hadoop NullWritable 源码
haddop NullWritable 代码
文件路径：/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/io/NullWritable.java
/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.io;
import java.io.*;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
/** Singleton Writable with no data. */
@InterfaceAudience.Public
@InterfaceStability.Stable
public class NullWritable implements WritableComparable<NullWritable> {
private static final NullWritable THIS = new NullWritable();
private NullWritable() {} // no public ctor
/**
* Returns the single instance of this class.
* @return the single instance of this class.
*/
public static NullWritable get() { return THIS; }
@Override
public String toString() {
return "(null)";
}
@Override
public int hashCode() { return 0; }
@Override
public int compareTo(NullWritable other) {
return 0;
}
@Override
public boolean equals(Object other) { return other instanceof NullWritable; }
@Override
public void readFields(DataInput in) throws IOException {}
@Override
public void write(DataOutput out) throws IOException {}
/** A Comparator "optimized" for NullWritable. */
public static class Comparator extends WritableComparator {
public Comparator() {
super(NullWritable.class);
}
/**
* Compare the buffers in serialized form.
*/
@Override
public int compare(byte[] b1, int s1, int l1,
byte[] b2, int s2, int l2) {
assert 0 == l1;
assert 0 == l2;
return 0;
}
}
static { // register this comparator
WritableComparator.define(NullWritable.class, new Comparator());
}
}
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