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Java PushbackInputStream

Jakob Jenkov
Last update: 2019-09-28

The PushbackInputStream is intended to be used when you parse data from an InputStream. Sometimes you need to read ahead a few bytes to see what is coming, before you can determine how to interpret the current byte. The PushbackInputStream allows you to do that. Well, actually it allows you to push the read bytes back into the stream. These bytes will then be read again the next time you call read().

The Java PushbackInputStream is a subclass of the Java InputStream so it inherits its public methods - read(), close() etc. The PushbackInputStream is similar to the Java PushbackReader, except the PushbackInputStream reads raw bytes, and the PushbackReader reads characters (text).

PushbackInputStream Example

Here is a simple PushbackInputStream example:

PushbackInputStream input = new PushbackInputStream(
                                new FileInputStream("c:\\data\\input.txt"));

int data = input.read();

input.unread(data);

The call to read() reads a byte just like from an InputStream. The call to unread() pushes a byte back into the PushbackInputStream. The next time read() is called the pushed back bytes will be read first. If you push back multiple bytes into the PushbackInputStream, the latest byte pushed back will be returned first from read(), just like on a stack.

Create a PushbackInputStream

To use a Java PushbackInputStream you must first create a PushbackInputStream instance. Here is an example of creating a Java PushbackInputStream:

PushbackInputStream input = new PushbackInputStream(
                                new FileInputStream("c:\\data\\input.txt"));

Notice how the PushbackInputStream needs another InputStream as parameter to its constructor. The PushbackInputStream will read its bytes from this underlying InputStream.

Setting the Push Back Limit of a PushbackInputStream

You can set the number of bytes you should be able to unread in the constructor of the PushbackInputStream. Here is how to set the push back limit via the PushbackInputStream constructor:

int pushbackLimit = 8;
PushbackInputStream input = new PushbackInputStream(
                                new FileInputStream("c:\\data\\input.txt"),
                                pushbackLimit);

This example sets an internal buffer of 8 bytes. That means you can unread at most 8 bytes at a time, before reading them again.

Read Bytes

You read bytes from the Java PushbackInputStream via its read() method, just like you do from a regular InputStream . Here is an example of reading a byte from a PushbackInputStream :

int aByte = pushbackInputStream.read();

while(aByte != -1) {
    //do something with aByte
    byte byteRead = (byte) aByte;

    aByte = pushbackInputStream.read();
}

Notice, that once the PushbackInputStream has no more bytes to read, from its underlying PushbackInputStream, the read() method will return -1.

Push a Byte Back

To push a byte back into the Java PushbackInputStream you use the unread() method. Here is an example of pushing a byte back into a PushbackInputStream :

int aByte = pushbackInputStream.read();

pushbackInputStream.unread(aByte);

aByte = pushbackInputStream.read()

This example first reads a byte from the PushbackInputStream, then pushes it back into the PushbackInputStream, and then reads that byte back again with the final read() call.

Closing a PushbackInputStream

When you are finished reading bytes from a Java PushbackInputStream you should remember to close it. Closing a PushbackInputStream will also close the InputStream instance from which the PushbackInputStream is reading.

Closing a PushbackInputStream is done by calling its close() method. Here is how closing a PushbackInputStream looks:

pushbackInputStream.close();

You can also use the try-with-resources construct introduced in Java 7. Here is how to use and close a PushbackInputStream looks with the try-with-resources construct:

InputStream inputStream = new FileInputStream("data/data.bin");

try(PushbackInputStream pushbackInputStream =
    new PushbackInputStream(inputStream)){

    int data = pushbackInputStream.read();
    while(data != -1) {
        byte aByte = (byte) data;
        data = pushbackInputStream.read();
    }
}

Notice how there is no longer any explicit close() method call. The try-with-resources construct takes care of that.

Notice also that the first FileInputStream instance is not created inside the try-with-resources block. That means that the try-with-resources block will not automatically close this FileInputStream instance. However, when the PushbackInputStream is closed it will also close the InputStream instance it reads from, so the FileInputStream instance will get closed when the PushbackInputStream is closed.

Jakob Jenkov

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