Java容器——Map接口-白红宇

Java容器——Map接口

阅读量：5369 次

发布时间：2019-06-15

本文共 5468 字，大约阅读时间需要 18 分钟。

1.定义

Map用于保存存在映射关系<key, value>的数据。其中key值不能重复（使用equals()方法比较），value值可以重复。

2.常用实现类

HashMap：和HashSet类似，键按键的HashCode()方法确定存储位置，无序

TreeMap：用于对键进行排序，方式与TreeSet相同

LinkedHashMap：和LinkedHashSet类似

3.方法

3.1 HashMap常用方法

void clear()：移除所有映射

Object clone()：浅拷贝原映射

boolean containsKey(Object key)：判断是否包含指定键

boolean containsValue(Object value)：判断是否包含指定值

Set<Map.Entry<K,V>> entrySet()：返回包含映射关系的set集合

V get(Object key)：查找给定键的值

boolean isEmpty()：判断是否为空

Set<K> keySet()：返回map中键的set集合

V put(K key, V value)：存入键值对

V putIfAbsent(K key, V value)：存入键值对，如果该键未存在map中

V remove(Object key)：移除给定键的键值对

boolean remove(Object key, Object value)：移除给定的键值对

V replace(K key, V value)：替换给定键的值

boolean replace(K key, V oldValue, V newValue)：如果键值对符合要求，替换新值

int size()：返回键值对数目

Collection<V> values()：返回value集合

注：Map接口没有继承Iterable接口，所以不能直接通过map.iterator进行遍历(List，Map拥有该接口，可以直接遍历)，需要先转化为set类型，使用entrySet()方法，Map.Entry<k,v>中含有方法getKey()和getValue()，获取对应的键和值。

4.示例

MapFunc.java

1 import java.util.*; 2  3 public class MapFunc { 4     public static void main(String[] args) { 5  6         HashMap
      
        hm1 = new HashMap<>(); 7         hm1.put("a", new Customer(1,"AA")); 8         hm1.put("b", new Customer(2,"BB")); 9         hm1.put("c", new Customer(3,"CC"));10         hm1.put("d", new Customer(4,"DD"));11         hm1.put("e", new Customer(5,"EE"));12 13         /*14         * Map的几种遍历方法15         * keySet、values、entrySet、entrySet.iterator16         * */17         // 利用keySet()遍历18         Set
       
         keys = hm1.keySet();19         System.out.println("keys= "+ keys);  // [a, b, c, d]20         for(String key: keys){21             System.out.println("key= "+ key + " and value= " + hm1.get(key));22         }23 24         // 利用values()遍历，无法遍历key25         Collection
        
          values = hm1.values();26         System.out.println("values= "+ values);   // [Customer：[Id=1, Name=AA],...]27         for(Customer cus: values){28             System.out.println("value= " + cus);29         }30 31         // 利用entrySet遍历32         Set
         
          
           > entries = hm1.entrySet();33         System.out.println("entries= "+ entries);    // [a=Customer：[Id=1, Name=AA],...]34         for(Map.Entry
           
             entry: entries){35 System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());36 }37 38 // 利用entrySet().iterator()遍历39 Iterator
            
             
              > it = hm1.entrySet().iterator();40 while (it.hasNext()) {41 Map.Entry
              
                entry = it.next();42 System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());43 }44 45 /*46 * HashMap常用方法示例47 * */48 // get49 Customer b = hm1.get("b");50 System.out.println(b); // Customer：[Id=2, Name=BB]51 52 // putIfAbsent, key值存在，不覆盖value53 hm1.putIfAbsent("e",new Customer(5,"EEE"));54 System.out.println(hm1);55 56 // containsKey57 boolean flag = hm1.containsKey("b");58 System.out.println(flag); // true59 60 // containsValue61 flag = hm1.containsValue(b);62 System.out.println(flag);63 64 // remove65 hm1.remove("c");66 System.out.println(hm1); // Customer：[Id=3, Name=CC]67 flag = hm1.remove("b", new Customer(1,"BB"));68 System.out.println(flag); // false69 70 // replace71 hm1.replace("b", new Customer(2,"BBB"));72 System.out.println(hm1);73 hm1.replace("d",new Customer(4,"D"),74 new Customer(4,"DDD")); // 注意!!!75 System.out.println(hm1);76 77 // clone78 HashMap
               
                 hm2 = (HashMap
                
                 )hm1.clone();79 System.out.println(hm2);80 // clear81 hm2.clear();82 System.out.println(hm2); // {}83 System.out.println(hm1);84 // isEmpty85 flag = hm2.isEmpty();86 System.out.println(flag); // true87 // size88 int size = hm1.size();89 System.out.println(size); // 490 91 }92 }

View Code

Customer.java

1 import java.util.Objects; 2  3 public class Customer implements Comparable
      
       { 4  5     private int customerId; 6     private String customerName; 7  8     public Customer(Integer customerId, String customerName) { 9         this.customerId = customerId;10         this.customerName = customerName;11     }12 13     public int getCustomerId() {14         return customerId;15     }16     public String getCustomerName() {17         return customerName;18     }19 20     @Override21     public String toString() {22         return "Customer：[Id=" + customerId + ", Name=" + customerName + "]";23     }24 25      /*26      * 重写compareTo方法27      * 按Id或者name排序28      * 可以对整体添加负号决定升降序29      * */30     @Override31     public int compareTo(Customer o) {32 //        return this.customerId - o.customerId;33         return this.customerName.compareTo(o.customerName);34     }35 36     /*37     * 重写equals和hashcode方法38     * 这里id和name相同则为同一对象39     * */40     @Override41     public boolean equals(Object o) {42         if (this == o) return true;43         if (!(o instanceof Customer)) return false;44         Customer customer = (Customer) o;45         return customerId == customer.customerId &&46                 Objects.equals(customerName, customer.customerName);47     }48 49     @Override50     public int hashCode() {51         return Objects.hash(customerId, customerName);52     }53 54 }

View Code

CustomerComparator.java

1 import java.util.Comparator; 2  3 public class CustomerComparator implements Comparator
      
        { 4  5     @Override 6     public int compare(Customer c1, Customer c2) { 7         // 按Id排序 8         return c1.getCustomerId() - c2.getCustomerId(); 9      }10 }

View Code

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转载于:https://www.cnblogs.com/jfl-xx/p/4707319.html

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