如何创建使用Java泛型的方法将Maps关键字列出？

Question

public <T> List<T> map2list(Map<T, T> map){

    List<T> list = new ArrayList<T>(map.keySet());
    return list;

}

My Method like this... But it does not work.

Answer 1

Maybe your map is empty, because the example below works:

public class Utils {

    public <T, K> List<T> map2list(Map<T, K> map) {
        List<T> list = new ArrayList<T>(map.keySet());
        return list;
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<String, Integer>();
        map.put("test1", 1);
        map.put("test2", 2);
        map.put("test3", 3);
        Utils t = new Utils();
        List<String> result = t.map2list(map);
        for (String key : result) {
            System.out.println(key);
        }
    }

}

printing the following:

test2
test3
test1

Answer 2

If your map in argument is of String as key & Integer as value, try following method,

public static <T, L> List<T> map2list (Map<T, L> map) {  
     List<T> list = new ArrayList<T>(map.keySet());  
     return list;  
}

Answer 3

<T> define only one type for the runtime type-aguments of your map. So if you have different types of arguments for the generics of your map, you can change it like this by defining two generic type T and for example K:

public static <T,K> List<T> map2list(Map<T, K> map){
    List<T> list = new ArrayList<T>(map.keySet());
    return list;
}

And it will work.

Also this situation supports that if you want to pass Map<String,String> or generally Map<T,T> too.

Hope this would help.