ArrayList 和数组 在mapreduce编程中序列化

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看看这个源码，应该就很明白了
1   /**
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10    *     http://www.apache.org/licenses/LICENSE-2.0 11    *
12    * Unless required by applicable law or agreed to in writing, software
13    * distributed under the License is distributed on an "AS IS" BASIS,
14    * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15    * See the License for the specific language governing permissions and
16    * limitations under the License.
17    */
18
19   package org.apache.hadoop.io;
20
21   import java.io;
22   import java.lang.reflect.Array;
23
24   /** A Writable for arrays containing instances of a class. */
25   public class ArrayWritable implements Writable {
26     private Class valueClass;
27     private Writable[] values;
28
29     public ArrayWritable() {
30       this.valueClass = null;
31     }
32
33     public ArrayWritable(Class valueClass) {
34       this.valueClass = valueClass;
35     }
36
37     public ArrayWritable(Class valueClass, Writable[] values) {
38       this(valueClass);
39       this.values = values;
40     }
41
42     public ArrayWritable(String[] strings) {
43       this(UTF8.class, new Writable[strings.length]);
44       for (int i = 0; i < strings.length; i++) {
45         values[i] = new UTF8(strings[i]);
46       }
47     }
48
49     public void setValueClass(Class valueClass) {
50       if (valueClass != this.valueClass) {
51         this.valueClass = valueClass;
52         this.values = null;
53       }
54     }
55
56     public Class getValueClass() {
57       return valueClass;
58     }
59
60     public String[] toStrings() {
61       String[] strings = new String[values.length];
62       for (int i = 0; i < values.length; i++) {
63         strings[i] = values[i].toString();
64       }
65       return strings;
66     }
67
68     public Object toArray() {
69       Object result = Array.newInstance(valueClass, values.length);
70       for (int i = 0; i < values.length; i++) {
71         Array.set(result, i, values[i]);
72       }
73       return result;
74     }
75
76     public void set(Writable[] values) { this.values = values; }
77
78     public Writable[] get() { return values; }
79
80     public void readFields(DataInput in) throws IOException {
81       values = new Writable[in.readInt()];          // construct values
82       for (int i = 0; i < values.length; i++) {
83         Writable value = WritableFactories.newInstance(valueClass);
84         value.readFields(in);                       // read a value
85         values[i] = value;                          // store it in values
86       }
87     }
88
89     public void write(DataOutput out) throws IOException {
90       out.writeInt(values.length);                 // write values
91       for (int i = 0; i < values.length; i++) {
92         values[i].write(out);
93       }
94     }
95
96   }
97

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