JAVA核心技术I---JAVA基础知识（映射Map）

一：映射Map分类

二：Hashtable（同步，慢，数据量小）

–K-V对，K和V都不允许为null
–同步，多线程安全
–无序的
–适合小数据量
–主要方法：clear, contains/containsValue, containsKey, get, 
put,remove, size

（一）基本使用方法

        Hashtable<Integer,String> ht=new Hashtable<Integer,String>();
        
        //ht.put(1, null);    //编译不报错  运行时报错
        //ht.put(null, "das");
        
        ht.put(1000, "adad");
        ht.put(200, "adsdad");
        ht.put(1, "adadge");
        
        //进行判断是否含有数据
        System.out.println(ht.contains("adad"));
        System.out.println(ht.containsValue("adad"));    //contains和containsValue一样
        System.out.println(ht.containsKey(1));
        
        //进行数据获取
        System.out.println(ht.get(1));
        
        //进行更新
        ht.put(1, "666");    //按照键，更新值
        
        //进行移除
        System.out.println(ht.containsKey(200));
        ht.remove(200);
        System.out.println(ht.containsKey(200));
        
        //获取大小
        System.out.println(ht.size());

true
true
true
adadge
true
false
2

（二）遍历方法

    public static void traverseByEntry(Hashtable<Integer,String> ht){
        long startTime = System.nanoTime();
        System.out.println("=======Entry迭代器遍历=======");
        Integer key;
        String value;
        Iterator<Entry<Integer,String>> iter=ht.entrySet().iterator();
        while(iter.hasNext()) {
            Map.Entry<Integer, String> ent= iter.next();
            key = ent.getKey();
            value=ent.getValue();
            System.out.println(key+":"+value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }

    
    public static void traverseByKeySet(Hashtable<Integer,String> ht){
        long startTime = System.nanoTime();
        System.out.println("=======KeySet迭代器遍历=======");
        Integer key;
        String value;
        Iterator<Integer> iter=ht.keySet().iterator();
        while(iter.hasNext()) {
            key = iter.next();
            value=ht.get(key);
            System.out.println(key+":"+value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }

    //Enumeration只在Hashtable中用到了，后面废弃不用
    public static void traverseByKeyEnumeration(Hashtable<Integer,String> ht){
        long startTime = System.nanoTime();
        System.out.println("=======KeyEnumeration迭代器遍历=======");
        Integer key;
        String value;
        Enumeration<Integer> keys=ht.keys();    //一次性获取所有的key值，
        while(keys.hasMoreElements()) {
            key = keys.nextElement();   
            // 获取value
            value = ht.get(key);
            //System.out.println("Key:" + key + ", Value:" + value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }

（三）性能测试：数据量越大，差距越小

        for(int i=0;i<10000;i++)
        {
            ht.put(i, "aaa");
        }
        
        
        //进行遍历
        traverseByEntry(ht);
        traverseByKeySet(ht);
        traverseByKeyEnumeration(ht);

=======Entry迭代器遍历=======
13470233纳秒
=======KeySet迭代器遍历=======
8841551纳秒
=======KeyEnumeration迭代器遍历=======
4933705纳秒

三：HashMap（不支持同步，快，数据量大）

–K-V对，K和V都允许为nul
–不同步，多线程不安全
    Map m = Collections.synchronizedMap(new HashMap(...));
–无序的
–主要方法：clear, containsValue, containsKey, get, put,remove, size

（一）基本使用方法一致

        HashMap<Integer,String> hm =new  HashMap<Integer,String>();
        hm.put(1, null); 
        hm.put(null, "abc");  
        hm.put(1000, "aaa");
        hm.put(2, "bbb");
        hm.put(30000, "ccc");
        System.out.println(hm.containsValue("aaa"));
        System.out.println(hm.containsKey(30000));
        System.out.println(hm.get(30000));
        
        hm.put(30000, "ddd");  //更新覆盖ccc
        System.out.println(hm.get(30000));
        
        hm.remove(2);
        System.out.println("size: " + hm.size());
        
        hm.clear();
        System.out.println("size: " + hm.size());

