MapReduce中ArrayWritable 使用指南
在编写MapReduce程序时，Map和Reduce之间传递的数据需要是ArrayList类型的，在调试运行时遇到了这样的一个错误：
java.lang.RuntimeException:java.lang.NoSuchMethodException:org.apache.hadoop.io.ArrayWritable.<init>()
经查询官网API文档后发现这样的一段话：
AWritableforarrayscontaininginstancesofaclass.Theelementsofthiswritablemustallbeinstancesofthesameclass.IfthiswritablewillbetheinputforaReducer,youwillneedtocreateasubclassthatsetsthevaluetobeofthepropertype.Forexample:publicclassIntArrayWritableextendsArrayWritable{publicIntArrayWritable(){super(IntWritable.class);}}
原来是要自己实现一个ArrayWritable类的派生类，使用时只要实现两个构造函数即可
publicstaticclassTextArrayWritableextendsArrayWritable{
publicTextArrayWritable(){
super(Text.class);
}
publicTextArrayWritable(String[]strings){
super(Text.class);
Text[]texts=newText[strings.length];
for(inti=0;i<strings.length;i++){
texts[i]=newText(strings[i]);
}
set(texts);
}
}