本博客为子墨原创，转载请注明出处！
http://blog.csdn.net/zimo2013/article/details/8867065
1.Map概述
Map<K,V> Map集合类用于存储元素对（称作键和值），其中每个键映射到一个值,该接口描述了从不重复的键到值的映射。
2.Map子类
1).HashTable,底层是哈希表数据结构，不可以存入null键和null值，线程同步，JDK1.0效率低； 2).HashMap,底层是哈希表数据结构，可以存入null键和null值，不同步，JDK1.2效率高； 3).TreeMap,底层是二叉树数据结构，线程不同步，可以用于Map键排序.
3.Map集合转为Set集合
1).Set<K>keySet(),将所有的键存入Set集合，再使用迭代器获取value值 2).Set<Map.Entry<K,V>>entrySet(), entrySet()方法返回一个实现Map.Entry接口的Set集合,集合中存放的是键/值对应关系，该关系是Map.Entry型。其中Entry是Map接口的内部接口。 Map.Entry提供的方法: A)getKey(): 返回条目的关键字 B)getValue(): 返回条目的值
4.Map常见操作
1).添加操作： V put(K key, V value):如果key已存在，在关联后，返回替换前该key对应的value值,如果不存在，则返回null; void putAll(Map t):将来自特定映像的所有元素添加给该映像. 2).删除操作： V remove(Object key):从此映射中移除指定键的映射关系（如果存在），不存在则返回null; void clear() :从此映射中移除所有映射关系. 3).查询操作： V get(key): 获得与关键字key相关的值，并且返回与关键字key相关的对象，如果没有该关键字，则返回null;判断key是否存在，可以通过返回值是否等于null boolean containsKey(key): 判断映像中是否存在关键字key; boolean containsValue(Object value): 判断映像中是否存在值value; int size(): 返回当前映像中映射的数量; boolean isEmpty(): 判断映像中是否有任何映射. Collection values():返回映像中所有value值的集，由于值多个，用Collection集合，对其操作可以使用Collection基本方法.
5.Map简单应用
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import java.util.*; class MapDemo {
public static void main(String[] args) { Map<String, String> mp =
new HashMap<String, String>(); mp.put(
"02", "zhangsan"); mp.put(
"01", "lisi"); mp.put(
"04", "wangwu"); Set<String> setmap = mp.keySet(); Iterator<String> it = setmap.iterator();
while(it.hasNext()) { System.out.println(mp.get(it.next())); } Set<Map.Entry<String, String>> entrySet = mp.entrySet(); Iterator<Map.Entry<String, String>> it2 = entrySet.iterator();
while(it2.hasNext()) { Map.Entry<String, String> e = it2.next(); System.out.println(e.getKey() +
":"+ e.getValue()); } } }
[java]
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import java.util.*; class Student {
private int age; private String name; Student(String name,
int age) {
this.name = name; this.age = age; }
public String getName() {
return name; }
public int getAge() {
return age; }
public int hashCode() {
return name.hashCode()+age*17; }
public boolean equals(Object obj) {
if(!(obj instanceof Student)) throw new ClassCastException("error!"); Student s = (Student)obj;
return this.age==s.age && this.name.equals(s.name); } }
class MapDemo2 {
public static void main(String[] args) { HashMap<Student, String> hm =
new HashMap<Student, String>(); hm.put(
new Student("java02", 10), "Beijing"); hm.put(
new Student("java04", 40), "Wuhan"); hm.put(
new Student("java01", 4), "Zhengzhou"); Set<Student> s1 = hm.keySet(); Iterator<Student> it1 = s1.iterator();
while(it1.hasNext()) { Student t1 = it1.next(); System.out.println(t1.getName()+
" "+ t1.getAge() +" "+ hm.get(t1)); } Set<Map.Entry<Student, String>> s2 = hm.entrySet(); Iterator<Map.Entry<Student, String>> it2 = s2.iterator();
while(it2.hasNext()) { Map.Entry<Student, String> mp = it2.next(); System.out.println(mp.getKey().getName()+
".."+ mp.getKey().getAge() +".."+ mp.getValue()); } } }
[java]
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import java.util.*; class MapDemo2 {
public static void main(String[] args) { String str =
"abddkdsadjljasdak"; char[] ch = str.toCharArray(); TreeMap<Character, Integer> hm =
new TreeMap<Character, Integer>(new Comp()); for(int i=0; i<str.length(); i++) {
if(hm.get(ch[i]) == null) hm.put(ch[i],
1); else hm.put(ch[i], hm.get(ch[i])+
1); } Set<Character> s = hm.keySet(); Iterator<Character> it = s.iterator();
while(it.hasNext()) {
char c = it.next(); System.out.print(c +
"("+ hm.get(c) +") "); } } }
class Comp implements Comparator<Character> {
public int compare(Character c1, Character c2) {
return c2.compareTo(c1); } }
[java]
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import java.util.*; class MapDemo2 {
public static void main(String[] args) { Comp comp =
new Comp(); TreeMap<String, TreeMap<String, String>> school =
new TreeMap<String, TreeMap<String, String>>(); TreeMap<String, String> room1 =
new TreeMap<String, String>(comp); TreeMap<String, String> room2 =
new TreeMap<String, String>(comp); TreeMap<String, String> room3 =
new TreeMap<String, String>(comp); school.put(
"class01", room1); school.put(
"class02", room2); school.put(
"class03", room3); room1.put(
"04","zhang"); room1.put(
"02","li"); room2.put(
"01","liu"); room2.put(
"02","zhang"); room3.put(
"07","wang"); room3.put(
"03","zhao"); Iterator<String> it1 = school.keySet().iterator();
while(it1.hasNext()) { String className = it1.next(); TreeMap<String, String> c = school.get(className); System.out.println(className +
"...."); Iterator<String> it2 = c.keySet().iterator();
while(it2.hasNext()) { String studentId = it2.next(); System.out.println(studentId +
","+ c.get(studentId)); } } } }
class Comp implements Comparator<String> {
public int compare(String s1, String s2) {
return s2.compareTo(s1); } }
转载于:https://www.cnblogs.com/hanfeihanfei/p/6123344.html
