The java.lang.throwable class is the root class of the Java exception hierarchy. It is further divided into two subclasses.

Hierarchy of Java Exception Classes

Types of Java Exception

1. Checked Exception 

• Since the checked exception are known at compilation time it is known as a caught exception.
• The checked exception occurs we must define the handler otherwise the compiler throws an unhandled exception error.
• The checked exception can be handled in two ways

Example:

package quipoin;

import java.io.File;
import java.io.IOException;

public class CheckedExceptionExmpl {
	public static void main(String[] args) {
		System.out.println("Program started");
		File f1 = new File("c://keshava//JECM84//sample.txt");
		try {
			f1.createNewFile(); 
			// without try catch block compiler throws exception in this line
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
}

Output:

Program started
java.io.IOException: The system cannot find the path specified
	at java.base/java.io.WinNTFileSystem.createFileExclusively(Native Method)
	at java.base/java.io.File.createNewFile(File.java:1043)
	at CheckedExceptionExmpl.main(CheckedExceptionExmpl.java:9)

2. Unchecked exception

• Since these exceptions are not identified by the compiler it is known as an uncaught exception. It is also called a runtime exception.
• All the exception classes which are a subclass of runtime exceptions come under the runtime exception category Ex. ArithmeticException, NullPointerException.
• Unchecked exceptions are known at execution time and occur at runtime.

Example:

package quipoin;

class UncheckedException {
	static void calc(int n1, int n2) {
		int res;
		res = n1 / n2;
		System.out.println(res);
	}

	public static void main(String[] args) {
		calc(15, 5);
	}
}

Output:

3

What are Error classes ?

• Generally, it has been noticed that most of the occurring errors are abnormal conditions and cannot be resolved by normal conditions.
• Error and its subclass are referred to as unchecked exceptions.
• Examples: AssertionError, AbstractMethodError, IllegalAccessError.   

Example:

package quipoin;

class StackOverflow {
	public static void check(int i) {
		if (i == 0) {
			return;
		} else {
			check(i++);
		}
	}

	public static void main(String[] args) {
		StackOverflow.check(5);
	}
}

Output:

Exception in thread "main" java.lang.StackOverflowError
	at com.StackOverflow.check(StackOverflow.java:6)
	at com.StackOverflow.check(StackOverflow.java:6)
	at com.StackOverflow.check(StackOverflow.java:6)
	at com.StackOverflow.check(StackOverflow.java:6)
	at com.StackOverflow.check(StackOverflow.java:6)
	at com.StackOverflow.check(StackOverflow.java:6)

In the above example, a StackOverflow error means stack memory has fully occupied its space.