1.11.Flink DataSetAPI、DataSet API之Data Sources、DataSet API之Transformations、DataSet Sink部分详解

电玩女神 2022-11-18 00:13 169阅读 0赞

1.11.Flink DataSetAPI  
1.11.1.DataSet API之Data Sources  
1.11.2.DataSet API之Transformations  
1.11.3.DataSet Sink部分详解

## 1.11.Flink DataSetAPI ##

### 1.11.1.DataSet API之Data Sources ###

基于文件  
readTextFile(path)  
基于集合  
fromCollection(Collection)

### 1.11.2.DataSet API之Transformations ###

Map：输入一个元素，然后返回一个元素，中间可以做一些清洗转换等操作。  
FlatMap：输入一个元素，可以返回零个，一个或者多个元素。  
MapPartition：类似map，一次处理一个分区的数据【如果在进行map处理的时候需要获取第三方资源链接，建议使用MapPartition】

import org.apache.flink.api.common.functions.MapPartitionFunction;
    import org.apache.flink.api.java.ExecutionEnvironment;
    import org.apache.flink.api.java.operators.DataSource;
    import org.apache.flink.api.java.tuple.Tuple2;
    import org.apache.flink.util.Collector;
    
    import java.util.ArrayList;
    import java.util.Iterator;
    
    /** * Hash-Partition * * Range-Partition * * Created by xxxx on 2020/10/09 . */
    public class BatchDemoHashRangePartition { 
    
        public static void main(String[] args) throws Exception{ 
    
            //获取运行环境
            ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
    
            ArrayList<Tuple2<Integer, String>> data = new ArrayList<>();
            data.add(new Tuple2<>(1,"hello1"));
            data.add(new Tuple2<>(2,"hello2"));
            data.add(new Tuple2<>(2,"hello3"));
            data.add(new Tuple2<>(3,"hello4"));
            data.add(new Tuple2<>(3,"hello5"));
            data.add(new Tuple2<>(3,"hello6"));
            data.add(new Tuple2<>(4,"hello7"));
            data.add(new Tuple2<>(4,"hello8"));
            data.add(new Tuple2<>(4,"hello9"));
            data.add(new Tuple2<>(4,"hello10"));
            data.add(new Tuple2<>(5,"hello11"));
            data.add(new Tuple2<>(5,"hello12"));
            data.add(new Tuple2<>(5,"hello13"));
            data.add(new Tuple2<>(5,"hello14"));
            data.add(new Tuple2<>(5,"hello15"));
            data.add(new Tuple2<>(6,"hello16"));
            data.add(new Tuple2<>(6,"hello17"));
            data.add(new Tuple2<>(6,"hello18"));
            data.add(new Tuple2<>(6,"hello19"));
            data.add(new Tuple2<>(6,"hello20"));
            data.add(new Tuple2<>(6,"hello21"));
    
            DataSource<Tuple2<Integer, String>> text = env.fromCollection(data);
    
            /*text.partitionByHash(0).mapPartition(new MapPartitionFunction<Tuple2<Integer,String>, Tuple2<Integer,String>>() { @Override public void mapPartition(Iterable<Tuple2<Integer, String>> values, Collector<Tuple2<Integer, String>> out) throws Exception { Iterator<Tuple2<Integer, String>> it = values.iterator(); while (it.hasNext()){ Tuple2<Integer, String> next = it.next(); System.out.println("当前线程id："+Thread.currentThread().getId()+","+next); } } }).print();*/
    
            text.partitionByRange(0).mapPartition(new MapPartitionFunction<Tuple2<Integer,String>, Tuple2<Integer,String>>() { 
                @Override
                public void mapPartition(Iterable<Tuple2<Integer, String>> values, Collector<Tuple2<Integer, String>> out) throws Exception { 
                    Iterator<Tuple2<Integer, String>> it = values.iterator();
                    while (it.hasNext()){ 
                        Tuple2<Integer, String> next = it.next();
                        System.out.println("当前线程id："+Thread.currentThread().getId()+","+next);
                    }
    
