java: How can I do dynamic casting of a variable from one type to another?

Question

I would like to do dynamic casting for a Java variable, the casting type is stored in a different variable.

This is the regular casting:

 String a = (String) 5;

This is what I want:

 String theType = 'String';
 String a = (theType) 5;

Is this possible, and if so how? Thanks!

Update

I'm trying to populate a class with a HashMap that I received.

This is the constructor:

public ConnectParams(HashMap<String,Object> obj) {

    for (Map.Entry<String, Object> entry : obj.entrySet()) {
        try {
            Field f =  this.getClass().getField(entry.getKey());                
            f.set(this, entry.getValue()); /* <= CASTING PROBLEM */
        } catch (NoSuchFieldException ex) {
            log.error("did not find field '" + entry.getKey() + '"');
        } catch (IllegalAccessException ex) {
            log.error(ex.getMessage());         
        }
    }

}

The problem here is that some of the class' variables are of type Double, and if the number 3 is received it sees it as Integer and I have type problem.

Decimeter

That doesn't make any sense. You want the variable name to be a type to cast a string to a string? What? – Arelus

I don't know an answer but I'm afraid this can become maintenance hell... Just learning Java myself but I'd avoid situations which require approach like this. I'm pretty sure whatever you do can be implemented in a better way... just my 2 cents. – Schlieren

ok i will provide more information to what i'm trying to achieve. – Decimeter

also updated my answer below! – Chinkiang

Answer 1

Regarding your update, the only way to solve this in Java is to write code that covers all cases with lots of if and else and instanceof expressions. What you attempt to do looks as if are used to program with dynamic languages. In static languages, what you attempt to do is almost impossible and one would probably choose a totally different approach for what you attempt to do. Static languages are just not as flexible as dynamic ones :)

Good examples of Java best practice are the answer by BalusC (ie ObjectConverter) and the answer by Andreas_D (ie Adapter) below.

---

That does not make sense, in

String a = (theType) 5;

the type of a is statically bound to be String so it does not make any sense to have a dynamic cast to this static type.

PS: The first line of your example could be written as Class<String> stringClass = String.class; but still, you cannot use stringClass to cast variables.

Holliman

I hope that the updated that i posted will explain what i'm trying to do. i come from a php background so maybe this thing is not possible to achieve in java. – Decimeter

Exactly, in Java you cannot be that dynamic, see my update as well. – Holliman

See BalusC's answer below, this is the length (and pain) to which you have to go... – Holliman

I know this is late but I think he meant [thetype] a = (thetype)some_object; which is pointless because you can just do Object a = some_object fine – Guib

Answer 2

Yes it is possible using Reflection

Object something = "something";
String theType = "java.lang.String";
Class<?> theClass = Class.forName(theType);
Object obj = theClass.cast(something);

but that doesn't make much sense since the resulting object must be saved in a variable of Object type. If you need the variable be of a given class, you can just cast to that class.

If you want to obtain a given class, Number for example:

Object something = new Integer(123);
String theType = "java.lang.Number";
Class<? extends Number> theClass = Class.forName(theType).asSubclass(Number.class);
Number obj = theClass.cast(something);

but there is still no point doing it so, you could just cast to Number.

Casting of an object does NOT change anything; it is just the way the compiler treats it.
The only reason to do something like that is to check if the object is an instance of the given class or of any subclass of it, but that would be better done using instanceof or Class.isInstance().

Update

according your last update the real problem is that you have an Integer in your HashMap that should be assigned to a Double. What you can do in this case, is check the type of the field and use the xxxValue() methods of Number

...
Field f =  this.getClass().getField(entry.getKey());
Object value = entry.getValue();
if (Integer.class.isAssignableFrom(f.getType())) {
    value = Integer.valueOf(((Number) entry.getValue()).intValue());
} else if (Double.class.isAssignableFrom(f.getType())) {
    value = Double.valueOf(((Number) entry.getValue()).doubleValue());
} // other cases as needed (Long, Float, ...)
f.set(this, value);
...

(not sure if I like the idea of having the wrong type in the Map)

Chinkiang

Answer 3

You'll need to write sort of ObjectConverter for this. This is doable if you have both the object which you want to convert and you know the target class to which you'd like to convert to. In this particular case you can get the target class by Field#getDeclaringClass().

You can find here an example of such an ObjectConverter. It should give you the base idea. If you want more conversion possibilities, just add more methods to it with the desired argument and return type.

Tuxedo

@Tuxedo - I find the ObjectConverter code interesting, could you please describe the use cases for it? – Midway

It's useful in cases when convention over configuration is preferred and the source type doesn't match the target type. I have used it 2-3 years ago in my (pure for hobby purposes) ORM and MVC frameworks. Also see the leading text of the blog article. – Tuxedo

Answer 4

You can do this using the Class.cast() method, which dynamically casts the supplied parameter to the type of the class instance you have. To get the class instance of a particular field, you use the getType() method on the field in question. I've given an example below, but note that it omits all error handling and shouldn't be used unmodified.

public class Test {

    public String var1;
    public Integer var2;
}

public class Main {

    public static void main(String[] args) throws Exception {
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("var1", "test");
        map.put("var2", 1);

        Test t = new Test();

        for (Map.Entry<String, Object> entry : map.entrySet()) {
            Field f = Test.class.getField(entry.getKey());

            f.set(t, f.getType().cast(entry.getValue()));
        }

        System.out.println(t.var1);
        System.out.println(t.var2);
    }
}

Ormand

What if the entry type isn't a supertype of the field type at all? You'll then really need to convert programmatically. – Tuxedo

Answer 5

You can write a simple castMethod like the one below.

