at Test.(Test.java:6)

public class Test {
   static {
      String s = null;
      System.out.println(s.length());
   }
   public static void main(String[] args) {
   }
}

Exception in thread “main” java.lang.ExceptionInInitializerError
Caused by: java.lang.NullPointerException: Cannot invoke “String.length()” because “” is null
at Test.(Test.java:4)

IllegalArgumentException

It is the child class of RuntimeException & hence it is an unchecked exception. It is raised explicitly either by the programmer to indicate that a method has been invoked with an illegal argument.

The valid range of thread priority is 1 to 10. If we are trying to set the priority with any other value then we will get RuntimeException:- IllegalArgumentException.

Thread t = new Thread();
t.setPriority(7);
t.setPriority(15);

Exception in thread “main” java.lang.IllegalArgumentException
at java.base/java.lang.Thread.setPriority(Thread.java:1135)
at Test.main(Test.java:5)

NumberFormatException

It is the direct child class of IllegalArgumentException which is the child class of RuntimeException & hence it is an unchecked exception. It is raised explicitly either by the programmer to indicate that we are trying to convert String to the number & string is not properly formatted.

• IllegalArgumentException
  • NumberFormatException

int i = Integer.parseInt("10");
int j = Integer.parseInt("ten"); // NumberFormatException

IllegalThreadStateException

It is the child class of RuntimeException and hence it is an unchecked exception. It is raised explicitly either by the programmer or by the API developer to indicate that a method has been invoked at the wrong time.

After storing a thread we are not allowed to restart the thread otherwise we will get the runtime exception:-  IllegalThreadStateException

Thread t = new Thread();
t.start();
// code
t.start(); // IllegalThreadStateException

AssertionError

It is the child class of Error and hence it is an unchecked exception. It is raised explicitly by the programmer to indicate that the assert statement fails. Example:-

assert(x > 10);

If x is not greater than 10 then we will get a runtime exception saying AssertionError.

Summary

The exceptions raised by JVM (JVM exception):-

1. ArrayIndexOutOfBoundException
2. NullPointerException
3. ClassCastException
4. StackOverflowError
5. NoClassDefFoundError
6. ExceptionInInitializerError

The below exceptions are explicitly raised by the programmer. Hence these are programmatic exceptions):-

1. IllegalArgumentException
2. NumberFormatException
3. IllegalThreadStateException
4. AssertionError

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