                }
            }).print();
        }
    
    }

import org.apache.flink.api.scala.ExecutionEnvironment
    import scala.collection.mutable.ListBuffer
    
    /** * Created by xxxx on 2020/10/09 */
    object BatchDemoHashRangePartitionScala { 
    
      def main(args: Array[String]): Unit = { 
    
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data1 = ListBuffer[Tuple2[Int,String]]()
        data1.append((1,"hello1"))
        data1.append((2,"hello2"))
        data1.append((2,"hello3"))
        data1.append((3,"hello4"))
        data1.append((3,"hello5"))
        data1.append((3,"hello6"))
        data1.append((4,"hello7"))
        data1.append((4,"hello8"))
        data1.append((4,"hello9"))
        data1.append((4,"hello10"))
        data1.append((5,"hello11"))
        data1.append((5,"hello12"))
        data1.append((5,"hello13"))
        data1.append((5,"hello14"))
        data1.append((5,"hello15"))
        data1.append((6,"hello16"))
        data1.append((6,"hello17"))
        data1.append((6,"hello18"))
        data1.append((6,"hello19"))
        data1.append((6,"hello20"))
        data1.append((6,"hello21"))
    
        val text = env.fromCollection(data1)
    
        /*text.partitionByHash(0).mapPartition(it=>{ while (it.hasNext){ val tu = it.next() println("当前线程id："+Thread.currentThread().getId+","+tu) } it }).print()*/
    
        text.partitionByRange(0).mapPartition(it=>{ 
          while (it.hasNext){ 
            val tu = it.next()
            println("当前线程id："+Thread.currentThread().getId+","+tu)
          }
          it
        }).print()
      }
    
    }

**再例如：**

import org.apache.flink.api.common.functions.MapPartitionFunction;
    import org.apache.flink.api.java.DataSet;
    import org.apache.flink.api.java.ExecutionEnvironment;
    import org.apache.flink.api.java.operators.DataSource;
    import org.apache.flink.util.Collector;
    
    import java.util.ArrayList;
    import java.util.Iterator;
    
    /** * Created by xxxx on 2020/10/09 . */
    public class BatchDemoMapPartition { 
    
        public static void main(String[] args) throws Exception{ 
            //获取运行环境
            ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
    
            ArrayList<String> data = new ArrayList<>();
            data.add("hello you");
            data.add("hello me");
    
            DataSource<String> text = env.fromCollection(data);
    
            /*text.map(new MapFunction<String, String>() { @Override public String map(String value) throws Exception { //获取数据库连接--注意，此时是每过来一条数据就获取一次链接 //处理数据 //关闭连接 return value; } });*/
            
            DataSet<String> mapPartitionData = text.mapPartition(new MapPartitionFunction<String, String>() { 
                @Override
                public void mapPartition(Iterable<String> values, Collector<String> out) throws Exception { 
                    //获取数据库连接--注意，此时是一个分区的数据获取一次连接【优点，每个分区获取一次链接】
                    //values中保存了一个分区的数据
                    //处理数据
                    Iterator<String> it = values.iterator();
                    while (it.hasNext()) { 
                        String next = it.next();
                        String[] split = next.split("\\W+");
                        for (String word : split) { 
                            out.collect(word);
                        }
                    }
                    //关闭链接
                }
            });
    
            mapPartitionData.print();
        }
        
    }

import org.apache.flink.api.scala.ExecutionEnvironment
    import scala.collection.mutable.ListBuffer
    
    /** * Created by xxxx on 2020/10/09 */
    object BatchDemoMapPartitionScala { 
    
      def main(args: Array[String]): Unit = { 
    
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data = ListBuffer[String]()
    
        data.append("hello you")
        data.append("hello me")
    
        val text = env.fromCollection(data)
    
        text.mapPartition(it=>{ 
          //创建数据库连接，建议吧这块代码放到try-catch代码块中
          val res = ListBuffer[String]()
          while(it.hasNext){ 
            val line = it.next()
            val words = line.split("\\W+")
            for(word <- words){ 
              res.append(word)
            }
          }
          res
          //关闭连接
        }).print()
      }
    
