Question

我正在尝试从HashMap个ArrayList个对象创建Student。我想从名称中选取字母作为键，并根据名称将每个数组列表附加到该键。

例如，如果输入为：

Student{roll no.,Name, Age} {105,Alex,23} {102,Alexander,24} {101,Becky,23}

HashMap应为：

{A,{105,Alex,23}}{102,Alexander,24}}{B,{101,Becky,23}}.

我的代码：

package moduletest;

import java.util.*;
import java.io.*;

class Simple {
    @SuppressWarnings("unchecked")
    public static void main(String args[]) {

        ArrayList al = new ArrayList();
        HashMap<Character, ArrayList<Student>> hm = new HashMap<Character, ArrayList<Student>>();
        al.add(new Student(101, "Vijay", 23));
        al.add(new Student(106, "Ajay", 27));
        al.add(new Student(105, "Jai", 21));

        System.out.println("Sorting ArrayList by Name...");

        Collections.sort(al, new NameComparator());
        Iterator itr = al.iterator();
        while (itr.hasNext()) {
            Student st = (Student) itr.next();
            Character key = st.name.charAt(0);
            hm.put(key, al);
            System.out.println(st.rollno + " " + st.name + " " + st.age);
        }

        System.out.println("HashMap.....");

        for (Map.Entry m : hm.entrySet()) {
            System.out.println(m.getKey() + " " + m.getValue());
        }
        /*
         * Character[] keys = new Character[hm.size()]; Object[] values = new
         * Object[hm.size()]; int index = 0; for (Map.Entry<Character,
         * ArrayList<Student>> mapEntry : hm.entrySet()) { keys[index] =
         * mapEntry.getKey(); values[index] = mapEntry.getValue(); index++; }
         * for(Character chr:keys) { System.out.println(chr); } for(Object
         * value:values) { Iterator itr1 = ((ArrayList) value).iterator(); while
         * (itr1.hasNext()) { Student st1 = (Student) itr1.next();
         * System.out.println(st1.rollno + " " + st1.name + " " + st1.age); } }
         */
    }
}

输出：

Sorting ArrayList by Name...
106 Ajay 27
105 Jai 21
101 Vijay 23
 HashMap.....
V [moduletest.Student@1f5b44d6, moduletest.Student@21044daf, moduletest.Student@21882d18]
A [moduletest.Student@1f5b44d6, moduletest.Student@21044daf, moduletest.Student@21882d18]
J [moduletest.Student@1f5b44d6, moduletest.Student@21044daf, moduletest.Student@21882d18]

Answer 1

那是错的：

        Character key = st.name.charAt(0);
        hm.put(key, al);

您将整个ArrayList作为每个键的值。

您应该为每个键创建一个新的ArrayList，并且只将名称以相同字符开头的学生放入其中。

        Character key = st.name.charAt(0);
        List l = null;
        if (hm.containsKey(key))
            l = hm.get(key);
        else {
            l = new ArrayList<Student>();
            hm.put(key, l);
        }
        l.add(st);

Answer 2

不是超级高效，但尝试这样的事情......

ArrayList<Student> al = new ArrayList<Student>();
al.add(new Student(101, "Vijay", 23));
al.add(new Student(106, "Ajay", 27));
al.add(new Student(105, "Jai", 21));

Map<Character, ArrayList<Student>> hm = new TreeMap<String, ArrayList<Student>>();

for (Student student : al) {
    Character key = student.name.charAt(0);
    ArrayList<Student> list = hm.get(key);  // get the existing list
    if (list == null) {
        list = new ArrayList<Student>();
    }
    list.add(student);  // add this student to it
    hm.put(key, list);  // replace it in the map, under the correct key
}

TreeMap将保持排序顺序。

使用唯一键和值asArrayList创建HashMap

2 个答案: