Java Object Oriented Design - Java Exception Throw

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If a piece of code may throw a checked exception, we have two options:

• Handle the checked exception with a try-catch block.
• Specify in your method/constructor declaration with throws clause.

Syntax

The general syntax for a throws clause is

<modifiers> <return type> <method name>(<params>) throws<List of Exceptions>{
    
}

The keyword throws is used to specify a throws clause.

The throws clause is placed after the closing parenthesis of the method's parameters list.

The throws keyword is followed by a comma-separated list of exception types.

Example

The following code shows how to use a throws Clause in a Method's Declaration

import java.io.IOException;

public class Main {
  public static void readChar() throws IOException {
    int input = System.in.read();
    
  }
}

Here is the code showing how to use it.

Example 2

import java.io.IOException;
//from ww w  .j  a  v  a 2s  .  c om
public class Main {
  public static void readChar() throws IOException {
    int input = System.in.read();
    System.out.println(input);
  }

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

}

The code above generates the following result.

Example 3

We can continue to throw exception.

import java.io.IOException;
/*from  w  w w  . j ava  2 s.  co  m*/
public class Main {
  public static void readChar() throws IOException {
    int input = System.in.read();
    System.out.println(input);
  }

  public static void main(String[] args) throws IOException {
    readChar();
  }
}

The code above generates the following result.

Throwing an Exception

We can throw an exception in our code using a throw statement.

The syntax for a throw statement is

throw <A throwable object reference>;

throw is a keyword, which is followed by a reference to a throwable object.

A throwable object is an instance of a class, which is a subclass of the Throwable class, or the Throwable class itself.

The following is an example of a throw statement, which throws an IOException:

// Create an  object of  IOException
IOException e1  = new IOException("File not  found");
// Throw the   IOException 
throw  e1;

We can create a throwable object and throw it in one statement.

// Throw an  IOException
throw  new IOException("File not  found");

If we throw a checked exception, we must handle it with a try-catch block, or using a throws clause in the method or constructor declaration.

These rules do not apply if you throw an unchecked exception.

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