    }

Filter:过滤函数，对传入的数据进行判断，符合条件的数据会被留下  
Reduce:对数据进行聚合操作，结合当前元素和上一次reduce返回的值进行聚合操作，然后返回一个新的值  
Aggregation:sum、max、min等。  
Distinct:返回一个数据集中去重之后的元素，data.distinct()

import org.apache.flink.api.common.functions.FlatMapFunction;
    import org.apache.flink.api.java.ExecutionEnvironment;
    import org.apache.flink.api.java.operators.DataSource;
    import org.apache.flink.api.java.operators.FlatMapOperator;
    import org.apache.flink.util.Collector;
    
    import java.util.ArrayList;
    
    /** * Created by xxxx on 2020/10/09 . */
    public class BatchDemoDistinct { 
    
        public static void main(String[] args) throws Exception { 
            //获取运行环境
            ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
    
            ArrayList<String> data = new ArrayList<>();
            data.add("hello you");
            data.add("hello me");
    
            DataSource<String> text = env.fromCollection(data);
            FlatMapOperator<String, String> flatMapData = text.flatMap(new FlatMapFunction<String, String>() { 
                @Override
                public void flatMap(String value, Collector<String> out) throws Exception { 
                    String[] split = value.toLowerCase().split("\\W+");
                    for (String word : split) { 
                        System.out.println("单词："+word);
                        out.collect(word);
                    }
                }
            });
    
            flatMapData.distinct()// 对数据进行整体去重
                    .print();
        }
    }

import org.apache.flink.api.scala.ExecutionEnvironment
    
    import scala.collection.mutable.ListBuffer
    
    /** * Created by xxxx on 2020/10/09 */
    object BatchDemoDistinctScala { 
    
      def main(args: Array[String]): Unit = { 
    
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data = ListBuffer[String]()
    
        data.append("hello you")
        data.append("hello me")
    
        val text = env.fromCollection(data)
    
        val flatMapData = text.flatMap(line=>{ 
          val words = line.split("\\W+")
          for(word <- words){ 
            println("单词："+word)
          }
          words
        })
    
        flatMapData.distinct().print()
      }
    
    }

Join:内连接

import org.apache.flink.api.scala.ExecutionEnvironment
    import scala.collection.mutable.ListBuffer
    
    /** * Created by xxxx on 2020/10/09 */
    object BatchDemoJoinScala { 
    
      def main(args: Array[String]): Unit = { 
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data1 = ListBuffer[Tuple2[Int,String]]()
        data1.append((1,"zs"))
        data1.append((2,"ls"))
        data1.append((3,"ww"))
        
        val data2 = ListBuffer[Tuple2[Int,String]]()
        data2.append((1,"beijing"))
        data2.append((2,"shanghai"))
        data2.append((3,"guangzhou"))
    
        val text1 = env.fromCollection(data1)
        val text2 = env.fromCollection(data2)
    
        text1.join(text2).where(0).equalTo(0).apply((first,second)=>{ 
          (first._1,first._2,second._2)
        }).print()
      }
    }

OuterJoin:外链接

import org.apache.flink.api.common.functions.JoinFunction;
    import org.apache.flink.api.java.ExecutionEnvironment;
    import org.apache.flink.api.java.operators.DataSource;
    import org.apache.flink.api.java.tuple.Tuple2;
    import org.apache.flink.api.java.tuple.Tuple3;
    
    import java.util.ArrayList;
    
    /** * 外连接 * * 左外连接 * 右外连接 * 全外连接 * * Created by xxxx on 2020/10/09 . */
    public class BatchDemoOuterJoin { 
    
        public static void main(String[] args) throws Exception{ 
            //获取运行环境
            ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
    
            //tuple2<用户id，用户姓名>
            ArrayList<Tuple2<Integer, String>> data1 = new ArrayList<>();
            data1.add(new Tuple2<>(1,"zs"));
            data1.add(new Tuple2<>(2,"ls"));
            data1.add(new Tuple2<>(3,"ww"));
    
