在下面的代码中如何生成对象？

本文介绍了在下面的代码中如何生成对象？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 我试图理解一个Java代码。 （Java的基础知识）

这里是

WordCountMapper Class

package company; 导入org.apache.hadoop.io.IntWritable; import org.apache.hadoop.io.LongWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.Mapper; import java.io.IOException; 公共类WordCountMapper扩展了Mapper< LongWritable，Text，Text，IntWritable> { @Override public void map（LongWritable key，Text value，Context context）throws IOException，InterruptedException { String line = value.toString（）; for（String word：line.split（））{ if（word.length（）> 0）{ context.write（new Text（word ），新的IntWritable（1））; $ b

Mapper Class

package org.apache.hadoop.mapreduce; import java.io.IOException; import org.apache.hadoop.classification.InterfaceAudience.Public; import org.apache.hadoop.classification.InterfaceStability.Stable; @ InterfaceAudience.Public @ InterfaceStability.Stable public class Mapper< KEYIN，VALUEIN，KEYOUT，VALUEOUT> { public Mapper（）{} 保护无效设置（Mapper< KEYIN，VALUEIN，KEYOUT，VALUEOUT> .Context上下文）抛出IOException，InterruptedException { $ b保护无效映射（KEYIN键，VALUEIN值，Mapper< KEYIN，VALUEIN，KEYOUT，VALUEOUT> .Context上下文）抛出IOException，InterruptedException { context.write（key，value）; 保护无效清理（Mapper< KEYIN，VALUEIN，KEYOUT，VALUEOUT> .Context上下文）抛出IOException，InterruptedException {} public void run（Mapper< KEYIN，VALUEIN，KEYOUT，VALUEOUT> .Context context）throws IOException，InterruptedException { setup（context）; $ context（），context.getCurrentValue（），context）; } cleanup（context）; 公共抽象类Context实现MapContext< KEYIN，VALUEIN，KEYOUT，VALUEOUT> { public Context（）{} }

主要方法类

package company ; 导入org.apache.hadoop.conf.Configuration; 导入org.apache.hadoop.fs.Path; import org.apache.hadoop.io.IntWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.Job; import org.apache.hadoop.mapreduce.lib.input.FileInputFormat; import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat; public class WordCount { public static void main（String [] args）throws Exception { if（args.length！= 2）{ System.err .println（Invalid Command）; System.err.println（Usage：WordCount< input path>< output path>）; System.exit（0）; } 配置conf = new Configuration（）; 工作职位=新职位（conf，wordcount）; job.setJarByClass（WordCount.class）; FileInputFormat.addInputPath（job，new Path（args [0]））; FileOutputFormat.setOutputPath（job，new Path（args [1]））; job.setMapperClass（WordCountMapper.class）; job.setReducerClass（WordCountReducer.class）; job.setOutputKeyClass（Text.class）; job.setOutputValueClass（IntWritable.class）; }

我怀疑WordCount类中的Text值是如何存在的？我的意思是它的一个对象，但是在它生成的地方，主类方法没有实例化Text类的实例。

这意味着什么 - 在创建类像下面的格式之前，我从来没有见过这样的事情

public class Mapper< KEYIN，VALUEIN，KEYOUT，VALUEOUT> {

有什么建议吗？

解决方案

您粘贴的代码是用。

基本上你有三个类：

• WordCount 映射器，它似乎将字符串拆分并将它们写入Hadoop流式上下文中
• 属于Hadoop流式库的 Mapper 类
• 将作业提交给Hadoop集群的 WordCount 驱动程序

其实我在你的问题中可能会出现一个 WordCountReducer 类，但似乎没有。

任何方式：文本将通过将其作为文件复制到您的Hadoop集群中并必须在HDFS（Hadoop文件系统）上运行之前存在。

此行代码指向一个HDFS路径：

FileInputFormat.addInputPath（job，new Path（args [0]））;

