Value of HashMap inside a Hashmap in Java

Question

I have this hashmap of students which stores the id, name and last name. So I created this :

Map<Interger, HashMap<String,String>> students = new HashMap<>();

where the second hashmap stores the name and lastname. My goal is to look for a student in a swing application, I succeed in searching with id because it's the key of the first hashmap, but i'd like to use all of them like this:

So my question is : If I want to search by name or last name, how can i get the value of the first hashmap and put it in a new hashmap ?

You could maintain 3 hashmaps (id as key, name as key etc) but there's a good chance of last name / name collision, so I would iterate for name and last name. Also you really should use a Student object — user180100
– user180100, Commented Sep 30, 2015 at 17:15
Why do you make your own task more complex? Java is an OO language. Define classes and use them. Use a Map<Integer, Student> instead of storing the fields of a student in a HashMap. — JB Nizet
– JB Nizet, Commented Sep 30, 2015 at 17:17
I think I'll create a Student class and go from because it seems really much easier, thank you @JBNizet :D — Naoufal EL BANTLI
– Naoufal EL BANTLI, Commented Sep 30, 2015 at 17:37

Halvra · Accepted Answer · 2015-09-30 17:15:35Z

1

You can iterate on the hashmap like this :

private int searchByName(String s) {
    for(Map.Entry<Integer, HashMap<String, String>> entry : students.entrySet()) {
        HashMap student = entry.getValue(); // Hashmap containing first and lastname
        if (student.containsKey(s)) // If name match
            return entry.getKey(); // Return the student ID
    }
    return 0; // Student not found
}

For the lastname just use containsValue(s) instead of containsKey(s)

answered Sep 30, 2015 at 17:15

Halvra

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NinjaCoder · Accepted Answer · 2015-09-30 17:18:15Z

0

You can use the Google Collections API Guava for that specifically a BiMap

A bimap (or "bidirectional map") is a map that preserves the uniqueness of its values as well as that of its keys. This constraint enables bimaps to support an "inverse view", which is another bimap containing the same entries as this bimap but with reversed keys and values.

With this you'will be able to search using first name and last name. This BiMap will be value to your first parent hashmap.

answered Sep 30, 2015 at 17:18

NinjaCoder

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Ashwin Sridhar · Accepted Answer · 2015-09-30 17:21:56Z

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I am not sure if the data structure is the best for your use-case but answering to your question, you should try using values() method which would return you the collection of values of the Map

http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html#values()

Collection res =  studensts.values();

Iterator<HashMap<String, String>> i = res.iterator();
Map<String,String> resM = null;
while(i.hasNext()){
   resM = i.next();
}

answered Sep 30, 2015 at 17:21

Ashwin Sridhar

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