            //tuple2<用户id，用户所在城市>
            ArrayList<Tuple2<Integer, String>> data2 = new ArrayList<>();
            data2.add(new Tuple2<>(1,"beijing"));
            data2.add(new Tuple2<>(2,"shanghai"));
            data2.add(new Tuple2<>(4,"guangzhou"));
    
            DataSource<Tuple2<Integer, String>> text1 = env.fromCollection(data1);
            DataSource<Tuple2<Integer, String>> text2 = env.fromCollection(data2);
    
            /** * 左外连接 * * 注意：second这个tuple中的元素可能为null * */
            text1.leftOuterJoin(text2)
                    .where(0)
                    .equalTo(0)
                    .with(new JoinFunction<Tuple2<Integer,String>, Tuple2<Integer,String>, Tuple3<Integer,String,String>>() { 
                        @Override
                        public Tuple3<Integer, String, String> join(Tuple2<Integer, String> first, Tuple2<Integer, String> second) throws Exception { 
                            if(second==null){ 
                                return new Tuple3<>(first.f0,first.f1,"null");
                            }else{ 
                                return new Tuple3<>(first.f0,first.f1,second.f1);
                            }
    
                        }
                    }).print();
    
            System.out.println("=============================================================================");
    
            /** * 右外连接 * * 注意：first这个tuple中的数据可能为null * */
            text1.rightOuterJoin(text2)
                    .where(0)
                    .equalTo(0)
                    .with(new JoinFunction<Tuple2<Integer,String>, Tuple2<Integer,String>, Tuple3<Integer,String,String>>() { 
                        @Override
                        public Tuple3<Integer, String, String> join(Tuple2<Integer, String> first, Tuple2<Integer, String> second) throws Exception { 
                            if(first==null){ 
                                return new Tuple3<>(second.f0,"null",second.f1);
                            }
                            return new Tuple3<>(first.f0,first.f1,second.f1);
                        }
                    }).print();
            
            System.out.println("=============================================================================");
    
            /** * 全外连接 * * 注意：first和second这两个tuple都有可能为null * */
            text1.fullOuterJoin(text2)
                    .where(0)
                    .equalTo(0)
                    .with(new JoinFunction<Tuple2<Integer,String>, Tuple2<Integer,String>, Tuple3<Integer,String,String>>() { 
                        @Override
                        public Tuple3<Integer, String, String> join(Tuple2<Integer, String> first, Tuple2<Integer, String> second) throws Exception { 
                            if(first==null){ 
                                return new Tuple3<>(second.f0,"null",second.f1);
                            }else if(second == null){ 
                                return new Tuple3<>(first.f0,first.f1,"null");
                            }else{ 
                                return new Tuple3<>(first.f0,first.f1,second.f1);
                            }
                        }
                    }).print();
        }
    }

import org.apache.flink.api.scala.ExecutionEnvironment
    import scala.collection.mutable.ListBuffer
    
    /** * Created by xxxx on 2020/10/09 */
    object BatchDemoOuterJoinScala { 
    
      def main(args: Array[String]): Unit = { 
    
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data1 = ListBuffer[Tuple2[Int,String]]()
        data1.append((1,"zs"))
        data1.append((2,"ls"))
        data1.append((3,"ww"))
        
        val data2 = ListBuffer[Tuple2[Int,String]]()
        data2.append((1,"beijing"))
        data2.append((2,"shanghai"))
        data2.append((4,"guangzhou"))
    
        val text1 = env.fromCollection(data1)
        val text2 = env.fromCollection(data2)
    
        text1.leftOuterJoin(text2).where(0).equalTo(0).apply((first,second)=>{ 
          if(second==null){ 
            (first._1,first._2,"null")
          }else{ 
            (first._1,first._2,second._2)
          }
        }).print()
    
        println("===============================")
    
        text1.rightOuterJoin(text2).where(0).equalTo(0).apply((first,second)=>{ 
          if(first==null){ 
            (second._1,"null",second._2)
          }else{ 
            (first._1,first._2,second._2)
          }
        }).print()
        
        println("===============================")
    
        text1.fullOuterJoin(text2).where(0).equalTo(0).apply((first,second)=>{ 
          if(first==null){ 
            (second._1,"null",second._2)
          }else if(second==null){ 
            (first._1,first._2,"null")
          }else{ 
            (first._1,first._2,second._2)
          }
        }).print()
      }
    