关于代码的问题：

public class Mapper< KEYIN，VALUEIN，KEYOUT，VALUEOUT>

这些是通用类型（请参阅 tutorial ），你必须在每次你映射一个映射器时声明它。

code> WordCount mapper实际上是这个 Mapper 类的子类并指定了四种类型：

public class WordCountMapper扩展了Mapper< LongWritable，Text，Text，IntWritable>

以下是信件：

KEYIN = LongWritable VALUEIN = Text KEYOUT = Text VALUEOUT = IntWritable

I'm trying to understand one java code. (Basic knowledge of Java)

Here its is

WordCountMapper Class

package company; import org.apache.hadoop.io.IntWritable; import org.apache.hadoop.io.LongWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.Mapper; import java.io.IOException; public class WordCountMapper extends Mapper<LongWritable, Text, Text, IntWritable> { @Override public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException { String line = value.toString(); for (String word : line.split(" ")) { if (word.length() > 0) { context.write(new Text(word), new IntWritable(1)); } }

Mapper Class

package org.apache.hadoop.mapreduce; import java.io.IOException; import org.apache.hadoop.classification.InterfaceAudience.Public; import org.apache.hadoop.classification.InterfaceStability.Stable; @InterfaceAudience.Public @InterfaceStability.Stable public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> { public Mapper() { } protected void setup(Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException { } protected void map(KEYIN key, VALUEIN value, Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException { context.write(key, value); } protected void cleanup(Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException { } public void run(Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>.Context context) throws IOException, InterruptedException { setup(context); while (context.nextKeyValue()) { map(context.getCurrentKey(), context.getCurrentValue(), context); } cleanup(context); } public abstract class Context implements MapContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> { public Context() { } }

}

Main method class

package company; import org.apache.hadoop.conf.Configuration; import org.apache.hadoop.fs.Path; import org.apache.hadoop.io.IntWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.Job; import org.apache.hadoop.mapreduce.lib.input.FileInputFormat; import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat; public class WordCount { public static void main(String[] args) throws Exception { if(args.length !=2){ System.err.println("Invalid Command"); System.err.println("Usage: WordCount <input path> <output path>"); System.exit(0); } Configuration conf = new Configuration(); Job job = new Job(conf, "wordcount"); job.setJarByClass(WordCount.class); FileInputFormat.addInputPath(job, new Path(args[0])); FileOutputFormat.setOutputPath(job, new Path(args[1])); job.setMapperClass(WordCountMapper.class); job.setReducerClass(WordCountReducer.class); job.setOutputKeyClass(Text.class); job.setOutputValueClass(IntWritable.class); }

My doubt is in WordCount class how Text value is coming into existance ? I mean its an object but where its getting generated, there is no sign in main method class to instantiate instance of Text class.

And what it means - , I have never seen this before creating class like in below format

public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {

Any suggestions ?

解决方案

The code you have pasted is meant to run using the Hadoop MapReduce framework.

Basically you have here three classes:

• The WordCount mapper which seems to split strings and write these to the Hadoop streaming context
• The Mapper class which is part of the Hadoop streaming libraries
• The WordCount driver which submits the job to the Hadoop cluster

Actually I would have expected a WordCountReducer class in your question, but that seems not to be there.

Any way: the text will "come to existence" by copying it as a file to your Hadoop cluster and must be on HDFS (Hadoop File System) before you run the job.

This line of code refers to one HDFS path:

FileInputFormat.addInputPath(job, new Path(args[0]));

And regarding the question about the code:

public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>

These are generic types (see this tutorial here) which have to be declared each time you subclass a mapper.

Your WordCount mapper actually subclasses this Mapper class and specifies the four types:

public class WordCountMapper extends Mapper<LongWritable,Text,Text,IntWritable>

These are the correspondences:

KEYIN = LongWritable VALUEIN = Text KEYOUT = Text VALUEOUT = IntWritable