Hierarchy of Java Exception Classes

• The java.lang.throwable class is the root class of the Java exception hierarchy inherited by two sub-classes: Exception and Error. 

Hierarchy of Java Exception classes:

Types of Java Exception:

• There are mainly two types of exceptions 

1. Checked Exception 

• The exception which is identified by the compiler during the compilation time is known as a 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

1. By using a try-catch block 
2. By using the throws declaration keyword

Key Points:

• The exception is a direct subclass of exception classes except the runtime exception class comes under the checked exception. e.g.. Exception, SQLException, ClassNotFoundExcegption.
• Checked exceptions are known at compile time and occur at runtime.

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:

• The unchecked exceptions are not identified by the compiler and occur at runtime
• 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?

• In Java, an error is a subclass of Throwable that tells something serious problem exists, and a reasonable Java application should not try to catch try error.
• 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.