7.27_java学习_map应用

一：练习一

/*每个学生都有归属地。学生属性：姓名，年龄。注意：姓名和年龄相同视为同一学生。保证学生的唯一性。1，描述学生2，定义map容器，将学生作为键，地址作为值，存入。3，获取map集合中的元素。*/import java.util.*;class Student implements Comparable<Student>{private String name;private int age;Student(String name,int age){this.name = name;this.age = age;}public int compareTo(Student s){int num = new Integer(this.age).compareTo(new Integer(s.age));if(num == 0)return this.name.compareTo(s.name);return num;}public int hashCode(){return age;}public boolean equals(Object obj){if(!(obj instanceof Student))throw new ClassCastException("类型不匹配");Student s = (Student)obj;return  this.age == s.age && this.name.equals(s.name); }public String getName(){return name;}public int getAge(){return age;}public String toString(){return name+":"+age;}}class MapTest {public static void main(String[] args) {HashMap<Student,String> hm = new HashMap<Student, String>();hm.put(new Student("lisi1",21),"beijing");hm.put(new Student("lisi1",21),"tianjin");hm.put(new Student("lisi2",22),"shanghai");hm.put(new Student("lisi3",23),"beijing2");hm.put(new Student("lisi4",24),"beijing3");//第一种取出方式 keySetSet<Student> keySet = hm.keySet();Iterator<Student> it = keySet.iterator();while(it.hasNext()){Student s = it.next();String addr = hm.get(s);System.out.println(s +":" + addr);}//第二种取出方式entrySetSet<Map.Entry<Student,String>> entrySet = hm.entrySet();Iterator<Map.Entry<Student,String>> iter = entrySet.iterator();while(iter.hasNext()){Map.Entry<Student,String> me = iter.next();Student s = me.getKey();String addr = me.getValue();System.out.println(s +":....。。。。。。。" + addr);}//对学生类进行排序，根据年龄！TreeMap<Student,String> tm = new TreeMap<Student,String>(new StuNameCompare());tm.put(new Student("zhangsha1",21),"beijing");tm.put(new Student("wanwu1",21),"tianjin");tm.put(new Student("lisi2",22),"shanghai");tm.put(new Student("lisi3",23),"beijing2");tm.put(new Student("lisi4",24),"beijing3");Set<Map.Entry<Student,String>> entrySet2 = tm.entrySet();Iterator<Map.Entry<Student,String>> iter2 = entrySet2.iterator();while(iter2.hasNext()){Map.Entry<Student,String> me2 = iter2.next();Student s2 = me2.getKey();String addr2 = me2.getValue();System.out.println(s2 +":....。。。。。。。" + addr2);}}}class StuNameCompare implements Comparator <Student>{public int compare(Student s1, Student s2){int num = s1.getName().compareTo(s2.getName());if(num == 0)return new Integer(s1.getAge()).compareTo(new Integer(s2.getAge()));return num;}}

二：练习二

/*练习：“sdfgzxcvasdfxcudf”获取字符串字母出现的次数。希望打印结果：a(1)c(2)通过结果发现，每一个字母都有对应的次数。说明字母和次数之间都有映射关系时，可以选择map集合。注意了，当发现有映射关系时，可以选择map集合。因为map集合中存放就是映射关系。当发现有映射关系时，可以选择map集合。因为map集合中存放就是映射关系。什么时候使用map集合当数据之间存在着映射关系时，就先想到map集合思路：1，将字符串转换成字符数组2，定义一个map集合，因为打印结果又顺序，所以选择treemap集合。3，遍历字符数组将每一个字母作为键去查map集合。如果返回null，将该字母和1存入map集合中。如果返回不是null，说明该字母在map集合已经存在并对应次数。那么就获取该次数并进行自增4，将map集合中的数据变成指定的字符串形式返回。*/import java.util.*;class MapTest2{public static void main(String[] args){String s =charCount("abbcddeffggg");System.out.println(s);}public static String charCount (String str){char [] chs = str.toCharArray();TreeMap<Character,Integer> tm = new TreeMap<Character,Integer>();for(int x =0;x<chs.length;x++){Integer value = tm.get(chs[x]);if(value == null){tm.put(chs[x],1);}else{tm.put(chs[x],++value);}}System.out.println(tm);StringBuilder sb = new StringBuilder();Set<Map.Entry<Character,Integer>> entrySet = tm.entrySet();Iterator<Map.Entry<Character,Integer>> it = entrySet.iterator();while(it.hasNext()){Map.Entry<Character,Integer> me= it.next();char ch = me.getKey();int value = me.getValue();sb.append(ch+"("+value+")");}return sb.toString();}}/*map扩展知识。map集合被使用时因为具备映射关系。*/