try with resources & AutoCloseable java 7 (example & class hierarchy)

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  • The try-with-resources is try statement that used to declare one or more resources.
    • e.g. resources like BufferedReader, Scanner, BufferedWriter can be used with try-with-resource statement.
  • Any resource class implementing AutoCloseable interface, can be used with try-with-resource statement.
    • Resources are closed automatically at the end of try statement.
  • We will look into the resources implementing AutoCloseable interface e.g. BufferedWriter, BufferedReader.
  • We will discuss Class hierarchy of custom BufferedWriter & BufferedReader classed with the help of example program.

Examples of AutoCloseable Interfaces:

1. BufferedReader class Implements AutoCloseable Interface:

  • BufferedReader class implements AutoCloseable.
  • BufferedReader class can be used with try-with-resource statement.
  • We will create our custom BufferedReader class by extending BufferedReader class, to demonstrate close method of AutoCloseable interface.
  • Override close method in CustomBufferedReader class.
    • close method will be called automatically at the end of try statement.
  • Class hierarchy of custom BufferedReader class is as follows:
BufferedReader autocloseable
Fig 1: CustomBufferedReader class hierarchy

Program – CustomBufferedReader class code is as follows:

class CustomBufferedReader extends BufferedReader {
 
    public CustomBufferedReader(Reader in) {
        super(in);     
    }
     
    @Override
    public void close() throws IOException {       
        super.close();
        System.out.println("AutoCloseable: Closing BufferReader resource");
    }
}

2. BufferedWriter class implements AutoCloseable interface:

  • BufferedWriter class implements AutoCloseable interface (similar to BufferedReader class)
    • BufferedWriter class can be used with try-with-resource statement.
  • Create CustomBufferWriter class by extending BufferedWriter class.
  • Override close method in CustomBufferedWriter class.
  • Class hierarchy of custom bufferedWriter class is as follows:
BufferedWriter try with resource
Fig 2: CustomBufferedWriter class hierarchy

Program – CustomBufferedWriter class code is as follows:

class CustomBufferedWriter extends BufferedWriter{
 
    public CustomBufferedWriter(Writer out) {
        super(out);    
    }
     
    @Override
    public void close() throws IOException {       
        super.close();
        System.out.println("AutoCloseable: Closing CustomBufferedWriter resource");
    }
}

AutoCloseable interface:

  • Resources implementing AutoCloseable interface will be closed automatically (on exiting the try statement).
  • Custom BufferedReader & BufferedWriter class implements AutoCloseable interface.
  • Let us look into the complete code to demonstrate AutoCloseable interface.
    • close method of CustomBufferedReader & CustomBufferedWriter class will be called automatically.

Program of Custom BufferedReader & BufferedWriter Classes

package org.learn.try_with_resource;
 
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.io.Writer;
 
public class DemoTryWithResource
{
    public static void main( String[] args ) throws IOException
    {
        writeUsingBufferWriter();
        readUsingBufferReader();   
    }
 
    private static void writeUsingBufferWriter() throws IOException {
        System.out.println("1. Start writing contents to file - CustomBufferedWriter");    
        try (FileOutputStream fileStream = new FileOutputStream(new File("sampleFile.txt"));
                OutputStreamWriter writer = new OutputStreamWriter(fileStream);
                CustomBufferedWriter customBufferWriter = new CustomBufferedWriter(writer)) {
            customBufferWriter.write("Soccer");
            customBufferWriter.newLine();
            customBufferWriter.write("Tennis");        
        }
        System.out.println("3. Successfully written contents to file - CustomBufferedWriter");
    }
     
    private static void readUsingBufferReader() throws IOException {
        System.out.println("\n4. Start Reading file using CustomBufferedReader");
        try (FileInputStream fileStream = new FileInputStream(new File("sampleFile.txt"));
                InputStreamReader reader = new InputStreamReader(fileStream);
                CustomBufferedReader customBufferedReader = new CustomBufferedReader(reader)) {
            String line;
            while ((line = customBufferedReader.readLine()) != null) {
                System.out.println(line);
            }
        }
        System.out.println("6. End Reading file using CustomBufferedReader");
    }
}
 
class CustomBufferedWriter extends BufferedWriter{
 
    public CustomBufferedWriter(Writer out) {
        super(out);    
    }
     
    @Override
    public void close() throws IOException {       
        super.close();
        System.out.println("2. AutoCloseable: Closing CustomBufferedWriter resource");
    }
}
 
class CustomBufferedReader extends BufferedReader {
 
    public CustomBufferedReader(Reader in) {
        super(in);     
    }
     
    @Override
    public void close() throws IOException {       
        super.close();
        System.out.println("5. AutoCloseable: Closing CustomBufferedReader resource");
    }
}

Output of AutoCloseable & try-with-resource is as follows:

1. Start writing contents to file - CustomBufferedWriter
2. AutoCloseable: Closing CustomBufferedWriter resource
3. Successfully written contents to file - CustomBufferedWriter
 
4. Start Reading file using CustomBufferedReader
Soccer
Tennis
5. AutoCloseable: Closing CustomBufferedReader resource
6. End Reading file using CustomBufferedReader

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