1。引发：做一个java读取hbase的注册成表的程序。但是读出来的是javaPairRDD，而网上都是javaRDD转成dataFrame,我只能自己摸索怎么转成javaRDD
2。方法
　　JavaRDD => JavaPairRDD: 通过mapToPair函数
　　JavaPairRDD => JavaRDD: 通过map函数转换
3。不管其他先运行一下

package com.lcc.spark.rdd.test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.apache.spark.SparkConf;
import org.apache.spark.api.java.JavaPairRDD;
import org.apache.spark.api.java.JavaRDD;
import org.apache.spark.api.java.JavaSparkContext;
import org.apache.spark.api.java.function.Function;
import org.apache.spark.api.java.function.Function2;
import org.apache.spark.api.java.function.PairFunction;
import org.apache.spark.api.java.function.VoidFunction;
import org.apache.spark.storage.StorageLevel;

import com.google.common.base.Optional;

import scala.Tuple2;
import shapeless.Tuple;

/**

• Hello world!

*/
public class App {

public static void main(String[] args) {  
    SparkConf conf = new SparkConf().setMaster("local").setAppName("Simple Application");  
    JavaSparkContext sc = new JavaSparkContext(conf);  

    // convert from other RDD  
    JavaRDD<String> line1 = sc.parallelize(Arrays.asList("1 aa", "2 bb", "4 cc", "3 dd")); 

    line1.foreach(new VoidFunction<String>(){

        @Override
        public void call(String num) throws Exception {
            // TODO Auto-generated method stub
            System.out.println("numbers;"+num);
        }
    });

    JavaPairRDD<String, String> prdd = line1.mapToPair(new PairFunction<String, String, String>() {  
        public Tuple2<String, String> call(String x) throws Exception {  
            return new Tuple2(x.split(" ")[0], x.split(" ")[1]);  
        }  
    });  
    System.out.println("111111111111mapToPair:");  
    prdd.foreach(new VoidFunction<Tuple2<String, String>>() {  
        public void call(Tuple2<String, String> x) throws Exception {  
            System.out.println(x);  
        }  
    });  

   /* JavaRDD => JavaPairRDD: 通过mapToPair函数
    JavaPairRDD => JavaRDD: 通过map函数转换*/

    System.out.println("===============1=========");  
    JavaRDD<String> javaprdd =prdd.map(new Function<Tuple2<String,String>,String>() {
        private static final long serialVersionUID = 1L;
        @Override
        public String call(Tuple2<String, String> arg0)  {
        // TODO Auto-generated method stub
            System.out.println("arg0======================"+arg0);
            System.out.println("arg0======================"+arg0._1);
             return arg0._1+" "+arg0._2;
        }

   });

    System.out.println("===============2=========");  

    javaprdd.foreach(new VoidFunction<String>(){

        @Override
        public void call(String num) throws Exception {
            // TODO Auto-generated method stub
            System.out.println("numbers;"+num);
        }
    });

}  

}

结果如下

numbers;1 aa
numbers;2 bb
numbers;4 cc
numbers;3 dd

111111111111mapToPair:
(1,aa)
(2,bb)
(4,cc)
(3,dd)

===============1=========
===============2=========
arg0======================(1,aa)
arg0======================1
numbers;1 aa
arg0======================(2,bb)
arg0======================2
numbers;2 bb
arg0======================(4,cc)
arg0======================4
numbers;4 cc
arg0======================(3,dd)
arg0======================3
numbers;3 dd
4。分析

JavaPairRDD<String, String> prdd = line1.mapToPair(new PairFunction<要返回JavaPairRDD的key类型, 要返回JavaPairRDD的value类型, javaRDD要输入的类型>() {

        public Tuple2<要返回JavaPairRDD的key类型, 要返回JavaPairRDD的value类型> call(javaRDD要输入的类型 x) throws Exception {  
            return new Tuple2(x.split(" ")[0], x.split(" ")[1]);  
        }  
    });  

JavaPairRDD<String, String> prdd = line1.mapToPair(new PairFunction<String, String, String>() {

        public Tuple2<String, String> call(String x) throws Exception {  
            return new Tuple2(x.split(" ")[0], x.split(" ")[1]);  
        }  
    });  

JavaRDD<要返回JavaRDD的类型> javaprdd =prdd.map(new Function<Tuple2<JavaPairRDD的key类型,JavaPairRDD的value类型>,要返回JavaRDD的类型>() {

        private static final long serialVersionUID = 1L;
        @Override
        public String call(Tuple2<String, String> arg0)  {
        // TODO Auto-generated method stub
            System.out.println("arg0======================"+arg0);
            System.out.println("arg0======================"+arg0._1);
             return arg0._1+" "+arg0._2;
        }

   });

    System.out.println("===============1=========");  

JavaRDD javaprdd =prdd.map(new Function<Tuple2<String,String>,String>() {

        private static final long serialVersionUID = 1L;
        @Override
        public String call(Tuple2<String, String> arg0)  {
        // TODO Auto-generated method stub
            System.out.println("arg0======================"+arg0);
            System.out.println("arg0======================"+arg0._1);
             return arg0._1+" "+arg0._2;
        }

   });

    System.out.println("===============2=========");  

从输出结果看

===============1=========
===============2=========
比下面的map函数体的内容先输出，我在这里坑了很久，
明明map函数写对了，但是我却没在下面打印输出，
结果总是map函数体的没有执行，我就很纳闷啊，
为什么啊，为什么不对啊，也没报错啊，
但是我一打印才发现里面竟然执行了，
这时候我突然想起spark的程序都是懒加载的，
你用的时候他才会执行，所以上面两行会先打印出来。
都是泪啊
arg0======================(1,aa)
arg0======================1
numbers;1 aa
5。由此总结javaRDD怎么转换成JavaPairRDD的就，反着怎么转换回去，互为逆运算