「java新建集合」java怎么定义集合

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本文目录一览：

1、Java编程实现创建一个集合类对象
2、eclipse如何新建一个集合类java
3、用java创建一个List集合
4、java 怎么创建集合

Java编程实现创建一个集合类对象

import java.util.ArrayList;

import java.util.HashMap;

import java.util.HashSet;

import java.util.Iterator;

import java.util.List;

import java.util.ListIterator;

import java.util.Map;

import java.util.Set;

public class Demo_02 {

public static V void main(String[] args) {

数组:数组的长度固定。

集合 :长度可变。

* 2. 几种集合的介绍（接口）

* 2.1、List（有序、可重复）

* 2.2、Set（无序、不能重复）

* 2.3、Map（键值对、键唯一、值不唯一）

//创建一个list集合 ArrayListString是这个list集合的实现类

ListString list=new ArrayListString();

list.add("1");

list.add("2");

list.add("4");

list.add("3");

//判断集合是否为空

boolean empty = list.isEmpty();

System.out.println(empty);

System.out.println(list);

//移除集合索引上的值

list.remove(0);

System.out.println(list);

list.add("1");

/* 遍历

四种的常见输出方式：

1.Iterator：迭代输出，是使用最多的输出方式。

2.ListIterator：是Iterator的子接口，专门用于输出List中的内容。

3.foreach输出：JDK1.5之后提供的新功能，可以输出数组或集合。

4.for循环

System.out.println("for遍历：");

for (int i = 0; i list.size(); i++) {

System.out.println(list.get(i));

}

System.out.println("foreach遍历:");

for (String s : list) {

System.out.println(s);

}

System.out.println("iterator的形式：");

Iterator it = list.iterator();

while(it.hasNext())

{

Object a = it.next();

System.out.println(a);

}

//ListIterator是Iterator的子接口，专门用于输出List中的内容。

System.out.println("ListIterator:");

ListIteratorString it2 = list.listIterator();

while(it2.hasNext())

{

Object a = it2.next();

System.out.println(a);

}

//创建一个Map集合

Map map=new HashMapString,String();

map.put("1", "a1");

map.put("2", "a2");

map.put("4", "a4");

map.put("3", "a3");

System.out.println(map);

//创建一个Set

Set set=new HashSetString();

set.add("1");

set.add("2");

set.add("4");

set.add("3");

System.out.println(set);

}

eclipse如何新建一个集合类java

1、下载Hadoop-eclipse-plugin-1.2.1.jar，并将之复制到eclipse/plugins下。

2、打开map-reduce视图

在eclipse中，打开window——open perspetive——other，选择map/reduce。

3、选择Map/Reduce Locations标签页，新建一个Location

4、在project exploer中，可以浏览刚才定义站点的文件系统

5、准备测试数据，并上传到hdfs中。

liaoliuqingdeMacBook-Air:Downloads liaoliuqing$ hadoop fs -mkdir in

liaoliuqingdeMacBook-Air:Downloads liaoliuqing$ hadoop fs -copyFromLocal maxTemp.txt in

liaoliuqingdeMacBook-Air:Downloads liaoliuqing$ hadoop fs -ls in

Found 1 items

-rw-r--r-- 1 liaoliuqing supergroup 953 2014-12-14 09:47 /user/liaoliuqing/in/maxTemp.txt

其中maxTemp.txt的内容如下：

123456798676231190101234567986762311901012345679867623119010123456798676231190101234561+00121534567890356

123456798676231190101234567986762311901012345679867623119010123456798676231190101234562+01122934567890456

123456798676231190201234567986762311901012345679867623119010123456798676231190101234562+02120234567893456

123456798676231190401234567986762311901012345679867623119010123456798676231190101234561+00321234567803456

123456798676231190101234567986762311902012345679867623119010123456798676231190101234561+00429234567903456

123456798676231190501234567986762311902012345679867623119010123456798676231190101234561+01021134568903456

123456798676231190201234567986762311902012345679867623119010123456798676231190101234561+01124234578903456

123456798676231190301234567986762311905012345679867623119010123456798676231190101234561+04121234678903456

123456798676231190301234567986762311905012345679867623119010123456798676231190101234561+00821235678903456

6、准备map-reduce程序

程序请见

7、运行程序

MaxTemperature.Java——run as——run configuration

在arguments中填入输入及输出目录，开始run。

此处是在hdfs中运行程序，事实上也可以在本地文件系统中运行程序，此方法可以方便的用于程序调试。

如在参数中填入：

/Users/liaoliuqing/in /Users/liaoliuqing/out

即可。

8、以下是eclise console中的输出内容

14/12/14 10:52:05 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable

14/12/14 10:52:05 WARN mapred.JobClient: Use GenericOptionsParser for parsing the arguments. Applications should implement Tool for the same.

14/12/14 10:52:05 WARN mapred.JobClient: No job jar file set. User classes may not be found. See JobConf(Class) or JobConf#setJar(String).

