Buffer reading in java

Class Buffer

A buffer is a linear, finite sequence of elements of a specific primitive type. Aside from its content, the essential properties of a buffer are its capacity, limit, and position:

A buffer’s capacity is the number of elements it contains. The capacity of a buffer is never negative and never changes.

A buffer’s limit is the index of the first element that should not be read or written. A buffer’s limit is never negative and is never greater than its capacity.

A buffer’s position is the index of the next element to be read or written. A buffer’s position is never negative and is never greater than its limit.

There is one subclass of this class for each non-boolean primitive type.

Transferring data

Each subclass of this class defines two categories of get and put operations:

Relative operations read or write one or more elements starting at the current position and then increment the position by the number of elements transferred. If the requested transfer exceeds the limit then a relative get operation throws a BufferUnderflowException and a relative put operation throws a BufferOverflowException ; in either case, no data is transferred.

Absolute operations take an explicit element index and do not affect the position. Absolute get and put operations throw an IndexOutOfBoundsException if the index argument exceeds the limit.

Data may also, of course, be transferred in to or out of a buffer by the I/O operations of an appropriate channel, which are always relative to the current position.

Marking and resetting

A buffer’s mark is the index to which its position will be reset when the reset method is invoked. The mark is not always defined, but when it is defined it is never negative and is never greater than the position. If the mark is defined then it is discarded when the position or the limit is adjusted to a value smaller than the mark. If the mark is not defined then invoking the reset method causes an InvalidMarkException to be thrown.

Invariants

The following invariant holds for the mark, position, limit, and capacity values:

A newly-created buffer always has a position of zero and a mark that is undefined. The initial limit may be zero, or it may be some other value that depends upon the type of the buffer and the manner in which it is constructed. Each element of a newly-allocated buffer is initialized to zero.

Additional operations

• clear() makes a buffer ready for a new sequence of channel-read or relative put operations: It sets the limit to the capacity and the position to zero.
• flip() makes a buffer ready for a new sequence of channel-write or relative get operations: It sets the limit to the current position and then sets the position to zero.
• rewind() makes a buffer ready for re-reading the data that it already contains: It leaves the limit unchanged and sets the position to zero.
• The slice() and slice(index,length) methods create a subsequence of a buffer: They leave the limit and the position unchanged.
• duplicate() creates a shallow copy of a buffer: It leaves the limit and the position unchanged.

Read-only buffers

Every buffer is readable, but not every buffer is writable. The mutation methods of each buffer class are specified as optional operations that will throw a ReadOnlyBufferException when invoked upon a read-only buffer. A read-only buffer does not allow its content to be changed, but its mark, position, and limit values are mutable. Whether or not a buffer is read-only may be determined by invoking its isReadOnly method.

Thread safety

Buffers are not safe for use by multiple concurrent threads. If a buffer is to be used by more than one thread then access to the buffer should be controlled by appropriate synchronization.

Invocation chaining

Methods in this class that do not otherwise have a value to return are specified to return the buffer upon which they are invoked. This allows method invocations to be chained; for example, the sequence of statements

b.flip(); b.position(23); b.limit(42);

Method Summary

Returns the offset within this buffer’s backing array of the first element of the buffer (optional operation).

Methods declared in class java.lang.Object

Method Details

capacity

position

position

Sets this buffer’s position. If the mark is defined and larger than the new position then it is discarded.

limit

limit

Sets this buffer’s limit. If the position is larger than the new limit then it is set to the new limit. If the mark is defined and larger than the new limit then it is discarded.

mark

reset

Resets this buffer’s position to the previously-marked position. Invoking this method neither changes nor discards the mark’s value.

clear

Clears this buffer. The position is set to zero, the limit is set to the capacity, and the mark is discarded. Invoke this method before using a sequence of channel-read or put operations to fill this buffer. For example:

buf.clear(); // Prepare buffer for reading in.read(buf); // Read data

This method does not actually erase the data in the buffer, but it is named as if it did because it will most often be used in situations in which that might as well be the case.

flip

Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded. After a sequence of channel-read or put operations, invoke this method to prepare for a sequence of channel-write or relative get operations. For example:

buf.put(magic); // Prepend header in.read(buf); // Read data into rest of buffer buf.flip(); // Flip buffer out.write(buf); // Write header + data to channel

This method is often used in conjunction with the compact method when transferring data from one place to another.

rewind

Rewinds this buffer. The position is set to zero and the mark is discarded. Invoke this method before a sequence of channel-write or get operations, assuming that the limit has already been set appropriately. For example:

out.write(buf); // Write remaining data buf.rewind(); // Rewind buffer buf.get(array); // Copy data into array

remaining

hasRemaining

isReadOnly

hasArray

Tells whether or not this buffer is backed by an accessible array. If this method returns true then the array and arrayOffset methods may safely be invoked.

array

Returns the array that backs this buffer (optional operation). This method is intended to allow array-backed buffers to be passed to native code more efficiently. Concrete subclasses provide more strongly-typed return values for this method. Modifications to this buffer’s content will cause the returned array’s content to be modified, and vice versa. Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.

arrayOffset

Returns the offset within this buffer’s backing array of the first element of the buffer (optional operation). If this buffer is backed by an array then buffer position p corresponds to array index p + arrayOffset() . Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.

isDirect

slice

Creates a new buffer whose content is a shared subsequence of this buffer’s content. The content of the new buffer will start at this buffer’s current position. Changes to this buffer’s content will be visible in the new buffer, and vice versa; the two buffers’ position, limit, and mark values will be independent. The new buffer’s position will be zero, its capacity and its limit will be the number of elements remaining in this buffer, its mark will be undefined. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

slice

Creates a new buffer whose content is a shared subsequence of this buffer’s content. The content of the new buffer will start at position index in this buffer, and will contain length elements. Changes to this buffer’s content will be visible in the new buffer, and vice versa; the two buffers’ position, limit, and mark values will be independent. The new buffer’s position will be zero, its capacity and its limit will be length , its mark will be undefined. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

duplicate

Creates a new buffer that shares this buffer’s content. The content of the new buffer will be that of this buffer. Changes to this buffer’s content will be visible in the new buffer, and vice versa; the two buffers’ position, limit, and mark values will be independent. The new buffer’s capacity, limit, position and mark values will be identical to those of this buffer. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

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