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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