使用唯一鍵和值創建一個HashMap asArrayList

Question

我試圖從Student對象的ArrayList創建一個HashMap。我想從名稱中選取字母作爲鍵，並根據名稱將每個數組列表附加到該鍵上。

例如，如果輸入是：

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

的HashMap應該是：

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

我的代碼：

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); } } 
     */ 
    } 
} 

輸出：

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

不是超級效率，但嘗試這樣的事情......

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 
} 

映像樹將保持有序。

Answer 2

這是錯誤的：

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

你把整個ArrayList中的每個鍵的值。

您應該爲每個鍵創建一個新的ArrayList，並且只將其名稱以相同字符開頭的學生放入其中。

 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);