Java Exception Handling

In the last tutorial, we learned about Java exceptions. We know that exceptions abnormally terminate the execution of a program.

This is why it is important to handle exceptions. Here's a list of different approaches to handle exceptions in Java.

• try...catch block
• finally block
• throw and throws keyword

1. Java try...catch block

The try-catch block is used to handle exceptions in Java. Here's the syntax of try...catch block:

try {
  // code
}
catch(Exception e) {
  // code
}

Here, we have placed the code that might generate an exception inside the try block. Every try block is followed by a catch block.

When an exception occurs, it is caught by the catch block. The catch block cannot be used without the try block.

Example: Exception handling using try...catch

class Main {
  public static void main(String[] args) {

    try {

      // code that generate exception
      int divideByZero = 5 / 0;
      System.out.println("Rest of code in try block");
    }
    
    catch (ArithmeticException e) {
      System.out.println("ArithmeticException => " + e.getMessage());
    }
  }
}

Output

ArithmeticException => / by zero

In the example, we are trying to divide a number by 0. Here, this code generates an exception.

To handle the exception, we have put the code, 5 / 0 inside the try block. Now when an exception occurs, the rest of the code inside the try block is skipped.

The catch block catches the exception and statements inside the catch block is executed.

If none of the statements in the try block generates an exception, the catch block is skipped.

2. Java finally block

In Java, the finally block is always executed no matter whether there is an exception or not.

The finally block is optional. And, for each try block, there can be only one finally block.

The basic syntax of finally block is:

try {
  //code
}
catch (ExceptionType1 e1) { 
  // catch block
}
finally {
  // finally block always executes
}

If an exception occurs, the finally block is executed after the try...catch block. Otherwise, it is executed after the try block. For each try block, there can be only one finally block.

Example: Java Exception Handling using finally block

class Main {
  public static void main(String[] args) {
    try {
      // code that generates exception
      int divideByZero = 5 / 0;
    }

    catch (ArithmeticException e) {
      System.out.println("ArithmeticException => " + e.getMessage());
    }
    
    finally {
      System.out.println("This is the finally block");
    }
  }
}

Output

ArithmeticException => / by zero
This is the finally block

In the above example, we are dividing a number by 0 inside the try block. Here, this code generates an ArithmeticException.

The exception is caught by the catch block. And, then the finally block is executed.

3. Java throw and throws keyword

The Java throw keyword is used to explicitly throw a single exception.

When we throw an exception, the flow of the program moves from the try block to the catch block.

Example: Exception handling using Java throw

class Main {
  public static void divideByZero() {

    // throw an exception
    throw new ArithmeticException("Trying to divide by 0");
  }

  public static void main(String[] args) {
    divideByZero();
  }
}

Output

Exception in thread "main" java.lang.ArithmeticException: Trying to divide by 0
        at Main.divideByZero(Main.java:5)
        at Main.main(Main.java:9)

In the above example, we are explicitly throwing the ArithmeticException using the throw keyword.

Similarly, the throws keyword is used to declare the type of exceptions that might occur within the method. It is used in the method declaration.

Example: Java throws keyword

import java.io.*;

class Main {
  // declareing the type of exception
  public static void findFile() throws IOException {

    // code that may generate IOException
    File newFile = new File("test.txt");
    FileInputStream stream = new FileInputStream(newFile);
  }

  public static void main(String[] args) {
    try {
      findFile();
    }
    catch (IOException e) {
      System.out.println(e);
    }
  }
}

Output

java.io.FileNotFoundException: test.txt (The system cannot find the file specified)

When we run this program, if the file test.txt does not exist, FileInputStream throws a FileNotFoundException which extends the IOException class.

The findFile() method specifies that an IOException can be thrown. The main() method calls this method and handles the exception if it is thrown.

If a method does not handle exceptions, the type of exceptions that may occur within it must be specified in the throws clause.

To learn more, visit Java throw and throws.