Java - ObjectStreamWriter flush() method

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Description

The Java ObjectStreamWriter flush() method flushes the stream.

Declaration

Following is the declaration for java.io.ObjectStreamWriter.flush() method.

public void flush()

Parameters

NA

Return Value

This method does not return a value.

Exception

IOException − If an I/O error occurs.

Example - Usage of ObjectStreamWriter flush() method

The following example shows the usage of ObjectStreamWriter flush() method.

OutputStreamWriterDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;

public class OutputStreamWriterDemo {
   public static void main(String[] args) {
      try {
         // create a new OutputStreamWriter
         OutputStream os = new FileOutputStream("test.txt");
         OutputStreamWriter writer = new OutputStreamWriter(os);

         // create a new FileInputStream to read what we write
         FileInputStream in = new FileInputStream("test.txt");

         // write something in the file
         writer.write(70);

         // flush the stream
         System.out.println("Flushing Stream...");
         writer.flush();
         System.out.println("Stream flushed.");

         // read what we write
         System.out.println("" + (char) in.read());

      } catch (Exception ex) {
         ex.printStackTrace();
      }
   }
}

Output

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

Flushing Stream...
Stream flushed.
F

Example - Using flush() to force data write before closing

The following example shows the usage of ObjectStreamWriter flush() method.

OutputStreamWriterDemo.java

package com.tutorialspoint;

import java.io.FileOutputStream;
import java.io.OutputStreamWriter;
import java.io.IOException;

public class OutputStreamWriterDemo {
   public static void main(String[] args) {
      try {
         FileOutputStream fos = new FileOutputStream("flush_output1.txt");
         OutputStreamWriter writer = new OutputStreamWriter(fos);

         writer.write("Flush ensures data is written immediately.");
         writer.flush();  // Forces characters to be written to file

         // Writer is still open here, more data could be written
         System.out.println("Data flushed to file.");
         writer.close();  // Finally close the stream
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

Data flushed to file.

Explanation

• write() writes to an internal buffer.

• flush() pushes that buffered data to the underlying FileOutputStream immediately, without closing the stream.

• Useful when you want to make sure data is written while keeping the stream open for more output later.

Example - Flushing data before waiting for user input

The following example shows the usage of ObjectStreamWriter flush() method.

OutputStreamWriterDemo.java

package com.tutorialspoint;

import java.io.OutputStreamWriter;
import java.io.IOException;
import java.util.Scanner;

public class OutputStreamWriterDemo {
   public static void main(String[] args) {
      try (
         OutputStreamWriter writer = new OutputStreamWriter(System.out);
         Scanner scanner = new Scanner(System.in)
      ) {
         writer.write("Enter your name: ");
         writer.flush();  // Ensure prompt is shown immediately

         String name = scanner.nextLine();

         writer.write("Hello, " + name + "! Welcome to OutputStreamWriter.\n");
         writer.flush();  // Flush greeting to console

      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output

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

Enter your name: Anand
Hello, Anand! Welcome to OutputStreamWriter.

Explanation

• This writes the prompt to System.out, which is a PrintStream wrapped by OutputStreamWriter.

• Without flush(), the prompt might be delayed or buffered, especially in buffered output streams.

• flush() ensures that the user sees the prompt right away.