14/12/14 10:52:05 INFO input.FileInputFormat: Total input paths to process : 1

14/12/14 10:52:05 WARN snappy.LoadSnappy: Snappy native library not loaded

14/12/14 10:52:06 INFO mapred.JobClient: Running job: job_local1815770300_0001

14/12/14 10:52:06 INFO mapred.LocalJobRunner: Waiting for map tasks

14/12/14 10:52:06 INFO mapred.LocalJobRunner: Starting task: attempt_local1815770300_0001_m_000000_0

14/12/14 10:52:06 INFO mapred.Task: Using ResourceCalculatorPlugin : null

14/12/14 10:52:06 INFO mapred.MapTask: Processing split: hdfs://localhost:9000/user/liaoliuqing/in/maxTemp.txt:0+953

14/12/14 10:52:06 INFO mapred.MapTask: io.sort.mb = 100

14/12/14 10:52:06 INFO mapred.MapTask: data buffer = 79691776/99614720

14/12/14 10:52:06 INFO mapred.MapTask: record buffer = 262144/327680

14/12/14 10:52:06 INFO mapred.MapTask: Starting flush of map output

14/12/14 10:52:06 INFO mapred.MapTask: Finished spill 0

14/12/14 10:52:06 INFO mapred.Task: Task:attempt_local1815770300_0001_m_000000_0 is done. And is in the process of commiting

14/12/14 10:52:06 INFO mapred.LocalJobRunner:

14/12/14 10:52:06 INFO mapred.Task: Task 'attempt_local1815770300_0001_m_000000_0' done.

14/12/14 10:52:06 INFO mapred.LocalJobRunner: Finishing task: attempt_local1815770300_0001_m_000000_0

14/12/14 10:52:06 INFO mapred.LocalJobRunner: Map task executor complete.

14/12/14 10:52:06 INFO mapred.Task: Using ResourceCalculatorPlugin : null

14/12/14 10:52:06 INFO mapred.LocalJobRunner:

14/12/14 10:52:06 INFO mapred.Merger: Merging 1 sorted segments

14/12/14 10:52:06 INFO mapred.Merger: Down to the last merge-pass, with 1 segments left of total size: 90 bytes

14/12/14 10:52:06 INFO mapred.LocalJobRunner:

14/12/14 10:52:06 INFO mapred.Task: Task:attempt_local1815770300_0001_r_000000_0 is done. And is in the process of commiting

14/12/14 10:52:06 INFO mapred.LocalJobRunner:

14/12/14 10:52:06 INFO mapred.Task: Task attempt_local1815770300_0001_r_000000_0 is allowed to commit now

14/12/14 10:52:06 INFO output.FileOutputCommitter: Saved output of task 'attempt_local1815770300_0001_r_000000_0' to hdfs://localhost:9000/user/liaoliuqing/out

14/12/14 10:52:06 INFO mapred.LocalJobRunner: reduce reduce

14/12/14 10:52:06 INFO mapred.Task: Task 'attempt_local1815770300_0001_r_000000_0' done.

14/12/14 10:52:07 INFO mapred.JobClient: map 100% reduce 100%

14/12/14 10:52:07 INFO mapred.JobClient: Job complete: job_local1815770300_0001

14/12/14 10:52:07 INFO mapred.JobClient: Counters: 19

14/12/14 10:52:07 INFO mapred.JobClient: File Output Format Counters

14/12/14 10:52:07 INFO mapred.JobClient: Bytes Written=43

14/12/14 10:52:07 INFO mapred.JobClient: File Input Format Counters

14/12/14 10:52:07 INFO mapred.JobClient: Bytes Read=953

14/12/14 10:52:07 INFO mapred.JobClient: FileSystemCounters

14/12/14 10:52:07 INFO mapred.JobClient: FILE_BYTES_READ=450

14/12/14 10:52:07 INFO mapred.JobClient: HDFS_BYTES_READ=1906

14/12/14 10:52:07 INFO mapred.JobClient: FILE_BYTES_WRITTEN=135618

14/12/14 10:52:07 INFO mapred.JobClient: HDFS_BYTES_WRITTEN=43

14/12/14 10:52:07 INFO mapred.JobClient: Map-Reduce Framework

14/12/14 10:52:07 INFO mapred.JobClient: Reduce input groups=5

14/12/14 10:52:07 INFO mapred.JobClient: Map output materialized bytes=94

14/12/14 10:52:07 INFO mapred.JobClient: Combine output records=0

14/12/14 10:52:07 INFO mapred.JobClient: Map input records=9

14/12/14 10:52:07 INFO mapred.JobClient: Reduce shuffle bytes=0

14/12/14 10:52:07 INFO mapred.JobClient: Reduce output records=5

14/12/14 10:52:07 INFO mapred.JobClient: Spilled Records=16

14/12/14 10:52:07 INFO mapred.JobClient: Map output bytes=72

14/12/14 10:52:07 INFO mapred.JobClient: Total committed heap usage (bytes)=329252864

14/12/14 10:52:07 INFO mapred.JobClient: SPLIT_RAW_BYTES=118

14/12/14 10:52:07 INFO mapred.JobClient: Map output records=8

14/12/14 10:52:07 INFO mapred.JobClient: Combine input records=0

14/12/14 10:52:07 INFO mapred.JobClient: Reduce input records=8

用java创建一个List集合

package test;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

/**

* 用java创建一个List集合用java创建一个list集合，保存'a b a b c a c b a a b'。

* 然后使用Map集合，key保存对应的字母，value保存对应字母的出现次数。

* @author ASUS

public class Test15 {

public static void main(String[] args) {

ListCharacter list = new ArrayListCharacter();

list.add('a');

list.add('b');

list.add('a');

list.add('b');

list.add('c');

list.add('a');

list.add('c');

list.add('b');

list.add('a');

list.add('b');

MapCharacter,Integer map = new HashMapCharacter,Integer();

for(Character ch : list){

if(map.containsKey(ch)){

int num = map.get(ch);

num+=1;

map.put(ch, num);

}else{

map.put(ch, 1);

}

System.out.println(map);

}

java 怎么创建集合

java的集合有List

List a=new ArrayList();//实例化一个对象

a.add("");

a.get(0);

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