throw vs throws vs Throwable In Java

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In Java exception handling, three commonly confused terms are: throw, throws, and Throwable. At first glance, these keywords and class names may seem related because they all deal with exceptions, but each plays a very different role.

If you’re new to Java or preparing for interviews, it’s essential to understand how throw, throws, and Throwable work — and when to use each one correctly.

In this article, we’ll break down these three concepts with simple language, real code examples, and a comparison table to help you master Java exception handling.

Quick Overview

Let’s now explore each one in detail.

🚀 1. What is throw in Java?

The throw keyword is used in Java to manually throw an exception. You can throw either checked or unchecked exceptions using throw.

Syntax:

throw new ExceptionType("Error message");

Example:

public class ThrowExample {
    public static void main(String[] args) {
        int age = -1;

        if (age < 0) {
            throw new IllegalArgumentException("Age cannot be negative");
        }

        System.out.println("Age is valid");
    }
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: Age cannot be negative

Key Points:

• You can only throw objects that are instances of Throwable or its subclasses.
• You must throw a new instance of an exception (throw new ...).
• Once throw is executed, the current method is immediately exited.

🚀 2. What is throws in Java?

The throws keyword is used in a method declaration to indicate that the method might throw a checked exception.

This is Java’s way of enforcing compile-time exception checking.

Syntax:

public void myMethod() throws IOException, SQLException {
    // code that might throw checked exceptions
}

Example:

import java.io.*;

public class ThrowsExample {
    public static void readFile() throws FileNotFoundException {
        FileReader file = new FileReader("file.txt"); // may throw exception
    }

    public static void main(String[] args) {
        try {
            readFile();
        } catch (FileNotFoundException e) {
            System.out.println("File not found: " + e.getMessage());
        }
    }
}

Key Points:

• throws is used only in method signatures.
• It tells the caller of the method that they need to handle or declare the exception.
• Commonly used for checked exceptions, not required for unchecked exceptions.

🚀 3. What is Throwable in Java?

Throwable is a class defined in the java.lang package. It is the superclass for all exceptions and errors in Java.

public class Throwable extends Object implements Serializable

All exceptions (like IOException, NullPointerException) and errors (like OutOfMemoryError, StackOverflowError) are subclasses of Throwable.

✅ Throwable Class Hierarchy:

java.lang.Object
  └── java.lang.Throwable
        ├── java.lang.Exception
        │     ├── IOException
        │     └── SQLException
        └── java.lang.Error
              ├── OutOfMemoryError
              └── StackOverflowError

Example:

public class ThrowableExample {
    public static void main(String[] args) {
        try {
            throw new Throwable("Custom throwable thrown");
        } catch (Throwable t) {
            System.out.println("Caught Throwable: " + t.getMessage());
        }
    }
}

Key Points:

• You can catch Throwable, but it’s usually discouraged.
• Catching Throwable means catching both Exception and Error types, which could hide serious problems like OutOfMemoryError.
• You should catch specific exceptions instead of Throwable in most cases.

📊 Comparison Table: throw vs throws vs Throwable

Practical Example Using All Three

import java.io.*;

public class ExceptionDemo {

    // Method declares it might throw an IOException
    public static void readFile() throws IOException {
        throw new IOException("File not found!"); // Throwing manually
    }

    public static void main(String[] args) {
        try {
            readFile();
        } catch (Throwable t) { // Catching using Throwable
            System.out.println("Caught: " + t);
        }
    }
}

Output:

Caught: java.io.IOException: File not found!

This example uses:

• throw to manually raise an exception.
• throws to declare the exception.
• Throwable to catch any throwable object.

⚠️ Common Mistakes to Avoid

❌ 1. Using throw without new

throw IOException; // ❌ Incorrect
throw new IOException(); // ✅ Correct

❌ 2. Catching Throwable instead of Exception

catch (Throwable t) { } // ❌ Only use this if you’re certain
catch (Exception e) { } // ✅ Prefer catching specific exceptions

❌ 3. Declaring throws but not handling

public void riskyMethod() throws IOException {
    // Must be handled where this method is called
}

Final Thoughts

To summarize:

• Use throw when you want to explicitly throw an exception from your code.
• Use throws when a method might throw an exception, and you want the caller to handle it.
• Throwable is the root class for all exceptions and errors, but you should avoid catching it unless absolutely necessary.