throw and throws in exception handling

 

🔹 throw Keyword in Java

The throw keyword is used to explicitly throw an exception from a method or block of code.

✅ Syntax:

throw new ExceptionType("Error message");

✅ Key Points:

• You can throw only one exception at a time using throw.

• You must throw an object of type Throwable (or subclass like Exception, RuntimeException, etc.).

• Common for manual exception generation.

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📘 Example 1: Using throw with unchecked exception

public class ThrowExample {
    public static void main(String[] args) {
        int age = 15;
        if (age < 18) {
            throw new ArithmeticException("You must be 18 or older.");
        }
        System.out.println("Access granted.");
    }
}

🔸 Output:

Exception in thread "main" java.lang.ArithmeticException: You must be 18 or older.

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🔹 throws Keyword in Java

The throws keyword is used in method declarations to indicate that a method may throw one or more exceptions.

✅ Syntax:

returnType methodName() throws ExceptionType1, ExceptionType2 {
    // method code
}

✅ Key Points:

• Used with checked exceptions.

• Propagates the responsibility to handle the exception to the caller.

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📘 Example 2: Using throws with checked exception

import java.io.*;

public class ThrowsExample {
    public static void main(String[] args) throws IOException {
        readFile();
    }

    static void readFile() throws IOException {
        FileReader fr = new FileReader("file.txt");  // file might not exist
        BufferedReader br = new BufferedReader(fr);
        System.out.println(br.readLine());
    }
}

🔸 Output (if file not found):

Exception in thread "main" java.io.FileNotFoundException: file.txt (No such file or directory)

If you don't handle or declare the checked exception, the compiler will throw an error.

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✅ Differences between throw and throws

Feature	throw	throws
Purpose	To explicitly throw an exception	To declare exceptions that a method might throw
Used in	Inside method body	In method signature
Can throw	One exception at a time	Multiple exceptions (comma separated)
Example	throw new IOException();	void read() throws IOException

📘 Example 3: Combining throw and throws

public class ThrowThrowsExample {
    public static void main(String[] args) {
        try {
            checkMarks(30);
        } catch (Exception e) {
            System.out.println("Exception: " + e.getMessage());
        }
    }

    static void checkMarks(int marks) throws Exception {
        if (marks < 40) {
            throw new Exception("Marks too low to pass.");
        }
        System.out.println("You passed!");
    }
}

🔸 Output:

Exception: Marks too low to pass.