Java Iteration through HashMap

Question

I'm struggling with a java HashMap. I want to return translationList as an array with strings. Example: word : "translated word".

Main class:

public static void main(String[] args) {
    Dictionary dictionary = new Dictionary();
    dictionary.add("apina", "monkey");
    dictionary.add("banaani", "banana");
    dictionary.add("cembalo", "harpsichord");

    ArrayList<String> translations = dictionary.translationList();
    for(String translation: translations) {
        System.out.println(translation);
    }
}

Dictionary class:

private HashMap<String, String> dictionary = new HashMap<String, String>();
public Dictionary(){};
public String translate(String word){
    if(dictionary.containsKey(word)){
        return dictionary.get(word);
    }
    return null;
}
public void add(String word, String translation){
    dictionary.put(word,translation);
}
public int amountOfWords() {
    return dictionary.size();
}
public ArrayList<String> translationList(){
    for ( String key : dictionary.keySet() ) {
        if(translationList().size()<dictionary.size()){
            translationList().add(key+" = "+dictionary.get(key));
        }
    }
    return translationList();
}

Java returns:

Exception in thread "main" java.lang.StackOverflowError
at java.base/java.util.HashMap$KeyIterator.<init>(HashMap.java:1515)
at java.base/java.util.HashMap$KeySet.iterator(HashMap.java:917)
at Dictionary.translationList(Dictionary.java:21)
at Dictionary.translationList(Dictionary.java:22)

Thanks for help :)

You are calling your translationList() method instead of creating an actual list and adding to it. Why? — RealSkeptic
– RealSkeptic, Commented May 21, 2018 at 12:22
If next time you include a runnable code, (with imports and class structure) help will be faster :) — Teifun2
– Teifun2, Commented May 21, 2018 at 12:27

Eran · Accepted Answer · 2018-05-21 12:22:51Z

5

Your translationList method is calling itself, leading to infinite recursive calls and StackOverflowError.

You probably intended to write something like:

public List<String> translationList(){
    List<String> list = new ArrayList<>();
    for ( String key : dictionary.keySet() ) {
        list.add(key+" = "+dictionary.get(key));
    }
    return list;
}

or

public List<String> translationList(){
    List<String> list = new ArrayList<>();
    for ( Map.Entry<String,String> entry : dictionary.entrySet() ) {
        list.add(entry.getKey()+" = "+entry.getValue());
    }
    return list;
}

You have to first create an ArrayList and then add Strings to it. Besides that, the if (translationList().size()<dictionary.size()) condition is pointless, even without the recursive call.

answered May 21, 2018 at 12:22

Eran

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Comments

ernest_k · Accepted Answer · 2018-05-21 12:30:59Z

1

You're having a recursive call with return translationList();. This is probably accidental. Your code was probably intended to look something like:

public ArrayList<String> translationList(){
    List<String> translationList = new ArrayList<>();
    for ( String key : dictionary.keySet() ) {
        if(translationList().size()<dictionary.size()){
            translationList.add(key+" = "+dictionary.get(key));
        }
    }
    return translationList();
}

But an even simpler implementation can avoid extra lookups:

public List<String> translationList(){
    return dictionary.entrySet()
                .stream()
                .map(entry -> entry.getKey() + " = " + entry.getValue())
                .collect(Collectors.toList());
}

edited May 21, 2018 at 12:30

answered May 21, 2018 at 12:29

ernest_k

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