Java 容器(四) Map

傷城~ 2022-08-21 14:51 165阅读 0赞

***Java 容器(四) Map***

**1.Map的使用估计是最常见的了： **

**1）HashMap：**存储的值可以重复，键不能；不能使用Comparator和Comparable接口排序；HashMap最多只允许一条记录的键为null，不允许多条记录的键为null

**2）TreeMap：**存储的值可以重复，键不能；自动排序或手动排序；

**3）LinkedHashMap：**存储的值可以重复，键不能；保持输入的顺序

其中它们的用法和Set对比，几乎是一一对应：

package roadArchitectWeb.Test;
    import java.util.Comparator;
    import java.util.HashMap;
    import java.util.LinkedHashMap;
    import java.util.Map;
    import java.util.TreeMap;
    class Student{
    	private Integer id;
    	private String name;
    	public Integer getId() {
    		return id;
    	}
    	public void setId(Integer id) {
    		this.id = id;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    	public Student(Integer id, String name) {
    		super();
    		this.id = id;
    		this.name = name;
    	}
    	@Override
    	public String toString() {
    		return "Student [id=" + id + ", name=" + name + "]";
    	}
    	@Override
    	public int hashCode() {
    		return this.id;
    	}
    	@Override
    	public boolean equals(Object obj) {
    		if(obj instanceof Student){
    			Student stu = (Student)obj;
    			if(stu.name == this.name && stu.id == this.id)
    				return true;
    		}
    		return false;
    	}
    }
    public class Test5 {
    		protected static Map<String,Student> hashMap = new HashMap<String,Student>();
    		/*Comparable的排序会失效,因为map只能按键排列*/
    		protected static Map<String,Student> treeMapComparable = new TreeMap<String,Student>();
    		protected static Map<String,Student> treeMapComparator = new TreeMap<String,Student>(new Comparator<String>() {
    			@Override
    			public int compare(String o1, String o2) {
    				return o2.compareTo(o1);
    			}
    		});
    		protected static Map<String,Student> linkedHashMap = new LinkedHashMap<String,Student>();
    		public static void main(String[] args) {
    		hashMap.put("1",new Student(3,"sun"));
    		hashMap.put("2",new Student(1,"zhao"));
    		hashMap.put("3",new Student(2,"qian"));
    		System.out.println("hashMap:"+hashMap);
    		/*
    		 *这里使用Comparable
    		 */
    		treeMapComparable.put("3",new Student(3,"sun"));
    		treeMapComparable.put("2",new Student(1,"zhao"));
    		treeMapComparable.put("1",new Student(2,"qian"));
    		System.out.println("treeMapComparable:"+treeMapComparable);
    		
    		/*这里使用Comparator
    		 */
    		treeMapComparator.put("3",new Student(3,"sun"));
    		treeMapComparator.put("1",new Student(1,"zhao"));
    		treeMapComparator.put("2",new Student(2,"qian"));
    		System.out.println("treeMapComparator:"+treeMapComparator);
    		
    		linkedHashMap.put("3",new Student(3,"sun"));
    		linkedHashMap.put("1",new Student(1,"zhao"));
    		linkedHashMap.put("2",new Student(2,"qian"));
    		System.out.println("linkedHashMap:"+linkedHashMap);
    	}
    }

结果如下：

hashMap:{1=Student [id=3, name=sun], 2=Student [id=1, name=zhao], 3=Student [id=2, name=qian]}
    treeMapComparable:{1=Student [id=2, name=qian], 2=Student [id=1, name=zhao], 3=Student [id=3, name=sun]}
    treeMapComparator:{3=Student [id=3, name=sun], 2=Student [id=2, name=qian], 1=Student [id=1, name=zhao]}
    linkedHashMap:{3=Student [id=3, name=sun], 1=Student [id=1, name=zhao], 2=Student [id=2, name=qian]}

**2.注意**

**1）**同Set，hashMap和LinkedHashMap不支持排序；

**2）**hashTable：基本上与hashMap相同，它不允许记录的键或者值为空；线程同步；

**3）**HashSet是通过HashMap实现的,TreeSet是通过TreeMap实现的,只不过Set用的只是Map的key；

**4）**另外注意到，treeMap当key是基础类，如String时候，似乎只能使用Comparator接口，基础类的Comparable接口是内置的（TreeMap排序使用Comparable没有实现过）

**3.Map中迭代器使用：**

/*遍历key*/
    		Iterator it1 = hashMap.keySet().iterator();
    		while(it1.hasNext()){
    			System.out.println("key:"+it1.next());
    		}
    		/*遍历entry，从而可以获得key或者value的值*/
    		Iterator it2 = hashMap.entrySet().iterator();
    		while(it2.hasNext()){
    			Map.Entry<String, Student> entry = null;
    			entry = (Entry<String, Student>) it2.next();
    			System.out.println("value:"+entry.getValue());
    		}

**结果如下：**

key:1
    key:2
    key:3
    value:Student [id=3, name=sun]
    value:Student [id=1, name=zhao]
    value:Student [id=2, name=qian]