When writing a MapReduce program, the data passed between Map and Reduce needs to be of ArrayList type. When debugging and running, I encountered such an error:
java.lang.RuntimeException: java.lang.NoSuchMethodException: org.apache.hadoop.io.ArrayWritable.<init>()
After checking the official website API documentation, I found this paragraph:
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); } }It turns out that you need to implement a derived class of the ArrayWritable class yourself. When using it, you only need to implement two constructors.
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); }}