Exception Handling Using Throw keyword

Sometimes we explicitly want to create an exception object and then throw it to halt the normal processing of the program.

The throw keyword is used to throw exceptions to the runtime to handle it. It is mainly used to throw a custom exception.

Types of Exceptions in Java

1. User-Defined Exceptions

2. Built-In Exceptions

We can throw either checked or unchecked exceptions in Java by throw keyword.

Build-In Exceptions are of two types:

1) Unchecked Exceptions

2) Checked Exceptions

Unchecked Exceptions

1. ArithmeticException

2. ClassCastException

3. NullPointerException

4. ArrayIndexOutOfBoundsException

5. ArrayStoreException

6. IllegalThreadStateException

Example

Java

package com.prac;

public class UncheckException {
    public static void main(String[] args) {
        validate(15);
    }

    public static void validate(int age) {
        if (age < 18) {
            // throw exception if not eligible to vote
            throw new ArithmeticException("Person is not eligible to vote");
        } else {
            System.out.println("Person is eligible to vote!!");
        }
    }
}

Output:

Exception in thread "main" java.lang.ArithmeticException: Person is not eligible to vote
	at com.prac.UncheckException.validate(UncheckException.java:11)
	

Checked Exceptions Throw

1.ClassNotFoundException

2.InterruptedException

3.IOException

4.InstantiationException

5.SQLException

6.FileNotFoundException

Example

Java

package com.prac;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class CheckedException {
    public static void main(String[] args) {
        readFile();
    }

    public static void readFile() {
        String fileName = "file";
        File file = new File(fileName);
        FileInputStream fileInputStream = null;
        try {
            fileInputStream = new FileInputStream(file);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Output

java.io.FileNotFoundException: file (The system cannot find the file specified)
	at java.base/java.io.FileInputStream.open0(Native Method)
	at java.base/java.io.FileInputStream.open(FileInputStream.java:211)
	at java.base/java.io.FileInputStream.<init>(FileInputStream.java:153)
	at com.prac.CheckedException.readFile(CheckedException.java:17)
	at com.prac.CheckedException.main(CheckedException.java:9)

User-defined Exception Throw

Approach

Create a User Defined Exception class

Java

UserDefinedException.java

// class represents user-defined exception  
class UserDefinedException extends Exception {
    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    public UserDefinedException(String str) {
        // Calling constructor of parent Exception
        super(str);
    }
}

Test User Defined Exceptions

Java

TestThrow.java

public class TestThrow {
    public static void main(String args[]) {
        try {
            // throw an object of user defined exception
            throw new UserDefinedException("This is user-defined exception");
        } catch (UserDefinedException ude) {
            System.out.println("Caught the exception");
            System.out.println(ude.getMessage());
        }
    }
}

Output

Caught the exception
This is user-defined exception

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