1, Handling Exceptions

Here's a program that divides two number and uses a try/catch statement to catch an exception if the second number turns out to be zero.

Main.java

public class Main {

	public static void main(String[] args) {

		int a = 5;
		int b = 0;

		try {
			int c = a / b;
		} catch (ArithmeticException e) {
			System.out.println("Oops, you can't divide by zero.");
		}
	}
}

When you run this program, the following is displayed on the console.

コンソール

Oops, you can't divide by zero.

2, Another example

This shows a simple example of a program that uses a method to get a valid integer from the user.

Main.java

import java.util.InputMismatchException;
import java.util.Scanner;

public class Main {

	static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {

		System.out.print("Enter an integer: ");
		int i = GetAnInteger();
		System.out.println("You entered " + i);

	}

	public static int GetAnInteger() {
		while (true) {
			try {
				return sc.nextInt();

			} catch (InputMismatchException e) {
				sc.next();
				System.out.print("That's not an integer. Try again: ");
			}
		}
	}
}

Here's what the console might look like for a typical execution of this program.

コンソール

Enter an integer: three
That's not an integer. Try again: 3.01
That's not an integer. Try again: 3
You entered 3

3, Using a finally Block

A finally block is a block that appears after any of the catch blocks for a statement.

Main.java

public class Main {

	public static void main(String[] args) {

		int answer = 0;

		try {
			answer = divideTheseNumbers(5, 0);

		} catch (Exception e) {
			System.out.println("Tried twice, still didn't work!");
		}

		System.out.println("Answer is: " + answer);

	}

	public static int divideTheseNumbers(int a, int b) {

		int c;
		try {

			c = a / b;
			System.out.println("It worked!");

		} catch (Exception e) {
			System.out.println("Didn't work the first time.");
			c = a / b;
			System.out.println("It worked the second time!");
		} finally {
			System.out.println("Better clean up my mess.");
		}

		System.out.println("It worked after all.");
		return c;

	}
}

Here's the console output the program.

コンソール

Didn't work the first time.
Better clean up my mess.
Tried twice, still didn't work!
Answer is: 0

4, Catching FileNotFoundException

One way to deal with the FileNotFoundException is to catch it by using an ordinary try statement.

Main.java

import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class Main {

	public static void main(String[] args) {

		openFile("C:\\test.txt");

	}

	public static void openFile(String name) {
		try {
			FileInputStream f = new FileInputStream(name);
			System.out.println(f);

		} catch (FileNotFoundException e) {
			System.out.println("File not found.");
		}
	}
}

In this example, the message "File not found." is displayed if the file doesn't exist.

コンソール

File not found.

5, Throwing Your Own Exceptions

Here's a program that demonstrates the basic structure of a method that throws an exception.

Main.java

public class Main {

	public static void main(String[] args) {

		try {
			doSomething(true);
		} catch (Exception e) {
			System.out.println("Exception!");
		}
	}

	public static void doSomething(boolean t) throws Exception {
		if (t) {
			throw new Exception();
		}
	}
}

Here the doSomething method accepts a boolean value as a parameter. If this value is true, it throws an exception.

コンソール

Exception!

参考資料

Chapter 8 Handling Exception

もくじ

1, Handling Exceptions

2, Another example

3, Using a finally Block

4, Catching FileNotFoundException

5, Throwing Your Own Exceptions

参考資料