[java] implements Closeable or implements AutoCloseable

I'm in the process of learning Java and I cannot find any good explanation on the implements Closeable and the implements AutoCloseable interfaces.

When I implemented an interface Closeable, my Eclipse IDE created a method public void close() throws IOException.

I can close the stream using pw.close(); without the interface. But, I cannot understand how I can implement theclose() method using the interface. And, what is the purpose of this interface?

Also I would like to know: how can I check if IOstream was really closed?

I was using the basic code below

import java.io.*;

public class IOtest implements AutoCloseable {

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

    File file = new File("C:\\test.txt");
    PrintWriter pw = new PrintWriter(file);

    System.out.println("file has been created");

    pw.println("file has been created");

}

@Override
public void close() throws IOException {


}

Closeable extends AutoCloseable, and is specifically dedicated to IO streams: it throws IOException instead of Exception, and is idempotent, whereas AutoCloseable doesn't provide this guarantee.

This is all explained in the javadoc of both interfaces.

Implementing AutoCloseable (or Closeable) allows a class to be used as a resource of the try-with-resources construct introduced in Java 7, which allows closing such resources automatically at the end of a block, without having to add a finally block which closes the resource explicitely.

Your class doesn't represent a closeable resource, and there's absolutely no point in implementing this interface: an IOTest can't be closed. It shouldn't even be possible to instantiate it, since it doesn't have any instance method. Remember that implementing an interface means that there is a is-a relationship between the class and the interface. You have no such relationship here.

Here is the small example

public class TryWithResource {

    public static void main(String[] args) {
        try (TestMe r = new TestMe()) {
            r.generalTest();
        } catch(Exception e) {
            System.out.println("From Exception Block");
        } finally {
            System.out.println("From Final Block");
        }
    }
}



public class TestMe implements AutoCloseable {

    @Override
    public void close() throws Exception {
        System.out.println(" From Close -  AutoCloseable  ");
    }

    public void generalTest() {
        System.out.println(" GeneralTest ");
    }
}

Here is the output:

GeneralTest 
From Close -  AutoCloseable  
From Final Block

AutoCloseable (introduced in Java 7) makes it possible to use the try-with-resources idiom:

public class MyResource implements AutoCloseable {

    public void close() throws Exception {
        System.out.println("Closing!");
    }

}

Now you can say:

try (MyResource res = new MyResource()) {
    // use resource here
}

and JVM will call close() automatically for you.

Closeable is an older interface. For some reason To preserve backward compatibility, language designers decided to create a separate one. This allows not only all Closeable classes (like streams throwing IOException) to be used in try-with-resources, but also allows throwing more general checked exceptions from close().

When in doubt, use AutoCloseable, users of your class will be grateful.

The try-with-resources Statement.

The try-with-resources statement is a try statement that declares one or more resources. A resource is an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.

The following example reads the first line from a file. It uses an instance of BufferedReader to read data from the file. BufferedReader is a resource that must be closed after the program is finished with it:

static String readFirstLineFromFile(String path) throws IOException {
    try (BufferedReader br =
                   new BufferedReader(new FileReader(path))) {
        return br.readLine();
    }
}

In this example, the resource declared in the try-with-resources statement is a BufferedReader. The declaration statement appears within parentheses immediately after the try keyword. The class BufferedReader, in Java SE 7 and later, implements the interface java.lang.AutoCloseable. Because the BufferedReader instance is declared in a try-with-resource statement, it will be closed regardless of whether the try statement completes normally or abruptly (as a result of the method BufferedReader.readLine throwing an IOException).

Prior to Java SE 7, you can use a finally block to ensure that a resource is closed regardless of whether the try statement completes normally or abruptly. The following example uses a finally block instead of a try-with-resources statement:

static String readFirstLineFromFileWithFinallyBlock(String path)
                                                     throws IOException {
    BufferedReader br = new BufferedReader(new FileReader(path));
    try {
        return br.readLine();
    } finally {
        if (br != null) br.close();
    }

}

Please refer to the docs.

Recently I have read a Java SE 8 Programmer Guide ii Book.

I found something about the difference between AutoCloseable vs Closeable.

The AutoCloseable interface was introduced in Java 7. Before that, another interface existed called Closeable. It was similar to what the language designers wanted, with the following exceptions:

• Closeable restricts the type of exception thrown to IOException.
• Closeable requires implementations to be idempotent.

The language designers emphasize backward compatibility. Since changing the existing interface was undesirable, they made a new one called AutoCloseable. This new interface is less strict than Closeable. Since Closeable meets the requirements for AutoCloseable, it started implementing AutoCloseable when the latter was introduced.