CODEWITHSUNDEEP
javahashmap
jsan
jsan

Reputation: 1057

Iterate through nested hashmap

How would I go about iterating through a nested HashMap?

The HashMap is setup like this:

HashMap<String, HashMap<String, Student>>

Where Student is an object containing a variable name. If for instance my HashMap looked like this (the following is not my code, it's just to simulate what the contents of the hashmap could be)

 hm => HashMap<'S', Hashmap<'Sam', SamStudent>>
       HashMap<'S', Hashmap<'Seb', SebStudent>>
       HashMap<'T', Hashmap<'Thomas', ThomasStudent>>

How could I iterate through all of the single letter keys, then each full name key, then pull out the name of the student?

Upvotes: 16

Views: 31167

Answers (2)

Brett Kail
Brett Kail

Reputation: 33946

for (Map.Entry<String, HashMap<String, Student>> letterEntry : students.entrySet()) {
    String letter = letterEntry.getKey();
    // ...
    for (Map.Entry<String, Student> nameEntry : letterEntry.getValue().entrySet()) {
        String name = nameEntry.getKey();
        Student student = nameEntry.getValue();
        // ...
    }
}

...and the var keyword in Java 10 can remove the generics verbosity:

for (var letterEntry : students.entrySet()) {
    String letter = letterEntry.getKey();
    // ...
    for (var nameEntry : letterEntry.getValue().entrySet()) {
        String name = nameEntry.getKey();
        Student student = nameEntry.getValue();
        // ...
    }
}

Upvotes: 27

Jeffrey Bosboom
Jeffrey Bosboom

Reputation: 13653

Java 8 lambdas and Map.forEach make bkail's answer more concise:

outerMap.forEach((letter, nestedMap) -> {
    //...
    nestedMap.forEach((name, student) -> {
        //...
    });
    //...
});

Upvotes: 14

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