java Map

Posted 2021-02-03 飞龙dragon

https://www.cnblogs.com/lzq198754/p/5780165.html

Map.一组成对的"键值对"对象,允许你使用键来查找值,ArrayList允许你使用数字来查找值,因此在某种意义上讲,它将数字与对象关联在了一起.映射表允许我们使用另一个对象,它也被称为"关联数组",因为它将某些对象与另外一些对象关联在了一起,或者被称为"字典",因为你可以使用键对象来查找值对象,就像在字典中使用单词来定义一样,Map是强大的编程工具

将对象映射到其它对象的能力是一种解决编程问题的杀手锏,例如考虑一个程序,它将接受来自Java的Random类到随机性,在理想状态下,Random可以产生理想的数字分布,但要向测试它,则需要产生大量的随机数,不对落入各种不同范围的数字进行计数,Map很容易可以解决该问题,

通用Map有:HashMap、Hashtable、Properties、LinkedHashMap、IdentityHashMap、TreeMap、WeakHashMap、ConcurrentHashMap

抽象Map:AbstractMap

java Map类

package java.util;

import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.io.Serializable;
public interface Map<K,V> {
    int size();
    boolean isEmpty();//判空
    boolean containsKey(Object key);  //检查容器中是否包含键key
    boolean containsValue(Object value);//检查容器中是否包含值value
    V get(Object key);        //查询键是否在容器中,如果键不在容器中,返回null,否则返回键对应的value
    V put(K key, V value); //设置键key,和键的值value,key(键)在容器中不能重复
    V remove(Object key);  //移除键key及其value
    void putAll(Map<? extends K, ? extends V> m);
    void clear();      //清空Map
    Set<K> keySet();   //返回键的Set
    Collection<V> values(); //返回值的Collection
    Set<Map.Entry<K, V>> entrySet(); //返回 Map 中所包含映射的 Set 视图。Set 中的每个元素都是一个 Map.Entry 对象
                //可以使用Iterator调用Map.Entry 对象,Map.Entry对象又可以使用Entry的方法获取键和值
   interface Entry<K,V> { //Entry的用法 for(Map.Entry<Integer,String> entry: map.entrySet())
                         // {entry.getValue();entry.getKey(　)}
       K getKey();
       V getValue();
       V setValue(V value);
     boolean equals(Object o);
       int hashCode();
        public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> c1.getKey().compareTo(c2.getKey());
        }
       public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> c1.getValue().compareTo(c2.getValue());
        }

        public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
            Objects.requireNonNull(cmp);
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
        }
       public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
            Objects.requireNonNull(cmp);
            return (Comparator<Map.Entry<K, V>> & Serializable)
                (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
        }
    }
    boolean equals(Object o);
    int hashCode();
    default V getOrDefault(Object key, V defaultValue) {
        V v;
        return (((v = get(key)) != null) || containsKey(key))
            ? v
            : defaultValue;
    }
    default void forEach(BiConsumer<? super K, ? super V> action) {
//用法 map.forEach((k,v)-> System.out.println(k+ " " + v));;

        Objects.requireNonNull(action);
        for (Map.Entry<K, V> entry : entrySet()) {
            K k;
            V v;
            try {
                k = entry.getKey();
                v = entry.getValue();
            } catch(IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                throw new ConcurrentModificationException(ise);
            }
            action.accept(k, v);
        }
    }
    default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        Objects.requireNonNull(function);
        for (Map.Entry<K, V> entry : entrySet()) {
            K k;
            V v;
            try {
                k = entry.getKey();
                v = entry.getValue();
            } catch(IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                throw new ConcurrentModificationException(ise);
            }

            // ise thrown from function is not a cme.
            v = function.apply(k, v);

            try {
                entry.setValue(v);
            } catch(IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                throw new ConcurrentModificationException(ise);
            }
        }
    }
    default V putIfAbsent(K key, V value) {
        V v = get(key);
        if (v == null) {
            v = put(key, value);
        }

        return v;
    }
    default boolean remove(Object key, Object value) {
        Object curValue = get(key);
        if (!Objects.equals(curValue, value) ||
            (curValue == null && !containsKey(key))) {
            return false;
        }
        remove(key);
        return true;
    }
    default boolean replace(K key, V oldValue, V newValue) {
        Object curValue = get(key);
        if (!Objects.equals(curValue, oldValue) ||
            (curValue == null && !containsKey(key))) {
            return false;
        }
        put(key, newValue);
        return true;
    }
   default V replace(K key, V value) {
        V curValue;
        if (((curValue = get(key)) != null) || containsKey(key)) {
            curValue = put(key, value);
        }
        return curValue;
    }
    default V computeIfAbsent(K key,
            Function<? super K, ? extends V> mappingFunction) {
        Objects.requireNonNull(mappingFunction);
        V v;
        if ((v = get(key)) == null) {
            V newValue;
            if ((newValue = mappingFunction.apply(key)) != null) {
                put(key, newValue);
                return newValue;
            }
        }

        return v;
    }
   default V computeIfPresent(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue;
        if ((oldValue = get(key)) != null) {
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                put(key, newValue);
                return newValue;
            } else {
                remove(key);
                return null;
            }
        } else {
            return null;
        }
    }
   default V compute(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue = get(key);

