Multiple Catch Blocks in Java: Rules, Order, and Best Practices

📌 Multiple Catch Blocks in Java (Professional & Beginner-Friendly Explanation)

When a programmer uses a try block followed by multiple catch blocks, Java follows strict rules to avoid ambiguity while handling exceptions.

🔹 Key Rules of Multiple Catch Blocks
1️⃣ No Duplicate Catch Blocks Allowed

Each catch block must handle a different exception type.

❌ Invalid Code (Duplicate Catch):

try {

    int a = 10 / 0;

}

catch (ArithmeticException e) {

    System.out.println("Arithmetic Exception");

}

catch (ArithmeticException e) { // ❌ Duplicate

    System.out.println("Duplicate Exception");

}

📌 Reason: 

The JVM will be confused about which catch block to execute, resulting in a compile-time error.

2️⃣ Superclass Catch Can Handle Subclass Exceptions

A catch block with a superclass exception can handle all its subclass exceptions.

📌 Example:

try {

    int[] arr = new int[5];

    arr[10] = 50;

}

catch (Exception e) {

    System.out.println("Handled by Exception class");

}

✔ Here, Exception handles ArrayIndexOutOfBoundsException because it is a subclass of Exception.

3️⃣ Order of Catch Blocks Is Very Important

When multiple catch blocks are used:

✅ Subclass exception must come before superclass exception

❌ Wrong Order (Compile-Time Error):

try {

    int[] arr = new int[5];

    arr[10] = 50;

}

catch (Exception e) { // ❌ Superclass first

    System.out.println("Exception");

}

catch (ArrayIndexOutOfBoundsException e) {

    System.out.println("Array Index Error");

}

📌 Reason:

The superclass Exception can already handle the subclass exception, so the subclass catch block becomes unreachable code.

✅ Correct Order (Subclass → Superclass)

try {

    int[] arr = new int[5];

    arr[10] = 50;

}

catch (ArrayIndexOutOfBoundsException e) {

    System.out.println("Array Index Error");

}

catch (Exception e) {

    System.out.println("General Exception");

}

✔ The JVM checks catch blocks top to bottom and executes the first matching block.

🔹 Real-Life Example
📌 ATM Transaction System

try {

    int balance = 5000;

    int withdraw = 6000;

    int result = balance / (balance - withdraw);

}

catch (ArithmeticException e) {

    System.out.println("Invalid arithmetic operation");

}

catch (Exception e) {

    System.out.println("Something went wrong");

}

✔ Specific issues are handled first

✔ General issues are handled later

📝 Summary Table

Rule                                 Explanation

Duplicate catch.           ❌ Not allowed

Superclass catch.     Can handle subclass exceptions

Catch order.               Subclass first, superclass last

JVM behavior.          Executes first matching catch