View Code

（二）遍历方法除了没有Enumeration，其他一致

    public static void traverseByEntry(HashMap<Integer,String> ht)
    {
        long startTime = System.nanoTime();
        System.out.println("============Entry迭代器遍历==============");
        Integer key;
        String value;
        Iterator<Entry<Integer, String>> iter = ht.entrySet().iterator();
        while(iter.hasNext()) {
            Map.Entry<Integer, String> entry = iter.next();
            // 获取key
            key = entry.getKey();
            // 获取value
            value = entry.getValue();
            //System.out.println("Key:" + key + ", Value:" + value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }

traverseByEntry

    
    public static void traverseByKeySet(HashMap<Integer,String> ht)
    {
        long startTime = System.nanoTime();
        System.out.println("============KeySet迭代器遍历=============="); 
        Integer key;
        String value;
        Iterator<Integer> iter = ht.keySet().iterator();
        while(iter.hasNext()) {
            key = iter.next();            
            // 获取value
            value = ht.get(key);
            //System.out.println("Key:" + key + ", Value:" + value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }

traverseByKeySet

四：Properties (同步，文件形式，数据量小）

–继承于Hashtable
–可以将K-V对保存在文件
–适用于数据量少的配置文件
–继承自Hashtable的方法：clear, contains/containsValue, containsKey, 
get, put,remove, size
–从文件加载的load方法， 写入到文件中的store方法
–获取属性 getProperty ，设置属性setProperty

（一）使用方法

import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Enumeration;
import java.util.Properties;

//关于Properties类常用的操作
public class PropertiesTest {
    //根据Key读取Value
    public static String GetValueByKey(String filePath, String key) {
        Properties pps = new Properties();
        try {
            InputStream in = new BufferedInputStream (new FileInputStream(filePath));  
            pps.load(in); //所有的K-V对都加载了
            String value = pps.getProperty(key);
            //System.out.println(key + " = " + value);
            return value;
            
        }catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }
    
    //读取Properties的全部信息
    public static void GetAllProperties(String filePath) throws IOException {
        Properties pps = new Properties();
        InputStream in = new BufferedInputStream(new FileInputStream(filePath));
        pps.load(in); //所有的K-V对都加载了
        Enumeration en = pps.propertyNames(); //得到配置文件的名字
        
        while(en.hasMoreElements()) {
            String strKey = (String) en.nextElement();
            String strValue = pps.getProperty(strKey);
            //System.out.println(strKey + "=" + strValue);
        }
        
    }
    
    //写入Properties信息
    public static void WriteProperties (String filePath, String pKey, String pValue) throws IOException {
        File file = new File(filePath);
        if(!file.exists())
        {
            file.createNewFile();
        }
        Properties pps = new Properties();
        
        InputStream in = new FileInputStream(filePath);
        //从输入流中读取属性列表（键和元素对） 
        pps.load(in);　　//先加载原来的文件键值对，在其基础上进行设置，再写入
        //调用 Hashtable 的方法 put。使用 getProperty 方法提供并行性。  
        //强制要求为属性的键和值使用字符串。返回值是 Hashtable 调用 put 的结果。
        OutputStream out = new FileOutputStream(filePath);
        pps.setProperty(pKey, pValue);
        //以适合使用 load 方法加载到 Properties 表中的格式，  
        //将此 Properties 表中的属性列表（键和元素对）写入输出流  
        pps.store(out, "Update " + pKey + " name");
        out.close();
    }
    
    public static void main(String [] args) throws IOException{
        System.out.println("写入Test.properties================");
        WriteProperties("Test.properties","name", "12345");
        
        System.out.println("加载Test.properties================");
        GetAllProperties("Test.properties");
        
        System.out.println("从Test.properties加载================");
        String value = GetValueByKey("Test.properties", "name");
        System.out.println("name is " + value);
    }
}

五：LinkedHashMap和TreeMap

LinkedHashMap
–基于双向链表的维持插入顺序的HashMap
TreeMap
–基于红黑树的Map，可以根据key的自然排序或者compareTo方法进行排序输出

import java.util.LinkedHashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;

public class LinkedHashMapTest {

    public static void main(String[] args) {
        LinkedHashMap<Integer,String> hm =new  LinkedHashMap<Integer,String>();
        hm.put(1, null); 
        hm.put(null, "abc");  
        hm.put(1000, "aaa");
        hm.put(2, "bbb");
        hm.put(30000, "ccc");
        System.out.println(hm.containsValue("aaa"));
        System.out.println(hm.containsKey(30000));
        System.out.println(hm.get(30000));
        
        hm.put(30000, "ddd");  //更新覆盖ccc
        System.out.println(hm.get(30000));
        
        hm.remove(2);
        System.out.println("size: " + hm.size());
        
        //hm.clear();
        //System.out.println("size: " + hm.size());
        
        System.out.println("遍历开始==================");
        
