WritableComparator (ODPS SDK Commons for Mapreduce/Graph 0.48.7-public API)

java.lang.Object
- com.aliyun.odps.io.WritableComparator

All Implemented Interfaces:

RawComparator, Comparator

Direct Known Subclasses:

BigDecimalWritable.Comparator, BooleanWritable.Comparator, BytesWritable.Comparator, ByteWritable.Comparator, DatetimeWritable.Comparator, DateWritable.Comparator, DoubleWritable.Comparator, FloatWritable.Comparator, IntervalDayTimeWritable.Comparator, IntervalYearMonthWritable.Comparator, IntWritable.Comparator, LongWritable.Comparator, NullWritable.Comparator, ShortWritable.Comparator, Text.Comparator, TimestampNtzWritable.Comparator, TimestampWritable.Comparator, TupleReaderWriter.TupleRawComparator
```
public class WritableComparator
extends Object
implements RawComparator
```
WritableComparator 提供对 WritableComparable 对象的通用比较函数.
通用比较函数使用对象的自然顺序进行排序，如果用户需要自定义对象顺序，则需要重载 compare(WritableComparable, WritableComparable)方法。
如果对排序性能敏感，可以重载 compare(byte[], int, int, byte[], int, int) 方法。

Method Summary

All Methods Static Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`int`	`compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)` 本方法是 `RawComparator` 的低效实现，如果能提供此方法的高效实现，请重载此方法.
`int`	`compare(Object a, Object b)` 重载 `Comparator.compare(Object, Object)` 方法，定义为 final，不允许子类重载.
`int`	`compare(WritableComparable a, WritableComparable b)` 比较两个 `WritableComparable` 对象.
`static int`	`compareBytes(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2)` 逐字节比较两组二进制数据.
`static void`	`define(Class c, WritableComparator comparator)` 此静态方法用于为指定类型注册更高效的 WritableComparator 实现，否则默认使用本实现.
`static WritableComparator`	`get(Class<? extends WritableComparable> c)` 此静态方法用于获取为类型 c 注册的 WritableComparator 实现.
`Class<? extends WritableComparable>`	`getKeyClass()` 返回待比较的 `WritableComparable` 实现类
`static int`	`hashBytes(byte[] bytes, int length)` 计算二进制数据的哈希值.
`WritableComparable`	`newKey()` 新建一个新的 `WritableComparable` 对象.
`static double`	`readDouble(byte[] bytes, int start)` 从 byte 数组读取double.
`static float`	`readFloat(byte[] bytes, int start)` 从 byte 数组读取 float.
`static int`	`readInt(byte[] bytes, int start)` 从 byte 数组读取 int.
`static long`	`readLong(byte[] bytes, int start)` 从 byte 数组读取 long.
`static int`	`readUnsignedShort(byte[] bytes, int start)` 从 byte 数组读取无符号 short.
`static int`	`readVInt(byte[] bytes, int start)` 从byte数组读取压缩编码过的 int.
`static long`	`readVLong(byte[] bytes, int start)` 从byte数组读取压缩编码过的 long.

Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.util.Comparator
comparing, comparing, comparingDouble, comparingInt, comparingLong, equals, naturalOrder, nullsFirst, nullsLast, reversed, reverseOrder, thenComparing, thenComparing, thenComparing, thenComparingDouble, thenComparingInt, thenComparingLong

- Method Detail
  - get
```
public static WritableComparator get(Class<? extends WritableComparable> c)
```
    此静态方法用于获取为类型 c 注册的 WritableComparator 实现.
    此方法返回通过 define(Class, WritableComparator) 方法为类型 c 注册的 WritableComparator 实现，如果没有注册，则返回 WritableComparator 这一通用实现。
    
    Parameters:
    
    c - 待比较的 WritableComparable 类型
    
    Returns:
    
    WritableComparator 实现
    
    See Also:
    
    JobConf#getOutputKeyComparator()
  - define
```
public static void define(Class c,
                          WritableComparator comparator)
```
    此静态方法用于为指定类型注册更高效的 WritableComparator 实现，否则默认使用本实现.
    注意：只能注册线程安全的比较器，注册的对象可能在多线程场景中使用
    
    Parameters:
    
    c - 待比较的 WritableComparable 类型
    
    comparator - 更高效的 WritableComparator 实现
    
    See Also:
    
    get(Class)
  - getKeyClass
```
public Class<? extends WritableComparable> getKeyClass()
```
    返回待比较的 WritableComparable 实现类
    
    Returns:
    
    WritableComparable 实现类
  - newKey
```
public WritableComparable newKey()
```
    新建一个新的 WritableComparable 对象.
    
