Why does using a for loop change the output for this dictionary class?

Question

I am essentially creating my own Dictionary class where it works as a stack. I can put key-value pairs (another class i created myself) into my dictionary which holds them in an array. When I manually put the key-value pairs by righting each call to put() line by line it works fine. But when i use a for loop it does not work properly. If i check to see if a key is in my dictionary it returns false with the dictionary created with the for loop and true with the other one. Both dictionaries print the exact same thing with the method i created. But the dictionary created with the for loop does not gives correct outputs when methods are used.

here is the code for the main :

Does anyone know why the two dictionaries are different?

Edit : my code for put() is as follows

public class Dictionary implements Map<String, Integer> {

    private final static int INITIAL_CAPACITY = 10;
    private final static int INCREMENT = 5;
    private int count;

    Pair[] elems;

    public int getCount() {
      return count;
    }

    public int getCapacity() {
      return elems.length;
    }

    public Dictionary() {
        elems = new Pair[INITIAL_CAPACITY];
        count = 0;
    }

    @Override
    public void put(String key, Integer value) {
        Pair elem = new Pair(key, value);
        elems[count] = elem;
        count++;

        if (count == elems.length) {
          increaseCapacity();
        }

    }

    private void increaseCapacity() {
      Pair[] newElems = new Pair[count + INCREMENT];
      for (int i = 0; i < count; i++) {
        newElems[i] = elems[i];
      }
      elems = newElems;
    }

    @Override
    public boolean contains(String key) {
        for (int i = 0; i < count; i++) {
          if (elems[i].getKey() == key) {
            return true;
          }
        }
        return false;
    }

    @Override
    public Integer get(String key) {
      boolean got = false;
      Integer value = 0;

        for (int i = count - 1; i > - 1; i--) {
          if (elems[i].getKey() == key && !got) {
            value = elems[i].getValue();
            got = true;
          }
        }

        return value;

    }

    @Override
    public void replace(String key, Integer value) {
      Pair newPair;
      for (int i = count - 1; i > - 1; i--) {
        if (elems[i].getKey() == key) {
          newPair = new Pair(key, value);
          elems[i] = newPair;
        }
      }
    }

    @Override
    public Integer remove(String key) {
      Integer saved = null;
      boolean removed = false;
      for (int i = count - 1; i > - 1; i--) {
        if (elems[i].getKey() == key && !removed) {
          removed = true;
          saved = elems[i].getValue();
          elems[i] = null;
        }
      }

      return saved;
    }

    @Override
    public String toString() {
      String res;
      res = "Dictionary: {elems = [";
      for (int i = count-1; i >= 0 ; i--) {
          res += elems[i];
          if (i > 0) {
              res += ", ";
          }
      }
      return res +"]}";
    }

}

Pair is another class I have created that just represents a key-value pair.

I just made your example using Hashmap and it works properly so the problem exists in your dictionary implementation. Please put code of this class — Rafał Sokalski, Feb 28 '19 at 05:40
Wild guess: your `Dictionary` class compares strings using `==` instead of using `equals()` — Thomas Kläger, Feb 28 '19 at 05:47
@RaV1oLLi put all your dictionary class code not only put method — Rafał Sokalski, Feb 28 '19 at 06:15

score 0 · Accepted Answer · answered Feb 28 '19 at 06:24

Ok, I just repeat your problem.

Dictionary:

public class Dictionary {
    Map<String, Integer> map = new HashMap<>();

    public boolean contains(String key) {
        for(String mapKey : map.keySet()) {
            if(mapKey == key) {
                return true;
            }
        }
        return false;
    }
}

Main:

public static void main(String[] args) {
    Dictionary dict = new Dictionary();

    dict.map.put("X" + 0, 0);
    dict.map.put("X" + 1, 1);
    dict.map.put("X" + 2, 2);
    dict.map.put("X" + 3, 3);
    System.out.println(dict.contains("X2")); // true

    Dictionary dict2 = new Dictionary();
    for(int i=0; i<4; i++) {
        dict2.map.put("X" + i, i);
    }
    System.out.println(dict2.contains("X2")); // false
}

Problem exists because I compare String values using == instead of equals method so I'm almost sure you do the same mistake. You have to use equals method to compare Strings otherwise you can expect weird situation like that

Is this map that you implements comes from java.util.Map ? Because "put" method should return Integer in your case instead of void — Rafał Sokalski, Feb 28 '19 at 06:36

Why does using a for loop change the output for this dictionary class?

1 Answers1