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
import java.util.*;
public class MapFunc {
public static void main(String[] args) {
HashMap<String, Customer> hm1 = new HashMap<>();
hm1.put("a", new Customer(1,"AA"));
hm1.put("b", new Customer(2,"BB"));
hm1.put("c", new Customer(3,"CC"));
hm1.put("d", new Customer(4,"DD"));
hm1.put("e", new Customer(5,"EE"));
/*
* Map的几种遍历方法
* keySet、values、entrySet、entrySet.iterator
* */
// 利用keySet()遍历
Set<String> keys = hm1.keySet();
System.out.println("keys= "+ keys); // [a, b, c, d]
for(String key: keys){
System.out.println("key= "+ key + " and value= " + hm1.get(key));
}
// 利用values()遍历，无法遍历key
Collection<Customer> values = hm1.values();
System.out.println("values= "+ values); // [Customer：[Id=1, Name=AA],...]
for(Customer cus: values){
System.out.println("value= " + cus);
}
// 利用entrySet遍历
Set<Map.Entry<String,Customer>> entries = hm1.entrySet();
System.out.println("entries= "+ entries); // [a=Customer：[Id=1, Name=AA],...]
for(Map.Entry<String, Customer> entry: entries){
System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}
// 利用entrySet().iterator()遍历
Iterator<Map.Entry<String, Customer>> it = hm1.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, Customer> entry = it.next();
System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}
/*
* HashMap常用方法示例
* */
// get
Customer b = hm1.get("b");
System.out.println(b); // Customer：[Id=2, Name=BB]
// putIfAbsent, key值存在，不覆盖value
hm1.putIfAbsent("e",new Customer(5,"EEE"));
System.out.println(hm1);
// containsKey
boolean flag = hm1.containsKey("b");
System.out.println(flag); // true
// containsValue
flag = hm1.containsValue(b);
System.out.println(flag);
// remove
hm1.remove("c");
System.out.println(hm1); // Customer：[Id=3, Name=CC]
flag = hm1.remove("b", new Customer(1,"BB"));
System.out.println(flag); // false
// replace
hm1.replace("b", new Customer(2,"BBB"));
System.out.println(hm1);
hm1.replace("d",new Customer(4,"D"),
new Customer(4,"DDD")); // 注意!!!
System.out.println(hm1);
// clone
HashMap<String, Customer> hm2 = (HashMap<String, Customer>)hm1.clone();
System.out.println(hm2);
// clear
hm2.clear();
System.out.println(hm2); // {}
System.out.println(hm1);
// isEmpty
flag = hm2.isEmpty();
System.out.println(flag); // true
// size
int size = hm1.size();
System.out.println(size); //
}
}
Customer.java
import java.util.Objects;
public class Customer implements Comparable<Customer>{
private int customerId;
private String customerName;
public Customer(Integer customerId, String customerName) {
this.customerId = customerId;
this.customerName = customerName;
}
public int getCustomerId() {
return customerId;
}
public String getCustomerName() {
return customerName;
}
@Override
public String toString() {
return "Customer：[Id=" + customerId + ", Name=" + customerName + "]";
}
/*
* 重写compareTo方法
* 按Id或者name排序
* 可以对整体添加负号决定升降序
* */
@Override
public int compareTo(Customer o) {
// return this.customerId - o.customerId;
return this.customerName.compareTo(o.customerName);
}
/*
* 重写equals和hashcode方法
* 这里id和name相同则为同一对象
* */
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Customer)) return false;
Customer customer = (Customer) o;
return customerId == customer.customerId &&
Objects.equals(customerName, customer.customerName);
}
@Override
public int hashCode() {
return Objects.hash(customerId, customerName);
}
}
CustomerComparator.java
import java.util.Comparator;
public class CustomerComparator implements Comparator<Customer> {
@Override
public int compare(Customer c1, Customer c2) {
// 按Id排序
return c1.getCustomerId() - c2.getCustomerId();
}
}
!!!