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.()
经查询官网API文档后发现这样的一段话：
A Writable for arrays containing instances of a class. The elements of this writable must all be instances of the same class. If this writable will be the input for a Reducer, you will need to create a subclass that sets the value to be of the proper type. For example: public class IntArrayWritable extends ArrayWritable { public IntArrayWritable() { 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
public static class TextArrayWritable extends ArrayWritable {
public TextArrayWritable() {
super(Text.class);
}
public TextArrayWritable(String[] strings) {
super(Text.class);
Text[] texts = new Text[strings.length];
for (int i = 0; i < strings.length; i++) {
texts[i] = new Text(strings[i]);
}
set(texts);
}
}