Java - HashMap

//HashMap： 底层是哈希表数据结构，允许使用null键或值，线程不同步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 void setName(String name)    {        this.name=name;    }    public void setAge(int Age)    {        this.age=age;    }    public String getName()    {        return this.name;    }    public int getAge()    {        return this.age;    }    public String toString()    {        return name+":"+age;    }    public int hashCode()    {        return name.hashCode()+this.age*23;    }    public boolean equals(Object obj)    {        if(!(obj instanceof Student))            throw new ClassCastException("类型不匹配");        Student s = (Student)obj;        return this.name.equals(s.name) && this.age==s.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;    }}class HashMapTest{    public static void main(String[] args){        HashMap<Student,String> hm=new HashMap<Student,String>() ;        hm.put(new Student("test1",21),"beijing"); //新增元素        hm.put(new Student("test2",22),"nanjing");        hm.put(new Student("test3",23),"shanghai");        hm.put(new Student("test4",24),"hangzhou");        //map 第一种取出方式：keySet        Set<Student> keyset =hm.keySet();        Iterator<Student> it=keyset.iterator();        while(it.hasNext())        {            Student stu= it.next();            String addr=hm.get(stu);            System.out.println(stu+" "+addr);        }        //map 第二种取出方式：entrySet        Set<Map.Entry<Student,String>> mapentry = hm.entrySet();        Iterator<Map.Entry<Student,String>> itr=mapentry.iterator();        while(itr.hasNext())        {            Map.Entry<Student,String> me= itr.next();            Student s = me.getKey();            String addr = me.getValue();            System.out.println(s+" "+addr);        }    }}