Try, Catch, and Finally

Java uses five keywords for exception handling: try, catch, finally, throw, and throws. This section covers the first three, which are used to build the core mechanism of handling exceptions.

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1. The try Block

The try block contains the code that might generate an exception. You must place suspicious code (code that can throw an exception) inside a try block.

• A try block must be followed by either a catch block or a finally block (or both).
• You cannot have a standalone try block.

2. The catch Block

The catch block is used to handle the exception. It must be preceded by a try block.

• The catch block takes a parameter that matches the type of exception it can handle.
• If an exception occurs in the try block, the rest of the code in the try block is skipped, and the JVM jumps to the appropriate catch block.

Example: Basic Try-Catch

public class LabException1 {

  public static void main(String args[]) {
    System.out.println("Program Started");

    try {
      int result = 10 / 0; // This will throw an ArithmeticException
      System.out.println("Result: " + result); // This line is skipped
    } catch (ArithmeticException e) {
      System.out.println("Exception caught: Cannot divide by zero!");
    }

    System.out.println("Program Ended Normaly");
  }
}

Output:

Program Started
Exception caught: Cannot divide by zero!
Program Ended Normaly

[!NOTE] Because we handled the exception, the program didn't crash and "Program Ended Normally" was printed successfully.

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3. Multiple catch Blocks

A single try block can be followed by multiple catch blocks to handle different types of exceptions in different ways.

• Rule of Ordering: When using multiple catch blocks, the catch block for the subclass exception must appear before the catch block for the superclass exception. Otherwise, you will get a compile-time error.

Example: Multiple Catch

public class LabException2 {

  public static void main(String args[]) {
    try {
      int[] numbers = { 1, 2, 3 };
      int result = numbers[5] / 0;
    } catch (ArithmeticException e) {
      System.out.println("Arithmetic Exception occurred");
    } catch (ArrayIndexOutOfBoundsException e) {
      System.out.println("Array Index Out Of Bounds Exception occurred");
    } catch (Exception e) {
      System.out.println("Some general exception occurred");
    }
  }
}

[!WARNING] If you put catch (Exception e) at the very top, the subsequent catch blocks will become unreachable code, resulting in a compile-time error.

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4. The finally Block

The finally block is used to execute crucial code, such as closing connections, closing files, or releasing resources.

• The finally block always executes, regardless of whether an exception was thrown or caught.
• It is placed after the try block and all catch blocks.

Example: Using Finally

public class LabException3 {

  public static void main(String args[]) {
    try {
      System.out.println("Inside try block");
      int result = 10 / 2;
    } catch (ArithmeticException e) {
      System.out.println("Inside catch block");
    } finally {
      System.out.println("Inside finally block - always executes");
    }
  }
}

Output:

Inside try block
Inside finally block - always executes

When does finally NOT execute?

There is only one main scenario where a finally block will not execute: if the program is forcefully terminated using System.exit(0) inside the try or catch block.

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5. Nested Try-Catch

You can place a try-catch block inside another try block. This is called nested try-catch. It is useful when a specific block of code within a try block can cause a completely different exception.

public class LabException4 {

  public static void main(String args[]) {
    try {
      System.out.println("Outer try block");

      try {
        System.out.println("Inner try block");
        int res = 10 / 0;
      } catch (ArithmeticException e) {
        System.out.println("Handled Inner ArithmeticException");
      }
    } catch (Exception e) {
      System.out.println("Handled Outer Exception");
    }
  }
}