Java - FileDescriptor valid() method

---

---

Description

The Java FileDescriptor valid() method checks whether the file descriptor is valid or invalid. It returns−

• true if the file descriptor is active and can be used.

• false if the file descriptor is invalid or closed.

Declaration

Following is the declaration for java.io.FileDescriptor.valid() method −

public boolean valid()

Parameters

NA

Return Value

The method returns true if the file descriptor object is valid, else the method returns false.

Exception

NA

Example - Usage of FileDescriptor valid() method

The following example shows the usage of Java FileDescriptor valid() method.

FileDescriptorDemo.java

package com.tutorialspoint;

import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.IOException;

public class FileDescriptorDemo {
   public static void main(String[] args) throws IOException {
      FileInputStream fis = null;
      FileDescriptor fd = null;
      boolean bool = false;   
      
      try {
         // create input stream
         fis = new FileInputStream("test.txt");
         
         // get file descriptor
         fd = fis.getFD();
         
         // tests file descriptor object's validity
         bool = fd.valid();
         
         // print
         System.out.print("is file descriptor valid?: "+bool);
         
      } catch(Exception e) {
         // if any error occurs
         e.printStackTrace();
      } finally {
         // releases systems resources
         if(fis!=null)
            fis.close();   
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

is file descriptor valid?: true

Example - Checking if a FileDescriptor is Valid

The following example shows the usage of Java FileDescriptor valid() method.

FileDescriptorDemo.java

package com.tutorialspoint;

import java.io.File;
import java.io.FileDescriptor;
import java.io.FileOutputStream;
import java.io.IOException;

public class FileDescriptorDemo {
   public static void main(String[] args) {
      try (FileOutputStream fos = new FileOutputStream(new File("valid_example.txt"))) {
         FileDescriptor fd = fos.getFD(); // Get the file descriptor

         // Check if the file descriptor is valid
         System.out.println("Is file descriptor valid? " + fd.valid()); // Expected: true
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Is file descriptor valid? true

Explanation

• A FileOutputStream is created for "valid_example.txt".

• The file's descriptor (FileDescriptor) is retrieved using getFD().

• The valid() method checks if the file descriptor is valid.

• Since the file is open, valid() returns true.

Example - Checking Validity After Closing the Stream

The following example shows the usage of Java FileDescriptor valid() method.

FileDescriptorDemo.java

package com.tutorialspoint;

import java.io.File;
import java.io.FileDescriptor;
import java.io.FileOutputStream;
import java.io.IOException;

public class FileDescriptorDemo {
   public static void main(String[] args) {
      FileDescriptor fd = null;

      try (FileOutputStream fos = new FileOutputStream(new File("invalid_example.txt"))) {
         fd = fos.getFD(); // Get the file descriptor

         // Check if the file descriptor is valid before closing
         System.out.println("Before closing, is file descriptor valid? " + fd.valid()); // Expected: true
      } catch (IOException e) {
         e.printStackTrace();
      }

      // Check if the file descriptor is valid after closing
      System.out.println("After closing, is file descriptor valid? " + fd.valid()); // Expected: false
   }
}

Output

Let us compile and run the above program, this will produce the following result−

Before closing, is file descriptor valid? True
After closing, is file descriptor valid? false

Explanation

• A FileOutputStream is created for "invalid_example.txt", and its FileDescriptor is retrieved.

• The valid() method checks if the file descriptor is valid before closing the stream (should return true).

• The try-with-resources block automatically closes the stream.

• After closing the stream, calling valid() again returns false because the file descriptor is no longer in use.