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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