Java: How to check if a Field is of type java.util.Collection

Question

I have a utility method that goes through various classes and recursively retrieves the fields. I want to check if that field is a Collection. Here is some sample code:

void myMethod(Class<?> classToCheck)

Field[] fields = classToCheck.getDeclaredFields();

for(Field field:fields)
{
   // check if field if a Collection<?>

}

Thanks in advance for the help.

Solution 1

if (Collection.class.isAssignableFrom(field.getType())) {

}

Solution 2

You should use Class.isAssignableFrom:

if (Collection.class.isAssignableFrom(field.getType())
    ...

Solution 3

Using the getType() method

Field field =  ...;
if ( Collection.class.isAssignableFrom( field.getType() ) ){
  //do something with your collection
}

Solution 4

//This execute if

 List<String> cashType = split(" ,AlL ");
 if(cashType instanceof Collection){
     System.out.println("cashType instanceof Collection");
 }else{
     System.out.println("cashType is not instanceof Collection");
 }

//This executes else

List<String> cashType = split(" ,AlL ");
 if(cashType instanceof Hashtable){
     System.out.println("cashType instanceof Collection");
 }else{
     System.out.println("cashType is not instanceof Collection");
 }

View more solutions

Comments

[DJClayworth]

Do you want to test whether the declared type of the field is an implementation of Collection, or whether the actual object referred to by the field in any given object implements Collection? The two results would differ if a) the Field was declared as Object, but an object implementing Collection was assigned to it, or b) the Field was declared as Collection (or a descendant) but it was null.

[Dave Jarvis]

possible duplicate of Howto find out if a field is instanceof a type via reflection?

[Bozho]

nope, that's not it. This will get the class that owns the field, rather than the field type

[Robin]

Indeed. I just noticed my mistake by actually reading the javadoc. So far relying on my memory. Corrected my answer

[DJClayworth]

isAssignableFrom takes a Class, where getType() returns a Type

[Bozho]

@DJClayworth nope, it returns Class<?>. Check the javadoc

[Kirk Woll]

@DJClayworth, the documentation that proves you wrong is pretty easy to google.

[Trevor Freeman]

@user979467 I just tested it, it works. Maybe you are doing something else wrong?

[DJClayworth]

@Bozho My apologies, I found documentation that said it returned Type. Don't know where I found it now, so I'll just withdraw the comment.

[DJClayworth]

Then it isn't an instanceof Collection. instanceof handles this case correctly, I believe.

[DJClayworth]

New edits make it clear this wasn't the question the OP was asking, but I'll leave this here as a reference.

[Radhesh Khanna]

Make sure you use the Correct Import for Collection. Shouldn't be from the org.hibernate package by mistake.

[user1079475]

And how do you actually iterate over that collection?