How to convert java lang Object to ArrayList

Question

I have a valid ArrayList object in the form of java lang Object  I have to again convert the Object to an ArrayList  I tried this   Object obj2   from some source       ArrayList al1   new ArrayList    al1    ArrayList  obj2  System out println  List2 Value    al1     But it is printing null  How can I do this

User · Answer

You can create a util method that converts any collection to a java list

public static List<?> convertObjectToList(Object obj) {
    List<?> list = new ArrayList<>();
    if (obj.getClass().isArray()) {
        list = Arrays.asList((Object[])obj);
    } else if (obj instanceof Collection) {
        list = new ArrayList<>((Collection<?>)obj);
    }
    return list;
}

you can also mix with this validation below:

public static boolean isCollection(Object obj) {
  return obj.getClass().isArray() || obj instanceof Collection;
}

User · Answer

The conversion fails  java lang ClassCastException  java lang String cannot be cast to java util ArrayList  because you have surely some objects that are not ArrayList  verify the types of your different objects

User · Answer

Rather Cast It to an Object Array   Object obj2   from some source       Object   objects  Object   obj2

User · Answer

This only results in null if obj2 was already null before the cast  so your problem is earlier than you think   Also  you need not construct a new ArrayList to initialize al1 if you re going to assign to it immediately  Just say ArrayList al1    ArrayList  obj2

User · Answer

I hope this will be help you import java util ArrayList   public class Demo     public static void main String   args        Object obj2  null      ArrayList al1    ArrayList  obj2      al1    ArrayList  obj2      System out println  quot List2 Value   quot    al1             obj2 Object is default null before you cast it to ArrayList  That s why print  al1  as null

User · Answer

An interesting note  it appears that attempting to cast from an object to a list on the JavaFX Application thread always results in a ClassCastException   I had the same issue as you  and no answer helped  After playing around for a while  the only thing I could narrow it down to was the thread  Running the code to cast on any other thread other than the UI thread succeeds as expected  and as the other answers in this section suggest   Thus  be careful that your source isn t running on the JavaFX application thread

User · Answer

Converting from java lang Object directly to ArrayList lt T gt  which has elements of T is not recommended as it can lead to casting Exceptions  The recommended way is to first convert to a primitive array of T and then use Arrays asList T    One of the ways how you get entity from a java javax ws rs core Response is as follows -       T   t array   response readEntity object       ArrayList lt T gt  t arraylist   Arrays asList t array     You will still get Unchecked cast warnings

User · Answer

Object object   new Object            First way     List objects1   new ArrayList lt Object gt         objects1 add object           second way     List lt Object gt  objects2   Arrays asList object           Third way     List lt Object gt  objects3   Collections singletonList object

[java] How to convert java.lang.Object to ArrayList?

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