        Integer key;
        String value;
        Iterator<Entry<Integer, String>> iter = hm.entrySet().iterator();
        while(iter.hasNext()) {
            Map.Entry<Integer, String> entry = iter.next();
            // 获取key
            key = entry.getKey();
            // 获取value
            value = entry.getValue();
            System.out.println("Key:" + key + ", Value:" + value);
        }
        System.out.println("遍历结束==================");        
        
        LinkedHashMap<Integer,String> hm2 =new  LinkedHashMap<Integer,String>();
        for(int i=0;i<100000;i++)
        {
            hm2.put(i, "aaa");
        }
        traverseByEntry(hm2);
        traverseByKeySet(hm2);        
    }
    
    public static void traverseByEntry(LinkedHashMap<Integer,String> ht)
    {
        long startTime = System.nanoTime();
        System.out.println("============Entry迭代器遍历==============");
        Integer key;
        String value;
        Iterator<Entry<Integer, String>> iter = ht.entrySet().iterator();
        while(iter.hasNext()) {
            Map.Entry<Integer, String> entry = iter.next();
            // 获取key
            key = entry.getKey();
            // 获取value
            value = entry.getValue();
            //System.out.println("Key:" + key + ", Value:" + value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }
    
    
    public static void traverseByKeySet(LinkedHashMap<Integer,String> ht)
    {
        long startTime = System.nanoTime();
        System.out.println("============KeySet迭代器遍历=============="); 
        Integer key;
        String value;
        Iterator<Integer> iter = ht.keySet().iterator();
        while(iter.hasNext()) {
            key = iter.next();            
            // 获取value
            value = ht.get(key);
            //System.out.println("Key:" + key + ", Value:" + value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }
}

LinkedHashMapTest

import java.util.TreeMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;

public class TreeMapTest {

    public static void main(String[] args) {
        TreeMap<Integer,String> hm =new  TreeMap<Integer,String>();
        hm.put(1, null); 
        //hm.put(null, "abc");  编译没错，运行报空指针异常  
        hm.put(1000, "aaa");
        hm.put(2, "bbb");
        hm.put(30000, "ccc");
        System.out.println(hm.containsValue("aaa"));
        System.out.println(hm.containsKey(30000));
        System.out.println(hm.get(30000));
        
        hm.put(30000, "ddd");  //更新覆盖ccc
        System.out.println(hm.get(30000));
        
        //hm.remove(2);
        System.out.println("size: " + hm.size());
        
        //hm.clear();
        //System.out.println("size: " + hm.size());
        
        System.out.println("遍历开始==================");
        
        Integer key;
        String value;
        Iterator<Entry<Integer, String>> iter = hm.entrySet().iterator();
        while(iter.hasNext()) {
            Map.Entry<Integer, String> entry = iter.next();
            // 获取key
            key = entry.getKey();
            // 获取value
            value = entry.getValue();
            System.out.println("Key:" + key + ", Value:" + value);
        }
        System.out.println("遍历结束==================");
        
        TreeMap<Integer,String> hm2 =new  TreeMap<Integer,String>();
        for(int i=0;i<100000;i++)
        {
            hm2.put(i, "aaa");
        }
        traverseByEntry(hm2);
        traverseByKeySet(hm2);        
    }
    
    public static void traverseByEntry(TreeMap<Integer,String> ht)
    {
        long startTime = System.nanoTime();
        System.out.println("============Entry迭代器遍历==============");
        Integer key;
        String value;
        Iterator<Entry<Integer, String>> iter = ht.entrySet().iterator();
        while(iter.hasNext()) {
            Map.Entry<Integer, String> entry = iter.next();
            // 获取key
            key = entry.getKey();
            // 获取value
            value = entry.getValue();
            //System.out.println("Key:" + key + ", Value:" + value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }
    
    
    public static void traverseByKeySet(TreeMap<Integer,String> ht)
    {
        long startTime = System.nanoTime();
        System.out.println("============KeySet迭代器遍历=============="); 
        Integer key;
        String value;
        Iterator<Integer> iter = ht.keySet().iterator();
        while(iter.hasNext()) {
            key = iter.next();            
            // 获取value
            value = ht.get(key);
            //System.out.println("Key:" + key + ", Value:" + value);
        }
        long endTime = System.nanoTime();
        long duration = endTime - startTime;
        System.out.println(duration + "纳秒");
    }
}

TreeMapTest

六：总结

–HashMap是最常用的映射结构
–如需要排序，考虑LinkedHashMap和TreeMap
–如需要将K-V存储为文件，可采用Properties类

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原文地址：https://www.cnblogs.com/ssyfj/p/10219982.html