Java - FilterInputStream read(byte[] b, int off, int len) method

---

---

Description

The Java FilterInputStream read(byte[] b, int off, int len) method reads up to len of data from this filter input stream into a buffer.

Key Points

• Reads up to len bytes and stores them in b[] starting from off index.

• Returns the number of bytes actually read (could be less than len).

• Returns -1 if the end of the file (EOF) is reached.

• Efficient for reading large files in chunks.

Declaration

Following is the declaration for java.io.FilterInputStream.read(byte[] b, int off, int len) method −

public int read(byte[] b, int off, int len)

Parameters

• b − The destination buffer.

• off − The start offset in the destination buffer.

• len − The maximum number of bytes read.

Return Value

The method returns the total number of bytes read into the buffer, or -1 if there is no more data to read.

Exception

• IOException − If an I/O error occurs.

• IndexOutOfBoundsException − If len is greater that b.length-off, if off is negative, or if len is negative.

• NullPointerException − If buffer b is null.

Example - Usage of FilterInputStream read(byte[] b, int off, int len) method

The following example shows the usage of Java FilterInputStream read(byte[] b, int off, int len) method.

FilterInputStreamDemo.java

package com.tutorialspoint;

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;

public class FilterInputStreamDemo {
   public static void main(String[] args) throws Exception {
      InputStream is = null; 
      FilterInputStream fis = null;
      int i = 0;
      char c;
      byte[] buffer = new byte[6];
      
      try {
         // create input streams
         is = new FileInputStream("test.txt");
         fis = new BufferedInputStream(is);
         
         // returns number of bytes read to buffer
         i = fis.read(buffer, 2,4);
         
         // prints
         System.out.println("Number of bytes read: "+i);
         
         // for each byte in buffer
         for(byte b:buffer) {
         
            // converts byte to character
            c = (char)b;
            
            // if byte is null
            if(b == 0)
               c = '-';
            // prints
            System.out.println("Char read from buffer b: "+c);
         }
         
      } catch(IOException e) {
         // if any I/O error occurs
         e.printStackTrace();
      } finally {
         // releases any system resources associated with the stream
         if(is!=null)
            is.close();
         if(fis!=null)
            fis.close();
      }
   }
}

Output(assuming test.txt contains ABCDEF)

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

Number of bytes read: 4
Char read from buffer b: -
Char read from buffer b: -
Char read from buffer b: A
Char read from buffer b: B
Char read from buffer b: C
Char read from buffer b: D

Example - Reading into a Byte Array with an Offset Using BufferedInputStream

The following example shows the usage of Java FilterInputStream read(byte[] b, int off, int len) method.

FilterInputStreamDemo.java

package com.tutorialspoint;

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FilterInputStream;
import java.io.IOException;

public class FilterInputStreamDemo {
   public static void main(String[] args) {
      try (FilterInputStream fis = new BufferedInputStream(new FileInputStream("example.txt"))) {
         byte[] buffer = new byte[10]; // Create a buffer of 10 bytes
         int bytesRead = fis.read(buffer, 2, 5); // Read 5 bytes starting from index 2

         if (bytesRead != -1) {
            System.out.println("Bytes read: " + bytesRead);
            System.out.println("Buffer content: " + new String(buffer)); // Print full buffer
         }
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output(assuming example.txt contains HelloWorld)

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

Bytes read: 5
Buffer content:   Hello

Explanation

• Uses BufferedInputStream, which is a FilterInputStream subclass.

• Reads 5 bytes from the file and stores them starting at index 2 in the buffer array.

• Converts and prints the entire buff.

Example - Reading a File in Chunks Using PushbackInputStream

The following example shows the usage of Java FilterInputStream read(byte[] b, int off, int len) method.

FilterInputStreamDemo.java

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.PushbackInputStream;

public class FilterInputStreamDemo {
   public static void main(String[] args) {
      try (FilterInputStream fis = new PushbackInputStream(new FileInputStream("example.txt"))) {
         byte[] buffer = new byte[10]; // Buffer of size 10
         int bytesRead;

         while ((bytesRead = fis.read(buffer, 3, 4)) != -1) { // Read 4 bytes at index 3
            System.out.println("Bytes read: " + bytesRead);
            System.out.println("Buffer content: " + new String(buffer)); // Print full buffer
         }
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

Output(assuming example.txt contains Microservices)

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

Bytes read: 4
Buffer content:    Micr
Bytes read: 4
Buffer content:    ices

Explanation

• Uses PushbackInputStream, another FilterInputStream subclass.

• Reads 4 bytes at a time and stores them starting at index 3 in buffer.

• Loop continues until EOF (-1 is returned).

• Prints entire buffer each time.