CODEWITHSUNDEEP
javagenericsarraylist
windravenii
windravenii

Reputation: 105

Generic list of lists Java

i'm trying to make a generic function in Java to find out the maximum similarity between an ArrayList and a list from an ArrayList of ArrayLists.

public static int maxSimilarity(ArrayList<?> g, 
        ArrayList<ArrayList<?>> groups){

    int maxSim = 0;
    for(ArrayList<?> g2:groups){
        int sim = similarity(g, (ArrayList<?>) g2);
        if(sim > maxSim)
            maxSim = sim;
    }
    return maxSim;
}

However, when i try to call it in my main function, it show an incompatible error

ArrayList<ArrayList<Points>> cannot be converted to ArrayList<ArrayList<?>>

I don't understand, i tought all objects can be represented by the ? sign. Also, it works in my similarity function, between two ArrayLists:

public static int similarity(ArrayList<?> g1, ArrayList<?> g2){
    int total = 0;
    for(Object o1:g1){
        for(Object o2:g2){
            if(o1.equals(o2))
                total++;
        }
    }
    return total;
}

Upvotes: 3

Views: 2329

Answers (4)

qqilihq
qqilihq

Reputation: 11454

Change your method signature to:

public static int maxSimilarity(ArrayList<?> g, ArrayList<? extends ArrayList<?>> groups)

And in general prefer using interface types, instead of actual implementations (more flexible, less code):

public static int maxSimilarity(List<?> g, List<? extends List<?>> groups)

[edit] Based on the suggestion with the type variables, to make this super-generic, it should be:

public static <T> int maxSimilarity(List<? extends T> g, List<? extends List<? extends T>> groups)

Notice that ? extends T. This way, you can use e.g.

List<List<StringBuilder>> groups = // ...
List<String> g = // ...
maxSimilarity(g, groups);

(StringBuilder and String are a CharSequence, so they can be compared).

Upvotes: 2

Smutje
Smutje

Reputation: 18143

If you want to compare lists of similar objects, you should introduce a method type parameter

public static <T> int maxSimilarity(List<T> g, List<List<T>> groups) {

because it's nearly useless comparing completely different objects.

Upvotes: 1

Rogue
Rogue

Reputation: 11483

Instead of a wildcard, declare a generic value:

public <T> static int maxSimilarity(List<T> g, List<? extends List<T>> gs);

Upvotes: 3

eltabo
eltabo

Reputation: 3807

Try declaring your method:

public static <T>int maxSimilarity(ArrayList<T> g,  ArrayList<ArrayList<T>> groups)

Hope it helps.

Upvotes: 0

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