org.apache.lucene.store

## Class DataInput

- java.lang.Object
- - org.apache.lucene.store.DataInput

- All Implemented Interfaces:
    Cloneable
  Direct Known Subclasses:
    ByteArrayDataInput, ByteBuffersDataInput, FST.BytesReader, IndexInput, InputStreamDataInput, PagedBytes.PagedBytesDataInput

      public abstract class DataInput
      extends Object
      implements Cloneable

  Abstract base class for performing read operations of Lucene's low-level data types.

  `DataInput` may only be used from one thread, because it is not thread safe (it keeps internal state like file position). To allow multithreaded use, every `DataInput` instance must be cloned before used in another thread. Subclasses must therefore implement `clone()`, returning a new `DataInput` which operates on the same underlying resource, but positioned independently.

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `DataInput()`

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `DataInput` | `clone()` Returns a clone of this stream.
    `abstract byte` | `readByte()` Reads and returns a single byte.
    `abstract void` | `readBytes(byte[] b, int offset, int len)` Reads a specified number of bytes into an array at the specified offset.
    `void` | `readBytes(byte[] b, int offset, int len, boolean useBuffer)` Reads a specified number of bytes into an array at the specified offset with control over whether the read should be buffered (callers who have their own buffer should pass in "false" for useBuffer).
    `int` | `readInt()` Reads four bytes and returns an int.
    `void` | `readLELongs(long[] dst, int offset, int length)` Read a specified number of longs with the little endian byte order.
    `long` | `readLong()` Reads eight bytes and returns a long.
    `Map<String,String>` | `readMapOfStrings()` Reads a Map<String,String> previously written with `DataOutput.writeMapOfStrings(Map)`.
    `Set<String>` | `readSetOfStrings()` Reads a Set<String> previously written with `DataOutput.writeSetOfStrings(Set)`.
    `short` | `readShort()` Reads two bytes and returns a short.
    `String` | `readString()` Reads a string.
    `int` | `readVInt()` Reads an int stored in variable-length format.
    `long` | `readVLong()` Reads a long stored in variable-length format.
    `int` | `readZInt()` Read a `zig-zag`-encoded `variable-length` integer.
    `long` | `readZLong()` Read a `zig-zag`-encoded `variable-length` integer.
    `void` | `skipBytes(long numBytes)` Skip over `numBytes` bytes.

    - ### Methods inherited from class java.lang.Object

      `equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### DataInput

          public DataInput()

  - ### Method Detail

    - #### readByte

          public abstract byte readByte()
                                 throws IOException

      Reads and returns a single byte.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeByte(byte)`

    - #### readBytes

          public abstract void readBytes(byte[] b,
                                         int offset,
                                         int len)
                                  throws IOException

      Reads a specified number of bytes into an array at the specified offset.
      Parameters:
        `b` - the array to read bytes into
        `offset` - the offset in the array to start storing bytes
        `len` - the number of bytes to read
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeBytes(byte[],int)`

    - #### readBytes

          public void readBytes(byte[] b,
                                int offset,
                                int len,
                                boolean useBuffer)
                         throws IOException

      Reads a specified number of bytes into an array at the specified offset with control over whether the read should be buffered (callers who have their own buffer should pass in "false" for useBuffer). Currently only `BufferedIndexInput` respects this parameter.
      Parameters:
        `b` - the array to read bytes into
        `offset` - the offset in the array to start storing bytes
        `len` - the number of bytes to read
        `useBuffer` - set to false if the caller will handle buffering.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeBytes(byte[],int)`

    - #### readShort

          public short readShort()
                          throws IOException

      Reads two bytes and returns a short.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeByte(byte)`

    - #### readInt

          public int readInt()
                      throws IOException

      Reads four bytes and returns an int.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeInt(int)`

    - #### readVInt

          public int readVInt()
                       throws IOException

      Reads an int stored in variable-length format. Reads between one and five bytes. Smaller values take fewer bytes. Negative numbers are supported, but should be avoided.

      The format is described further in `DataOutput.writeVInt(int)`.

      Throws:
        `IOException`
      See Also:
        `DataOutput.writeVInt(int)`

    - #### readZInt

          public int readZInt()
                       throws IOException

      Read a `zig-zag`-encoded `variable-length` integer.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeZInt(int)`

    - #### readLong

          public long readLong()
                        throws IOException

      Reads eight bytes and returns a long.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeLong(long)`

    - #### readLELongs

          public void readLELongs(long[] dst,
                                  int offset,
                                  int length)
                           throws IOException

      Read a specified number of longs with the little endian byte order.

      This method can be used to read longs whose bytes have been `reversed` at write time:

           for (long l : longs) {
             output.writeLong(Long.reverseBytes(l));
           }

      Throws:
        `IOException`
      WARNING: This API is experimental and might change in incompatible ways in the next release.

    - #### readVLong

          public long readVLong()
                         throws IOException

      Reads a long stored in variable-length format. Reads between one and nine bytes. Smaller values take fewer bytes. Negative numbers are not supported.

      The format is described further in `DataOutput.writeVInt(int)`.

      Throws:
        `IOException`
      See Also:
        `DataOutput.writeVLong(long)`

    - #### readZLong

          public long readZLong()
                         throws IOException

      Read a `zig-zag`-encoded `variable-length` integer. Reads between one and ten bytes.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeZLong(long)`

    - #### readString

          public String readString()
                            throws IOException

      Reads a string.
      Throws:
        `IOException`
      See Also:
        `DataOutput.writeString(String)`

    - #### clone

          public DataInput clone()

      Returns a clone of this stream.

      Clones of a stream access the same data, and are positioned at the same point as the stream they were cloned from.

      Expert: Subclasses must ensure that clones may be positioned at different points in the input from each other and from the stream they were cloned from.

      Overrides:
        `clone` in class `Object`

    - #### readMapOfStrings

          public Map<String,String> readMapOfStrings()
                                              throws IOException

      Reads a Map<String,String> previously written with `DataOutput.writeMapOfStrings(Map)`.
      Returns:
        An immutable map containing the written contents.
      Throws:
        `IOException`

    - #### readSetOfStrings

          public Set<String> readSetOfStrings()
                                       throws IOException

      Reads a Set<String> previously written with `DataOutput.writeSetOfStrings(Set)`.
      Returns:
        An immutable set containing the written contents.
      Throws:
        `IOException`

    - #### skipBytes

          public void skipBytes(long numBytes)
                         throws IOException

      Skip over `numBytes` bytes. The contract on this method is that it should have the same behavior as reading the same number of bytes into a buffer and discarding its content. Negative values of `numBytes` are not supported.
      Throws:
        `IOException`

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