KeySet跟entrySet为什么会有两种不同的结果。。

Question

import java.util.*;

class Student implements Comparable
{
private String id;
private String name;
Student(String id,String name)
{
this.id = id;
this.name = name;
}
public String getId()
{
return id;
}
public String getName()
{
return name;
}
public String toString()
{
return id +"::" + name;
}
public int hashCode()
{
return this.id.hashCode() + this.name.hashCode()*30;
}
public boolean equals(Object obj)
{
if(obj instanceof Student)
throw new ClassCastException("类型不符合");

    Student s = (Student)obj;
    return this.name.equals(s.name) && this.id.equals(s.id);
}
public int compareTo(Student s)
{
    if(id.compareTo(s.id)==0)
        return this.name.compareTo(s.name);
    return this.id.compareTo(s.id);
}

}
class MyCmpt implements Comparator
{
public int compare(String o1,String o2)
{
return 1;
}
}
class MapDemo3
{
public static void demo()
{
TreeMap> czbk = new TreeMap>(new MyCmpt());

    List<Student> yure = new ArrayList<Student>();
    List<Student> jiuye = new ArrayList<Student>();

    czbk.put("yure",yure);
    czbk.put("jiuye",jiuye);

    yure.add(new Student("01","zhangsan"));
    yure.add(new Student("02","lisisan"));

    jiuye.add(new Student("01","zhangsansan"));
    jiuye.add(new Student("02","zhangxiaosan"));

    Set<Map.Entry<String,List<Student>>> e = czbk.entrySet();

    for(Iterator<Map.Entry<String,List<Student>>> it = e.iterator(); it.hasNext();)
    {
        Map.Entry<String,List<Student>> me = it.next();
        String st = me.getKey();
        List<Student> stu = me.getValue();
        System.out.println(st);
        getInfos(stu);
    }

    /*Set<String> s = czbk.keySet();
    for(Iterator<String> it = s.iterator();it.hasNext();)
    {
        String str = it.next();
        List<Student> stu = czbk.get(str);
        System.out.println(str);
        System.out.println(stu);
    }*/
}

public static void getInfos(List<Student> list)
{
    for(Iterator<Student> it = list.iterator();it.hasNext();)
    {
        Student s = it.next();
        System.out.println(s);
    }
}

public static void main(String[] args) 
{
    demo();
}

}
上面的代码我加入了比较器之后，用entrySet方法用迭代器去遍历键还是指向了值，但是我用keySet方法去遍历，键却指向了空，求解惑？

Answer 1

问题在于

 czbk = new TreeMap>(new MyCmpt());//这里设置了比较器
 但你的MyCmpt定义只有一种返回值
 class MyCmpt implements Comparator
{
public int compare(String o1,String o2)
{
return 1;
}
也就是不管什么key值，都取右节点接着找，直到末尾，最终因为找不到，所以返回null。
这里改成key值相等的时候就返回
public int compare(Object o1,Object o2)
{
return ((String)o1).equals(o2)?0:1;
}