Creating a method to check if a value exists in a generic HashMap in Java?

Question

I was trying to create a generic method for checking if an item exists within a HashMap. Simple enough concept, but Java does not like what I'm trying to do. Is this even possible? This was my attempt.

private static boolean inMap(String file, HashMap<String, Object> map) {
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        if (entry.getKey().equals(file)) return true;
    }
    return false;
}

This is called in the following section of code:

private static Map<String, String> files = new HashMap();
private static Map<String, BufferedImage> images = new HashMap();
private static Map<String, Texture> textures = new HashMap();
private static Map<String, Model> models = new HashMap();

public static TexturedModel loadOBJModel(String file) {
    if (inMap(file, files)) ...
}

public static Texture loadTexture(String file) {
    if (inMap(file, textures)) ...
}

I wanted this method to be generic for saving code and simplicity, as i have 4 different HashMaps to check. All of them are String keys, but they are all of different Value types as shown above.

If there is any way to make this generic, any help is welcome!

Why can't you use `files.containsKey('someStringKey');` and likewise for others? — Raf, Nov 08 '18 at 01:55
There already is a question on how to use `Hashmap.containsKey()` see here https://stackoverflow.com/questions/3626752/key-existence-check-in-hashmap — Raf, Nov 08 '18 at 02:06
Are you actually looking to see if has a key? The way I read your question I thought you were looking to see if the map contains a value. — Katie.Sun, Nov 08 '18 at 02:19

MeetTitan · Answer 1 · 2018-11-08T01:58:39.053

1

Let's make use of Map.get(Object) here, and the fact that it will return null if the key doesn't exist.

public static boolean contains(Map<?, ?> inMap, Object key) {
    return inMap.get(key) == null;
}

This will also take advantage of the O(1) access time of hashmaps without relying on an O(n) loop over entries.

edited Nov 08 '18 at 01:58

answered Nov 08 '18 at 01:54

MeetTitan

3,383
1
13
26

2

You don't need to use `.get()`, the `containsKey()` is enough and is `O(1)` – Raf Nov 08 '18 at 01:58
Perfect, thank you. works perfectly! – Kris Rice Nov 08 '18 at 01:58
@Raf post as answer, didn't even know this exists. – Kris Rice Nov 08 '18 at 01:59

Ryuzaki L · Answer 2 · 2018-11-08T02:08:36.820

1

you can write generic method using T type parameter, you can use this method for all different types of HashMap

public static <T> boolean contains(String file, T a) {
    return a.keySet().contains(file); //or a.containsKey(file);

}

edited Nov 08 '18 at 02:08

answered Nov 08 '18 at 01:56

Ryuzaki L

37,302
12
68
98

score 1 · Accepted Answer · answered Nov 08 '18 at 02:06

1

Why are you re-inventing the wheel? Map has containsKey() which does exactly as you want.

answered Nov 08 '18 at 02:06

Bernie

2,253
1
16
18

Creating a method to check if a value exists in a generic HashMap in Java?

3 Answers3