    Returns:
    
    新创建的 WritableComparable 对象
  - compare
```
public int compare(byte[] b1,
                   int s1,
                   int l1,
                   byte[] b2,
                   int s2,
                   int l2)
```
    本方法是 RawComparator 的低效实现，如果能提供此方法的高效实现，请重载此方法.
    本方法的默认实现是：先通过 Writable.readFields(DataInput) 将二进制表示反序列化为 WritableComparable 对象，然后调用 compare(WritableComparable, WritableComparable) 进行比较。
    
    Specified by:
    
    compare in interface RawComparator
    
    Parameters:
    
    b1 - 对象1二进制内容所在的byte数组
    
    s1 - 对象1二进制内容在b1的起始位置
    
    l1 - 对象1二进制内容长度
    
    b2 - 对象2二进制内容所在的byte数组
    
    s2 - 对象2二进制内容在b2的起始位置
    
    l2 - 对象2二进制内容长度
    
    Returns:
    
    对象1>对象2，返回1，对象1=对象2，返回0，对象1<对象2，返回-1
  - compare
```
public int compare(WritableComparable a,
                   WritableComparable b)
```
    比较两个 WritableComparable 对象.
    按自然顺序比较两个 WritableComparable 对象，直接调用 Comparable.compareTo(Object).
    
    Parameters:
    
    a - 左边 WritableComparable 对象
    
    b - 右边 WritableComparable 对象
    
    Returns:
    
    a.compareTo(b)
  - compare
```
public final int compare(Object a,
                         Object b)
```
    重载 Comparator.compare(Object, Object) 方法，定义为 final，不允许子类重载.
    本比较函数直接调用 compare(WritableComparable, WritableComparable).
    
    Specified by:
    
    compare in interface Comparator
  - compareBytes
```
public static int compareBytes(byte[] b1,
                               int s1,
                               int l1,
                               byte[] b2,
                               int s2,
                               int l2)
```
    逐字节比较两组二进制数据.
    
    Parameters:
    
    b1 -
    
    s1 -
    
    l1 -
    
    b2 -
    
    s2 -
    
    l2 -
    
    Returns:
    
    See Also:
    
    RawComparator.compare(byte[], int, int, byte[], int, int)
  - hashBytes
```
public static int hashBytes(byte[] bytes,
                            int length)
```
    计算二进制数据的哈希值.
    
    Parameters:
    
    bytes - byte数组
    
    length - 数据长度
    
    Returns:
    
    哈希值
  - readUnsignedShort
```
public static int readUnsignedShort(byte[] bytes,
                                    int start)
```
    从 byte 数组读取无符号 short.
    
    Parameters:
    
    bytes - 字节数组
    
    start - 起始位置，读取内容bytes[start, start+1]
    
    Returns:
  - readInt
```
public static int readInt(byte[] bytes,
                          int start)
```
    从 byte 数组读取 int.
    
    Parameters:
    
    bytes - 字节数组
    
    start - 起始位置，读取内容bytes[start, start+3]
    
    Returns:
  - readFloat
```
public static float readFloat(byte[] bytes,
                              int start)
```
    从 byte 数组读取 float.
    
    Parameters:
    
    bytes - 字节数组
    
    start - 起始位置，读取内容bytes[start, start+3]
    
    Returns:
  - readLong
```
public static long readLong(byte[] bytes,
                            int start)
```
    从 byte 数组读取 long.
    
    Parameters:
    
    bytes - 字节数组
    
    start - 起始位置，读取内容bytes[start, start+7]
    
    Returns:
  - readDouble
```
public static double readDouble(byte[] bytes,
                                int start)
```
    从 byte 数组读取double.
    
    Parameters:
    
    bytes - 字节数组
    
    start - 起始位置，读取内容bytes[start, start+7]
    
    Returns:
  - readVLong
```
public static long readVLong(byte[] bytes,
                             int start)
                      throws IOException
```
    从byte数组读取压缩编码过的 long.
    
    Parameters:
    
    bytes - 字节数组
    
    start - 起始位置，读取内容长度依 long 值的压缩编码特征而定
    
    Returns:
    
    Throws:
    
    IOException
  - readVInt
```
public static int readVInt(byte[] bytes,
                           int start)
                    throws IOException
```
    从byte数组读取压缩编码过的 int.
    
    Parameters:
    
    bytes - 字节数组
    
    start - 起始位置，读取内容长度依 int 值的压缩编码特征而定
    
    Returns:
    
    Throws:
    
    IOException

Class WritableComparator

Method Summary

Methods inherited from class java.lang.Object

Methods inherited from interface java.util.Comparator

Method Detail

get

define

getKeyClass

newKey

compare

compare

compare

compareBytes

hashBytes

readUnsignedShort

readInt

readFloat

readLong

readDouble

readVLong

readVInt