如何检查Object是否是Java中的集合类型？

Question

By using java reflection, we can easily know if an object is an array. What's the easiest way to tell if an object is a collection(Set,List,Map,Vector...)?

Answer 1

if (x instanceof Collection<?>){
}

if (x instanceof Map<?,?>){
}

Answer 2

Update: there are two possible scenarios here:

1. You are determining if an object is a collection;

2. You are determining if a class is a collection.

The solutions are slightly different but the principles are the same. You also need to define what exactly constitutes a "collection". Implementing either Collection or Map will cover the Java Collections.

Solution 1:

public static boolean isCollection(Object ob) {
  return ob instanceof Collection || ob instanceof Map;
}

Solution 2:

public static boolean isClassCollection(Class c) {
  return Collection.class.isAssignableFrom(c) || Map.class.isAssignableFrom(c);
}

(1) can also be implemented in terms of (2):

public static boolean isCollection(Object ob) {
  return ob != null && isClassCollection(ob.getClass());
}

I don't think the efficiency of either method will be greatly different from the other.

Answer 3

Since you mentioned reflection in your question;

boolean isArray = myArray.getClass().isArray();
boolean isCollection = Collection.class.isAssignableFrom(myList.getClass());
boolean isMap = Map.class.isAssignableFrom(myMap.getClass());

Answer 4

Java conveniently has the instanceof operator ( JLS 15.20.2 ) to test if a given object is of a given type.

 if (x instanceof List<?>) {   
    List<?> list = (List<?>) x;
    // do something with list
 } else if (x instanceof Collection<?>) {
    Collection<?> col = (Collection<?>) x;
    // do something with col
 }

One thing should be mentioned here: it's important in these kinds of constructs to check in the right order . You will find that if you had swapped the order of the check in the above snippet, the code will still compile, but it will no longer work . That is the following code doesn't work:

 // DOESN'T WORK! Wrong order!
 if (x instanceof Collection<?>) {
    Collection<?> col = (Collection<?>) x;
    // do something with col
 } else if (x instanceof List<?>) { // this will never be reached!
    List<?> list = (List<?>) x;
    // do something with list
 }

The problem is that a List<?> is-a Collection<?> , so it will pass the first test, and the else means that it will never reach the second test. You have to test from the most specific to the most general type .

Answer 5

Test if the object implements either java.util.Collection or java.util.Map . ( Map has to be tested separately because it isn't a sub-interface of Collection .)

Answer 6

Have you thinked about using instanceof ? Like, say

if(myObject instanceof Collection) {
     Collection myCollection = (Collection) myObject;

Although not that pure OOP style, it is however largely used for so-called "type escalation".