implements Closeable or implements AutoCloseable

Question

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

It seems to me that you are not very familiar with interfaces  In the code you have posted  you don t need to implement AutoCloseable   You only have to  or should  implement Closeable or AutoCloseable if you are about to implement your own PrintWriter  which handles files or any other resources which needs to be closed   In your implementation  it is enough to call pw close    You should do this in a finally block   PrintWriter pw   null  try      File file   new File  C   test txt       pw   new PrintWriter file     catch  IOException e       System out println  bad things happen      finally      if  pw    null          try            pw close            catch  IOException e                     The code above is Java 6 related  In Java 7 this can be done more elegantly  see this answer

User · Answer

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

User · Answer

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

[java] implements Closeable or implements AutoCloseable

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