    }

Cross:获取两个数据集的笛卡尔积

import org.apache.flink.api.java.ExecutionEnvironment;
    import org.apache.flink.api.java.operators.CrossOperator;
    import org.apache.flink.api.java.operators.DataSource;
    
    import java.util.ArrayList;
    
    /** * 获取笛卡尔积 * * Created by xxxx on 2020/10/09 . */
    public class BatchDemoCross { 
    
        public static void main(String[] args) throws Exception{ 
            //获取运行环境
            ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
    
            //tuple2<用户id，用户姓名>
            ArrayList<String> data1 = new ArrayList<>();
            data1.add("zs");
            data1.add("ww");
    
            //tuple2<用户id，用户所在城市>
            ArrayList<Integer> data2 = new ArrayList<>();
            data2.add(1);
            data2.add(2);
    
            DataSource<String> text1 = env.fromCollection(data1);
            DataSource<Integer> text2 = env.fromCollection(data2);
    
            CrossOperator.DefaultCross<String, Integer> cross = text1.cross(text2);
            cross.print();
    
            /** * 输出结果为: * (zs,1) * (zs,2) * (ww,1) * (ww,2) */
        }
    }

import org.apache.flink.api.scala.ExecutionEnvironment
    
    /** * Created by xxxx on 2020/10/09 on 2018/10/30. */
    object BatchDemoCrossScala { 
    
      def main(args: Array[String]): Unit = { 
    
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data1 = List("zs","ww")
        
        val data2 = List(1,2)
    
        val text1 = env.fromCollection(data1)
        val text2 = env.fromCollection(data2)
    
        text1.cross(text2).print()
      }
    }

Union:返回两个数据集的总和，数据类型需要一致。

import org.apache.flink.api.java.ExecutionEnvironment;
    import org.apache.flink.api.java.operators.DataSource;
    import org.apache.flink.api.java.operators.UnionOperator;
    import org.apache.flink.api.java.tuple.Tuple2;
    
    import java.util.ArrayList;
    
    /** * Created by xxxx on 2020/10/09 . */
    public class BatchDemoUnion { 
    
        public static void main(String[] args) throws Exception{ 
            //获取运行环境
            ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
    
            ArrayList<Tuple2<Integer, String>> data1 = new ArrayList<>();
            data1.add(new Tuple2<>(1,"zs"));
            data1.add(new Tuple2<>(2,"ls"));
            data1.add(new Tuple2<>(3,"ww"));
    
            ArrayList<Tuple2<Integer, String>> data2 = new ArrayList<>();
            data2.add(new Tuple2<>(1,"lili"));
            data2.add(new Tuple2<>(2,"jack"));
            data2.add(new Tuple2<>(3,"jessic"));
    
            DataSource<Tuple2<Integer, String>> text1 = env.fromCollection(data1);
            DataSource<Tuple2<Integer, String>> text2 = env.fromCollection(data2);
    
            UnionOperator<Tuple2<Integer, String>> union = text1.union(text2);
    
            union.print();
    }
    
    }

import org.apache.flink.api.scala.ExecutionEnvironment
    import scala.collection.mutable.ListBuffer
    
    /** * Created by xxxx on 2020/10/09 */
    object BatchDemoUnionScala { 
    
      def main(args: Array[String]): Unit = { 
        
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data1 = ListBuffer[Tuple2[Int,String]]()
        data1.append((1,"zs"))
        data1.append((2,"ls"))
        data1.append((3,"ww"))
    
        val data2 = ListBuffer[Tuple2[Int,String]]()
        data2.append((1,"jack"))
        data2.append((2,"lili"))
        data2.append((3,"jessic"))
    
        val text1 = env.fromCollection(data1)
        val text2 = env.fromCollection(data2)
    
        text1.union(text2).print()
    