        V newValue = remappingFunction.apply(key, oldValue);
        if (newValue == null) {
            // delete mapping
            if (oldValue != null || containsKey(key)) {
                // something to remove
                remove(key);
                return null;
            } else {
                // nothing to do. Leave things as they were.
                return null;
            }
        } else {
            // add or replace old mapping
            put(key, newValue);
            return newValue;
        }
    }
   default V merge(K key, V value,
            BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        Objects.requireNonNull(value);
        V oldValue = get(key);
        V newValue = (oldValue == null) ? value :
                   remappingFunction.apply(oldValue, value);
        if(newValue == null) {
            remove(key);
        } else {
            put(key, newValue);
        }
        return newValue;
    }
}

 

//: holding/Statistics.java
// Simple demonstration of HashMap.
package object;
import java.util.*;

public class Statistics {
  public static void main(String[] args) {
    Random rand = new Random(47);
    Map<Integer,Integer> m =
      new HashMap<Integer,Integer>();
    for(int i = 0; i < 10000; i++) {
      // Produce a number between 0 and 20:
      int r = rand.nextInt(20);
      Integer freq = m.get(r);//get()返回键的值,put()设置键和值
      m.put(r, freq == null ? 1 : freq + 1);//键是Random产生的值,值是该数字的字数
    }
    System.out.println(m);
  }
} /* Output:
{15=497, 4=481, 19=464, 8=468, 11=531, 16=533, 18=478, 3=508, 7=471, 12=521, 17=509, 2=489, 13=506, 9=549, 6=519, 1=502, 14=477, 10=513, 5=503, 0=481}
*///:~

下列示例演示了String

//: holding/PetMap.java
package object;
import typeinfo.pets.*;
import java.util.*;
import static net.mindview.util.Print.*;

public class PetMap {
  public static void main(String[] args) {
    Map<String,Pet> petMap = new HashMap<String,Pet>();
    petMap.put("My Cat", new Cat("Molly"));
    petMap.put("My Dog", new Dog("Ginger"));
    petMap.put("My Hamster", new Hamster("Bosco"));
    print(petMap);
    Pet dog = petMap.get("My Dog");
    print(dog);
    print(petMap.containsKey("My Dog"));//检查容器中是否包含键"My Dog"
    print(petMap.containsValue(dog));   //检查容器中是否包含value"dog"

} } /* Output: {My Cat=Cat Molly, My Hamster=Hamster Bosco, My Dog=Dog Ginger} Dog Ginger true true *///:~

map可以很容易扩展到多维,而我们只需要将其值设置为map(这些Map的值可以是其它容器,甚至是map),因此我们能很容易将容器组合起来从而快速生成强大的数据结构,下面是一个示例

//: holding/MapOfList.java
package object;
import typeinfo.pets.*;
import java.util.*;
import static net.mindview.util.Print.*;

public class MapOfList {
  public static Map<Person, List<? extends Pet>>
    petPeople = new HashMap<Person, List<? extends Pet>>();
  static {
    petPeople.put(new Person("Dawn"),
      Arrays.asList(new Cymric("Molly"),new Mutt("Spot")));
    petPeople.put(new Person("Kate"),
      Arrays.asList(new Cat("Shackleton"),
        new Cat("Elsie May"), new Dog("Margrett")));
    petPeople.put(new Person("Marilyn"),
      Arrays.asList(
       new Pug("Louie aka Louis Snorkelstein Dupree"),
       new Cat("Stanford aka Stinky el Negro"),
       new Cat("Pinkola")));    
    petPeople.put(new Person("Luke"),
      Arrays.asList(new Rat("Fuzzy"), new Rat("Fizzy")));
    petPeople.put(new Person("Isaac"),
      Arrays.asList(new Rat("Freckly")));
  }
  public static void main(String[] args) {
    print("People: " + petPeople.keySet());  //返回键的Set
    print("Pets: " + petPeople.values());    //返回值的Collection
    for(Person person : petPeople.keySet()) {
      print(person + " has:");
      for(Pet pet : petPeople.get(person))
        print("    " + pet);
    }
  }
} /* Output:    
People: [Person Luke, Person Marilyn, Person Isaac, Person Dawn, Person Kate]
Pets: [[Rat Fuzzy, Rat Fizzy], [Pug Louie aka Louis Snorkelstein Dupree, Cat Stanford aka Stinky el Negro, Cat Pinkola], [Rat Freckly], [Cymric Molly, Mutt Spot], [Cat Shackleton, Cat Elsie May, Dog Margrett]]
Person Luke has:
    Rat Fuzzy
    Rat Fizzy
Person Marilyn has:
    Pug Louie aka Louis Snorkelstein Dupree
    Cat Stanford aka Stinky el Negro
    Cat Pinkola
Person Isaac has:
    Rat Freckly
Person Dawn has:
    Cymric Molly
    Mutt Spot
Person Kate has:
    Cat Shackleton
    Cat Elsie May
    Dog Margrett
*///:~