Creating a HashMap with unique keys and values asArrayList

Question

I am trying to create a HashMap from an ArrayList of Student objects. I want to pick up alphabets from the name as keys and attach each array list to that key based on the name.

For example, if the input is:

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

The HashMap should be:

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

My code:

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); } }
         */
    }
}

Output:

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]

Add a toString method to you Student class to avoid output like moduletest.Student@1f5b44d6. And you may use a MultiMap (docs.guava-libraries.googlecode.com/git/javadoc/com/google/…). — Tom
– Tom, Commented Oct 7, 2014 at 18:40

Eran · Accepted Answer · 2014-10-07 18:39:22Z

4

That's wrong :

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

You are putting the entire ArrayList as the value of each of the keys.

You should create a new ArrayList for every key, and put in it only the Students whose name starts with the same character.

        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);

answered Oct 7, 2014 at 18:39

Eran

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Comments

spudone · Accepted Answer · 2014-10-07 18:56:35Z

1

Not super efficient, but try something like this...

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
}

the TreeMap will maintain sorted order.

edited Oct 7, 2014 at 18:56

answered Oct 7, 2014 at 18:46

spudone

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2 Comments

Eran Over a year ago

There's no need to replace the entry in the map on each iteration. You only have to call hm.put(key, list) when you create a new list.

spudone Over a year ago

@Eran ah yeah... when I originally wrote it, I forgot the null check. I added it later, but didn't change the put.

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