      }
    
    }

First-n:获取集合中的前N个元素。

import org.apache.flink.api.common.operators.Order;
    import org.apache.flink.api.java.ExecutionEnvironment;
    import org.apache.flink.api.java.operators.DataSource;
    import org.apache.flink.api.java.tuple.Tuple2;
    
    import java.util.ArrayList;
    
    /** * 获取集合中的前N个元素 * Created by xxxx on 2020/10/09 . */
    public class BatchDemoFirstN { 
    
        public static void main(String[] args) throws Exception{ 
            //获取运行环境
            ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
    
            ArrayList<Tuple2<Integer, String>> data = new ArrayList<>();
            data.add(new Tuple2<>(2,"zs"));
            data.add(new Tuple2<>(4,"ls"));
            data.add(new Tuple2<>(3,"ww"));
            data.add(new Tuple2<>(1,"xw"));
            data.add(new Tuple2<>(1,"aw"));
            data.add(new Tuple2<>(1,"mw"));
    
            DataSource<Tuple2<Integer, String>> text = env.fromCollection(data);
    
            //获取前3条数据，按照数据插入的顺序
            text.first(3).print();
            System.out.println("==============================");
    
            //根据数据中的第一列进行分组，获取每组的前2个元素
            text.groupBy(0).first(2).print();
            System.out.println("==============================");
    
            //根据数据中的第一列分组，再根据第二列进行组内排序[升序]，获取每组的前2个元素
            text.groupBy(0).sortGroup(1, Order.ASCENDING).first(2).print();
            System.out.println("==============================");
    
            //不分组，全局排序获取集合中的前3个元素，针对第一个元素升序，第二个元素倒序
            text.sortPartition(0,Order.ASCENDING).sortPartition(1,Order.DESCENDING).first(3).print();
        }
    
    }

import org.apache.flink.api.common.operators.Order
    import org.apache.flink.api.scala.ExecutionEnvironment
    import scala.collection.mutable.ListBuffer
    
    /** * Created by xxxx on 2020/10/09 */
    object BatchDemoFirstNScala { 
    
      def main(args: Array[String]): Unit = { 
    
        val env = ExecutionEnvironment.getExecutionEnvironment
        import org.apache.flink.api.scala._
    
        val data = ListBuffer[Tuple2[Int,String]]()
        data.append((2,"zs"))
        data.append((4,"ls"))
        data.append((3,"ww"))
        data.append((1,"xw"))
        data.append((1,"aw"))
        data.append((1,"mw"))
    
        val text = env.fromCollection(data)
    
        //获取前3条数据，按照数据插入的顺序
        text.first(3).print()
        println("==============================")
    
        //根据数据中的第一列进行分组，获取每组的前2个元素
        text.groupBy(0).first(2).print()
        println("==============================")
    
    
        //根据数据中的第一列分组，再根据第二列进行组内排序[升序]，获取每组的前2个元素
        text.groupBy(0).sortGroup(1,Order.ASCENDING).first(2).print()
        println("==============================")
    
    
        //不分组，全局排序获取集合中的前3个元素，
        text.sortPartition(0,Order.ASCENDING).sortPartition(1,Order.DESCENDING).first(3).print()
      }
    
    }

Sort Partition:在本地对数据集的所有分区进行排序，通过sortPartition()的链接调用来完成对多个字段的排序。  
Rebalance:对数据集进行再平衡，重分区，消除数据倾斜。  
Hash-Partition:根据指定key的哈希值对数据集进行分区（例如：partitionByHash（））  
Range-Partiton:根据指定的key对数据集进行范围分区（例如：partitionByRange（））  
Custom Partitioning:自定义分区规则  
一、自定义分区需要实现Partitioner接口  
二、partitionCustom(partitioner, “someKey”)  
三、或者partitionCustom(partitioner, 0)

### 1.11.3.DataSet Sink部分详解 ###

writeAsText():将元素以字符串形式逐行写入，这些字符串通过调用每个元素的toString()方法来获取  
writeAsCsv():将元组以逗号分隔写入文件中，行及字段之间的分隔是可配置的。每个字段的值来自对象的toString()方法  
print():打印每个元素的toString()方法的值到标准输出或者标准错误输出流中。