class

Exceptions in Constructors

In this example we shall show you how to handle exceptions in constructors. To handle exceptions in constructors we have performed the following steps:

  • We have created a class, InputFile, that has a BufferedReader field.
  • In its constructor, it gets a String and it creates a new FileReader with the given String name of path to file to read from. A FileNotFoundException might be thrown here, that needs to be caught, but since the file is not found the BufferedReader is not opened so does not need to close.
  • If any other exception occurs after that, the FileReader has opened so it has to close.
  • The InputFile class also has two methods, getLine(), that gets the line of the text in the BufferedReader, and dispose() that closes the BufferedReader. The methods throw RuntimeException that has to be caught.
  • We create a new instance of InputFile, with a given String. Since the constructor of the class is called if the file is not found the exception will be thrown,

as described in the code snippet below.  

package com.javacodegeeks.snippets.core;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class ExceptionInConstructor {

    public static void main(String[] args) {

  

  try {


InputFile inputFile = new InputFile("Cleanup.java");


String string;


int i = 1;


while ((string = inputFile.getLine()) != null)

  ; // Perform line-by-line processing here...


inputFile.dispose();

  } catch (Exception e) {


System.err.println("Caught Exception in main");


e.printStackTrace();

  }
    }
}

class InputFile {

    private BufferedReader input;

    public InputFile(String fileName) throws Exception {

  try {


input = new BufferedReader(new FileReader(fileName));


// Other code that might throw exceptions

  } catch (FileNotFoundException e) {


System.err.println("Could not open " + fileName);


// Wasn't open, so don't close it


throw e;

  } catch (Exception e) {


// All other exceptions must close it


try {


    input.close();


} catch (IOException e2) {


    System.err.println("in.close() unsuccessful");


}


throw e; // Rethrow

  } finally {


// Don't close it here!!!

  }
    }

    public String getLine() {

  String s;

  try {


s = input.readLine();

  } catch (IOException e) {


throw new RuntimeException("readLine() failed");

  }

  return s;
    }

    public void dispose() {

  try {


input.close();


System.out.println("dispose() successful");

  } catch (IOException e2) {


throw new RuntimeException("in.close() failed");

  }
    }
}

Output:

dispose() successful

  
This was an example of how to handle exceptions in constructors in Java.

Ilias Tsagklis

Ilias is a software developer turned online entrepreneur. He is co-founder and Executive Editor at Java Code Geeks.
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