JAVA - cast using class obtained by its name (string value)

Question

this is what my code looks if I use a predefined class:

List<MyClass> result = new ArrayList<MyClass>();
try {
    Query query = mgr.newQuery(MyClass.class);
    for(Object obj : (List<Object>) query.execute()) {
        result.add(((MyClass) obj));
    }
}
.... 
return result;

now I need to be more generic: starting from a generic class name (as a string, ie "TheChosenOne") I need to do the same thing, but I can't figure out how the cast part should be done..

Just to make an example of what I'm trying to do:

String str = "TheChosenOne"; //this value may change
Class cls;
List<Object> result = new ArrayList<Object>();
try {
    if(str.equals("TheChosenOne"))
        cls = Class.forName("com.blabla.TheChosenOne");
    else if(str.equals("Rabbit"))
        cls = Class.forName("com.blabla.Rabbit");
    else if(str.equals("Batman"))
        cls = Class.forName("com.heroes.Batman");
    else
        cls = null;

    if(cls != null){
        Query query = mgr.newQuery(MyClass.class);
        for(Object obj : (List<Object>) query.execute()) {
            result.add(((???) obj)); //I need help here!
        }
    }
}
...
return result;

I took the "Class.forName()..." part here.

Any suggestions? Thanks in advance, best regards

Denis Rosca · Accepted Answer · 2013-02-20 08:48:15Z

25

What you are trying to do is unnecessary because your list is declared as List<Object> so the cast is not needed.

-- Before Edit --

Not sure I understand what you need, but did you try to use:

Class.cast(object)

This is a method of the java.lang.Class

edited Feb 20, 2013 at 8:48

answered Feb 20, 2013 at 8:37

Denis Rosca

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1 Comment

cowls Over a year ago

result.add((Class.cast(obj))) is the same as result.add(obj). Because list is of type Object

cowls · Accepted Answer · 2013-02-20 08:47:43Z

3

I think what you are trying to do can be achieved with generics.

For example a method like this:

     public <E> List<E> getList(Class<E> clazz) {
         List<E> result = new ArrayList<E>();
         if(clazz != null){
             Query query = mgr.newQuery(MyClass.class);
             for(E obj : (List<E>)query.execute()) {
                 result.add(obj); 
             }
         }

         return result;
     }

Can be called with:

 getList(TheChosenOne.class)

And it will return a List<TheChosenOne> object

answered Feb 20, 2013 at 8:47

cowls

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Comments

Ian Roberts · Accepted Answer · 2013-02-20 08:57:09Z

2

If result is a List<Object> then you don't need a cast at all, simply add(obj) will work fine.

If you need compile time type safety then you will have to pass in the Class object rather than a string containing its name, and use a generic method signature

public <T> List<T> doQuery(Class<T> theClass) {
  List<T> result = new ArrayList<T>();
  try {
    Query query = mgr.newQuery(theClass);
    for(T obj : (List<T>) query.execute()) {
      result.add(obj);
    }
  }
  .... 
  return result;
}

If you go this route, and have the option of modifying the Query class then you might want to consider making that class parameterized

public class Query<E> {
  public List<E> execute() { ... }
}

// and in mgr
public <T> Query<T> newQuery(Class<T> cls)

which would then let you say

    Query<T> query = mgr.newQuery(theClass);
    for(T obj : query.execute()) {
      result.add(obj);

with no casting at all.

edited Feb 20, 2013 at 8:57

answered Feb 20, 2013 at 8:38

Ian Roberts

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1 Comment

BeNdErR Over a year ago

I need to return a list of TheChosenOne class, or a list of Rabbit class or a list of Batman class, based on the value of the str string..

user2089674 · Accepted Answer · 2013-02-20 08:37:37Z

0

I believe cls.cast(obj) is what you're looking for.

answered Feb 20, 2013 at 8:37

user2089674

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Comments

asifsid88 · Accepted Answer · 2013-02-20 08:40:48Z

0

I think you need to do it in this way: using instance of operator to check the type of the class and then do the type casting

for(Object obj : (List<Object>) query.execute()) {
  if(obj instance of Batman)
    result.add(((Batman) obj));
  else if(obj instance of Cat)
    result.add(((Cat) obj));

  // and so on
  ...
  ...
}

answered Feb 20, 2013 at 8:40

asifsid88

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Comments

Jayamohan · Accepted Answer · 2013-02-20 08:43:00Z

0

You dont need cast while putting object in the ArrayList,

 result.add(obj);

If you want you can cast while getting from the list

 cls .cast(result.get(1));

answered Feb 20, 2013 at 8:43

Jayamohan

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