public abstract class

InputStream

extends Object
implements Closeable
java.lang.Object
   ↳ java.io.InputStream
Known Direct Subclasses
Known Indirect Subclasses

Class Overview

The base class for all input streams. An input stream is a means of reading data from a source in a byte-wise manner.

Some input streams also support marking a position in the input stream and returning to this position later. This abstract class does not provide a fully working implementation, so it needs to be subclassed, and at least the read() method needs to be overridden. Overriding some of the non-abstract methods is also often advised, since it might result in higher efficiency.

Many specialized input streams for purposes like reading from a file already exist in this package.

See Also

Summary

Public Constructors
InputStream()
This constructor does nothing.
Public Methods
int available()
Returns an estimated number of bytes that can be read or skipped without blocking for more input.
void close()
Closes this stream.
void mark(int readlimit)
Sets a mark position in this InputStream.
boolean markSupported()
Indicates whether this stream supports the mark() and reset() methods.
int read(byte[] b)
Reads bytes from this stream and stores them in the byte array b.
abstract int read()
Reads a single byte from this stream and returns it as an integer in the range from 0 to 255.
int read(byte[] buffer, int offset, int length)
Reads at most length bytes from this stream and stores them in the byte array b starting at offset.
synchronized void reset()
Resets this stream to the last marked location.
long skip(long byteCount)
Skips at most n bytes in this stream.
[Expand]
Inherited Methods
From class java.lang.Object
From interface java.io.Closeable

Public Constructors

public InputStream ()

Since: API Level 1

This constructor does nothing. It is provided for signature compatibility.

Public Methods

public int available ()

Since: API Level 1

Returns an estimated number of bytes that can be read or skipped without blocking for more input.

Note that this method provides such a weak guarantee that it is not very useful in practice.

Firstly, the guarantee is "without blocking for more input" rather than "without blocking": a read may still block waiting for I/O to complete — the guarantee is merely that it won't have to wait indefinitely for data to be written. The result of this method should not be used as a license to do I/O on a thread that shouldn't be blocked.

Secondly, the result is a conservative estimate and may be significantly smaller than the actual number of bytes available. In particular, an implementation that always returns 0 would be correct. In general, callers should only use this method if they'd be satisfied with treating the result as a boolean yes or no answer to the question "is there definitely data ready?".

Thirdly, the fact that a given number of bytes is "available" does not guarantee that a read or skip will actually read or skip that many bytes: they may read or skip fewer.

It is particularly important to realize that you must not use this method to size a container and assume that you can read the entirety of the stream without needing to resize the container. Such callers should probably write everything they read to a ByteArrayOutputStream and convert that to a byte array. Alternatively, if you're reading from a file, length() returns the current length of the file (though assuming the file's length can't change may be incorrect, reading a file is inherently racy).

The default implementation of this method in InputStream always returns 0. Subclasses should override this method if they are able to indicate the number of bytes available.

Returns
  • the estimated number of bytes available
Throws
IOException if this stream is closed or an error occurs

public void close ()

Since: API Level 1

Closes this stream. Concrete implementations of this class should free any resources during close. This implementation does nothing.

Throws
IOException if an error occurs while closing this stream.

public void mark (int readlimit)

Since: API Level 1

Sets a mark position in this InputStream. The parameter readlimit indicates how many bytes can be read before the mark is invalidated. Sending reset() will reposition the stream back to the marked position provided readLimit has not been surpassed.

This default implementation does nothing and concrete subclasses must provide their own implementation.

Parameters
readlimit the number of bytes that can be read from this stream before the mark is invalidated.
See Also

public boolean markSupported ()

Since: API Level 1

Indicates whether this stream supports the mark() and reset() methods. The default implementation returns false.

Returns
  • always false.
See Also

public int read (byte[] b)

Since: API Level 1

Reads bytes from this stream and stores them in the byte array b.

Parameters
b the byte array in which to store the bytes read.
Returns
  • the number of bytes actually read or -1 if the end of the stream has been reached.
Throws
IOException if this stream is closed or another IOException occurs.

public abstract int read ()

Since: API Level 1

Reads a single byte from this stream and returns it as an integer in the range from 0 to 255. Returns -1 if the end of the stream has been reached. Blocks until one byte has been read, the end of the source stream is detected or an exception is thrown.

Returns
  • the byte read or -1 if the end of stream has been reached.
Throws
IOException if the stream is closed or another IOException occurs.

public int read (byte[] buffer, int offset, int length)

Since: API Level 1

Reads at most length bytes from this stream and stores them in the byte array b starting at offset.

Parameters
buffer the byte array in which to store the bytes read.
offset the initial position in buffer to store the bytes read from this stream.
length the maximum number of bytes to store in b.
Returns
  • the number of bytes actually read or -1 if the end of the stream has been reached.
Throws
IndexOutOfBoundsException if offset < 0 or length < 0, or if offset + length is greater than the length of b.
IOException if the stream is closed or another IOException occurs.

public synchronized void reset ()

Since: API Level 1

Resets this stream to the last marked location. Throws an IOException if the number of bytes read since the mark has been set is greater than the limit provided to mark, or if no mark has been set.

This implementation always throws an IOException and concrete subclasses should provide the proper implementation.

Throws
IOException if this stream is closed or another IOException occurs.

public long skip (long byteCount)

Since: API Level 1

Skips at most n bytes in this stream. This method does nothing and returns 0 if n is negative, but some subclasses may throw.

Note the "at most" in the description of this method: this method may choose to skip fewer bytes than requested. Callers should always check the return value.

This default implementation reads bytes into a temporary buffer. Concrete subclasses should provide their own implementation.

Parameters
byteCount the number of bytes to skip.
Returns
  • the number of bytes actually skipped.
Throws
IOException if this stream is closed or another IOException occurs.