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Java IO: InputStream

Jakob Jenkov
Last update: 2018-09-08

The Java InputStream class represents an ordered stream of bytes. In other words, you can read data from a Java InputStream as an ordered sequence of bytes. This is useful when reading data from a file, or received over the network.

The Java InputStream class is the base class (superclass) of all input streams in the Java IO API. InputStream Subclasses include the FileInputStream, BufferedInputStream and the PushbackInputStream among others. To see a full list of InputStream subclasses, go to the bottom table of the Java IO Overview page.

InputStreams and Sources

A Java InputStream is typically connected to some data source, like a file, network connection, pipe etc. This is also explained in more detail in the Java IO Overview text.

Java InputStream Example

Java InputStream's are used for reading byte based data, one byte at a time. Here is a Java InputStream example which reads all the bytes from a file:

InputStream inputstream = new FileInputStream("c:\\data\\input-text.txt");

int data = inputstream.read();
while(data != -1) {
  //do something with data...
  doSomethingWithData(data);

  data = inputstream.read();
}
inputstream.close();

This example creates a new FileInputStream instance. FileInputStream is a subclass of InputStream so it is safe to assign an instance of FileInputStream to an InputStream variable (the inputstream variable).

read()

The read() method of an InputStream returns an int which contains the byte value of the byte read. Here is an InputStream read() example:

int data = inputstream.read();

You can cast the returned int to a char like this:

char aChar = (char) data;

Subclasses of InputStream may have alternative read() methods. For instance, the DataInputStream allows you to read Java primitives like int, long, float, double, boolean etc. with its corresponding methods readBoolean(), readDouble() etc.

End of Stream

If the read() method returns -1, the end of stream has been reached, meaning there is no more data to read in the InputStream. That is, -1 as int value, not -1 as byte or short value. There is a difference here!

When the end of stream has been reached, you can close the InputStream.

read(byte[])

The InputStream class also contains two read() methods which can read data from the InputStream's source into a byte array. These methods are:

  • int read(byte[])
  • int read(byte[], int offset, int length)

Reading an array of bytes at a time is much faster than reading one byte at a time, so when you can, use these read methods instead of the read() method.

The read(byte[]) method will attempt to read as many bytes into the byte array given as parameter as the array has space for. The read(byte[]) method returns an int telling how many bytes were actually read. In case less bytes could be read from the InputStream than the byte array has space for, the rest of the byte array will contain the same data as it did before the read started. Remember to inspect the returned int to see how many bytes were actually read into the byte array.

The read(byte[], int offset, int length) method also reads bytes into a byte array, but starts at offset bytes into the array, and reads a maximum of length bytes into the array from that position. Again, the read(byte[], int offset, int length) method returns an int telling how many bytes were actually read into the array, so remember to check this value before processing the read bytes.

For both methods, if the end of stream has been reached, the method returns -1 as the number of bytes read.

Here is an example of how it could looke to use the InputStream's read(byte[]) method:

InputStream inputstream = new FileInputStream("c:\\data\\input-text.txt");

byte[] data      = new byte[1024];
int    bytesRead = inputstream.read(data);

while(bytesRead != -1) {
  doSomethingWithData(data, bytesRead);

  bytesRead = inputstream.read(data);
}
inputstream.close();

First this example create a byte array. Then it creates an int variable named bytesRead to hold the number of bytes read for each read(byte[]) call, and immediately assigns bytesRead the value returned from the first read(byte[]) call.

Inside the while loop the doSomethingWithData() method is called, passing along the data byte array as well as how many bytes were read into the array as parameters. At the end of the while loop data is read into the byte array again.

It should not take much imagination to figure out how to use the read(byte[], int offset, int length) method instead of read(byte[]). You pretty much just replace the read(byte[]) calls with read(byte[], int offset, int length) calls.

mark() and reset()

The InputStream class has two methods called mark() and reset() which subclasses of InputStream may or may not support.

If an InputStream subclass supports the mark() and reset() methods, then that subclass should override the markSupported() to return true. If the markSupported() method returns false then mark() and reset() are not supported.

The mark() sets a mark internally in the InputStream which marks the point in the stream to which data has been read so far. The code using the InputStream can then continue reading data from it. If the code using the InputStream wants to go back to the point in the stream where the mark was set, the code calls reset() on the InputStream. The InputStream then "rewinds" and go back to the mark, and start returning (reading) data from that point again. This will of course result in some data being returned more than once from the InputStream.

The methods mark() and reset() methods are typically used when implementing parsers. Sometimes a parser may need to read ahead in the InputStream and if the parser doesn't find what it expected, it may need to rewind back and try to match the read data against something else.

Closing an InputStream

When you are done with a Java InputStream you must close it. You close an InputStream by calling the InputStream close() method. Here is an example of opening an InputStream, reading all data from it, and then closing it:

InputStream inputstream = new FileInputStream("c:\\data\\input-text.txt");

int data = inputstream.read();
while(data != -1) {
  data = inputstream.read();
}
inputstream.close();

Notice how the while loop continues until a -1 value is read from the InputStream read() method. After that, the while loop exits, and the InputStream close() method is called.

The above code is not 100% robust. If an exception is thrown while reading data from the InputStream, the close() method is never called. To make the code more robust, you will have to use the Java try-with-resources construct. Proper exception handling for use of Java IO classes is also explained in my tutorial on Java IO Exception Handling.

Here is an example of closing a Java InputStream using the try-with-resources construct:

try( InputStream inputstream = new FileInputStream("file.txt") ) {

    int data = inputstream.read();
    while(data != -1){
        data = inputstream.read();
    }
}

Notice how the InputStream is now declared inside the parantheses after the try keyword. This signals to Java that this InputStream is to be managed by the try-with-resources construct.

Once the executing thread exits the try block, the inputstream variable is closed. If an exception is thrown from inside the try block, the exception is caught, the InputStream is closed, and then the exception is rethrown. You are thus guaranteed that the InputStream is closed, when used insde a try-with-resources block.

Jakob Jenkov




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