# InputStreams and OutputStreams

# Closing Streams

Most streams must be closed when you are done with them, otherwise you could introduce a memory leak or leave a file open. It is important that streams are closed even if an exception is thrown.

try(FileWriter fw = new FileWriter("outfilename");
    BufferedWriter bw = new BufferedWriter(fw);
    PrintWriter out = new PrintWriter(bw))
{
    out.println("the text");
    //more code
    out.println("more text");
    //more code
} catch (IOException e) {
    //handle this however you 
}

Remember: try-with-resources guarantees, that the resources have been closed when the block is exited, whether that happens with the usual control flow or because of an exception.

Sometimes, try-with-resources is not an option, or maybe you're supporting older version of Java 6 or earlier. In this case, proper handling is to use a finally block:

FileWriter fw = null;
BufferedWriter bw = null;
PrintWriter out = null;
try {
    fw = new FileWriter("myfile.txt");
    bw = new BufferedWriter(fw);
    out = new PrintWriter(bw);
    out.println("the text");
    out.close();
} catch (IOException e) {
    //handle this however you want
}
finally {
    try {
        if(out != null)
            out.close();
    } catch (IOException e) {
        //typically not much you can do here...
    }
}

Note that closing a wrapper stream will also close its underlying stream. This means you cannot wrap a stream, close the wrapper and then continue using the original stream.

# Reading InputStream into a String

Sometimes you may wish to read byte-input into a String. To do this you will need to find something that converts between byte and the "native Java" UTF-16 Codepoints used as char. That is done with a InputStreamReader (opens new window).

To speed the process up a bit, it's "usual" to allocate a buffer, so that we don't have too much overhead when reading from Input.

public String inputStreamToString(InputStream inputStream) throws Exception {
     StringWriter writer = new StringWriter();

     char[] buffer = new char[1024];
     try (Reader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"))) {
           int n;
           while ((n = reader.read(buffer)) != -1) {
                // all this code does is redirect the output of `reader` to `writer` in
                // 1024 byte chunks
                writer.write(buffer, 0, n);
           }
     }
     return writer.toString();
}

Transforming this example to Java SE 6 (and lower)-compatible code is left out as an exercise for the reader.

# Wrapping Input/Output Streams

OutputStream and InputStream have many different classes, each of them with a unique functionality. By wrapping a stream around another, you gain the functionality of both streams.

You can wrap a stream any number of times, just take note of the ordering.

# Useful combinations

Writing characters to a file while using a buffer

File myFile = new File("targetFile.txt");
PrintWriter writer = new PrintWriter(new BufferedOutputStream(new FileOutputStream(myFile)));

Compressing and encrypting data before writing to a file while using a buffer

Cipher cipher = ... // Initialize cipher
File myFile = new File("targetFile.enc");
BufferedOutputStream outputStream = new BufferedOutputStream(new DeflaterOutputStream(new CipherOutputStream(new FileOutputStream(myFile), cipher)));

# List of Input/Output Stream wrappers

Wrapper	Description
BufferedOutputStream/ BufferedInputStream	While `OutputStream` writes data one byte at a time, `BufferedOutputStream` writes data in chunks. This reduces the number of system calls, thus improving performance.
DeflaterOutputStream/ DeflaterInputStream	Performs data compression.
InflaterOutputStream/ InflaterInputStream	Performs data decompression.
CipherOutputStream/ CipherInputStream	Encrypts/Decrypts data.
DigestOutputStream/ DigestInputStream	Generates Message Digest to verify data integrity.
CheckedOutputStream/ CheckedInputStream	Generates a CheckSum. CheckSum is a more trivial version of Message Digest.
DataOutputStream/ DataInputStream	Allows writing of primitive data types and Strings. Meant for writing bytes. Platform independent.
PrintStream	Allows writing of primitive data types and Strings. Meant for writing bytes. Platform dependent.
OutputStreamWriter	Converts a OutputStream into a Writer. An OutputStream deals with bytes while Writers deals with characters
PrintWriter	Automatically calls OutputStreamWriter. Allows writing of primitive data types and Strings. Strictly for writing characters and best for writing characters

# Writing bytes to an OutputStream

Writing bytes to an OutputStream one byte at a time

OutputStream stream = object.getOutputStream();

byte b = 0x00;
stream.write( b );

Writing a byte array

byte[] bytes = new byte[] { 0x00, 0x00 };

stream.write( bytes );

Writing a section of a byte array

int offset = 1;
int length = 2;
byte[] bytes = new byte[] { 0xFF, 0x00, 0x00, 0xFF };

stream.write( bytes, offset, length );

# Copying Input Stream to Output Stream

This function copies data between two streams -

void copy(InputStream in, OutputStream out) throws IOException {
    byte[] buffer = new byte[8192];
    while ((bytesRead = in.read(buffer)) > 0) {
        out.write(buffer, 0, bytesRead);
    }
}

Example -


// reading from System.in and writing to System.out
copy(System.in, System.out);

# DataInputStream Example

package com.streams;  
import java.io.*;    
public class DataStreamDemo {  
  public static void main(String[] args) throws IOException {  
    InputStream input = new FileInputStream("D:\\datastreamdemo.txt");  
    DataInputStream inst = new DataInputStream(input);  
    int count = input.available();  
    byte[] arr = new byte[count];  
    inst.read(arr);  
    for (byte byt : arr) {  
      char ki = (char) byt;  
      System.out.print(ki+"-");  
    }  
  }  
}

# Syntax

int read(byte[] b) throws IOException

# Remarks

Note that most of the time you do NOT use InputStreams directly but use BufferedStreams, or similar. This is because InputStream reads from the source every time the read method is called. This can cause significant CPU usage in context switches into and out of the kernel.

← Streams Readers and Writers →