NullPointerException when indexing HashMap with character

Question

I'm writing an English to Morse translator and every time I run it an error appears:

Exception in thread "main" java.lang.NullPointerException
    at java.lang.String.concat(Unknown Source)
    at Code.translate(Code.java:49)
    at Code.<init>(Code.java:34)
    at Morse.main(Morse.java:8)

I've tried debugging. Here's the function with the problem:

String translate(String phrase, HashMap map) {   // line 37
        //String to contain the morse code
        String morse = "";
        //Char array to contain the English phrase
        char[] phraseList = new char[(int)phrase.length()];
        //Fill the char array
        phraseList = phrase.toCharArray();
        //Loop through the array and concat the morse string with the value returned from the key
        for (int x =0; x < phrase.length(); x++) {
            if(phraseList[x] == ' ') {
                morse.concat("  ");
            } else {
                //Here's where the error is produced
                morse.concat((String) map.get(phraseList[x]));
            }
        }
        return morse;
    }

The error is produced when the code reaches the "else" condition. Here is the constructor that sets up my HashMap that's passed to this function:

HashMap<String,String> codeMap = new HashMap<>();
//Gets passed a string phrase from the main class
public Code(String phrase) throws FileNotFoundException {   //line 14
        String filePath;
        String letter;
        String code;
        //Creates the object that reads the file location
        Scanner fileLocate = new Scanner(System.in);
        System.out.println("Enter file path of morse code");
        //Object reads file location
        filePath = fileLocate.nextLine();
        //Create file object
        filePath=filePath.replace("\\","/");
        File morse = new File(filePath);
        //Create scanner object that reads the file
        Scanner fileRead = new Scanner(morse);
        //Loop to read the file and store the info as a key/value pair in a hashmap
        while (fileRead.hasNext()) {
            letter = fileRead.next().toLowerCase();
            code = fileRead.next();
            codeMap.put(letter, code);
        }   
        translate(phrase,codeMap);
    }

The HashMap is full of the correct, lowercase values and the char array is full of the lowercase chars of the phrase, but for some reason it still produces the error. Any advice on what I'm doing wrong would be greatly appreciated.

Hint: that line contains 3 potential reasons that could throw an NPE. So, start by **dissecting** those statements to understand which of the objects you are using is actually null. — GhostCat, Jul 08 '16 at 14:09
Could you please tell us which line number `Code` and `translate` begin on? — Addison, Jul 08 '16 at 14:11
And hint on code quality: constructors **never** do "real work". Use use them to create new objects, not to open files, read content and translate that. Instead: you put many small methods on your class to do all those things. Bonus included: if you do so, all of a sudden, you will be able to **test** all those little methods ... one by one, with a very clear scope, and much less effort. Long story short: read about https://en.wikipedia.org/wiki/SOLID_(object-oriented_design) ... and especially **SRP**. — GhostCat, Jul 08 '16 at 14:11
Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — GhostCat, Jul 08 '16 at 14:12
`System.out.println(morse + " " + map + " " + phraseList[x] + " " + map.get(phraseList[x]);` which one is null? — Compass, Jul 08 '16 at 14:14

score 3 · Accepted Answer · edited May 23 '17 at 12:08

3

Try

morse.concat((String) map.get( Character.toString(phraseList[x]) ));
                          //  ^^^^^^^^^^^^^^^^^^^^ added this

Your map is indexed by String but translate doesn't know that due to type erasure. You are indexing with char instead of String. That means you are getting an autoboxed Character, which will not be the String you put in. As a result, the map won't contain the entry you expect, and will return null — whence the NullPointerException.

Explanation: Per this answer, hashCode is a first check in the HashMap. After that, Object.equals() is used to look for the key in the map. Both String.equals(Character) and Character.equals(String) will return false. Therefore, even though the hashCode values may be the same for String("a") and Character('a'), those two are not considered equal, so they cannot be used interchangeably as keys.

edited May 23 '17 at 12:08

Community

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answered Jul 08 '16 at 14:19

cxw

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I tried this but it said `Cannot cast from char to String`. – CS2016 Jul 08 '16 at 14:21
@CS2016 Sorry --- getting ahead of myself. Try the edited version. – cxw Jul 08 '16 at 14:25
@CS2016 Glad to hear it! I learned some new things myself as well :) . – cxw Jul 08 '16 at 14:35

NullPointerException when indexing HashMap with character

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