private <T> T castObject(Class<T> clazz, Object object) {
  return (T) object;
}

In your method you should be using it like

public ConnectParams(HashMap<String,Object> object) {

for (Map.Entry<String, Object> entry : object.entrySet()) {
    try {
        Field f =  this.getClass().getField(entry.getKey());                
        f.set(this, castObject(entry.getValue().getClass(), entry.getValue()); /* <= CASTING PROBLEM */
    } catch (NoSuchFieldException ex) {
        log.error("did not find field '" + entry.getKey() + '"');
    } catch (IllegalAccessException ex) {    
        log.error(ex.getMessage());          
    }    
}

}

Charlot

Yes, I think this answer is what the asker want. He/She just get a Class<?>, and wants to convert an instance to the class "?". By default java do not support this. But using <T> we can do this. – Megasporangium

@Megasporangium This is wrong. T in which case is a generic. Generics do nothing during runtime. (T)blah would just be (Object)blah during runtime because of type erasure. In short, generics -> compile-time, and has no effect during runtime. Since dynamic -> runtime and generics -> compile time, generics are useless. – Guib

Answer 6

It works and there's even a common pattern for your approach: the Adapter pattern. But of course, (1) it does not work for casting java primitives to objects and (2) the class has to be adaptable (usually by implementing a custom interface).

With this pattern you could do something like:

Wolf bigBadWolf = new Wolf();
Sheep sheep = (Sheep) bigBadWolf.getAdapter(Sheep.class);

and the getAdapter method in Wolf class:

public Object getAdapter(Class clazz) {
  if (clazz.equals(Sheep.class)) {
    // return a Sheep implementation
    return getWolfDressedAsSheep(this);
  }

  if (clazz.equals(String.class)) {
    // return a String
    return this.getName();
  }

  return null; // not adaptable
}

For you special idea - that is impossible. You can't use a String value for casting.

Deprecative

Answer 7

Your problem is not the lack of "dynamic casting". Casting Integer to Double isn't possible at all. You seem to want to give Java an object of one type, a field of a possibly incompatible type, and have it somehow automatically figure out how to convert between the types.

This kind of thing is anathema to a strongly typed language like Java, and IMO for very good reasons.

What are you actually trying to do? All that use of reflection looks pretty fishy.

Howland

@name: concerning the edit you keep suggesting: note that I am talking about not about primitive values but about the wrapper classes (signified by the capitalization and the styling as code), and casting between those is definitely not possible. – Howland

Answer 8

Don't do this. Just have a properly parameterized constructor instead. The set and types of the connection parameters are fixed anyway, so there is no point in doing this all dynamically.

Anarchist

Answer 9

For what it is worth, most scripting languages (like Perl) and non-static compile-time languages (like Pick) support automatic run-time dynamic String to (relatively arbitrary) object conversions. This CAN be accomplished in Java as well without losing type-safety and the good stuff statically-typed languages provide WITHOUT the nasty side-effects of some of the other languages that do evil things with dynamic casting. A Perl example that does some questionable math:

print ++($foo = '99');  # prints '100'
print ++($foo = 'a0');  # prints 'a1'

In Java, this is better accomplished (IMHO) by using a method I call "cross-casting". With cross-casting, reflection is used in a lazy-loaded cache of constructors and methods that are dynamically discovered via the following static method:

Object fromString (String value, Class targetClass)

Unfortunately, no built-in Java methods such as Class.cast() will do this for String to BigDecimal or String to Integer or any other conversion where there is no supporting class hierarchy. For my part, the point is to provide a fully dynamic way to achieve this - for which I don't think the prior reference is the right approach - having to code every conversion. Simply put, the implementation is just to cast-from-string if it is legal/possible.

So the solution is simple reflection looking for public Members of either:

STRING_CLASS_ARRAY = (new Class[] {String.class});

a) Member member = targetClass.getMethod(method.getName(),STRING_CLASS_ARRAY); b) Member member = targetClass.getConstructor(STRING_CLASS_ARRAY);

You will find that all of the primitives (Integer, Long, etc) and all of the basics (BigInteger, BigDecimal, etc) and even java.regex.Pattern are all covered via this approach. I have used this with significant success on production projects where there are a huge amount of arbitrary String value inputs where some more strict checking was needed. In this approach, if there is no method or when the method is invoked an exception is thrown (because it is an illegal value such as a non-numeric input to a BigDecimal or illegal RegEx for a Pattern), that provides the checking specific to the target class inherent logic.

There are some downsides to this:

1) You need to understand reflection well (this is a little complicated and not for novices). 2) Some of the Java classes and indeed 3rd-party libraries are (surprise) not coded properly. That is, there are methods that take a single string argument as input and return an instance of the target class but it isn't what you think... Consider the Integer class:

static Integer getInteger(String nm)
      Determines the integer value of the system property with the specified name.

The above method really has nothing to do with Integers as objects wrapping primitives ints. Reflection will find this as a possible candidate for creating an Integer from a String incorrectly versus the decode, valueof and constructor Members - which are all suitable for most arbitrary String conversions where you really don't have control over your input data but just want to know if it is possible an Integer.

To remedy the above, looking for methods that throw Exceptions is a good start because invalid input values that create instances of such objects should throw an Exception. Unfortunately, implementations vary as to whether the Exceptions are declared as checked or not. Integer.valueOf(String) throws a checked NumberFormatException for example, but Pattern.compile() exceptions are not found during reflection lookups. Again, not a failing of this dynamic "cross-casting" approach I think so much as a very non-standard implementation for exception declarations in object creation methods.

If anyone would like more details on how the above was implemented, let me know but I think this solution is much more flexible/extensible and with less code without losing the good parts of type-safety. Of course it is always best to "know thy data" but as many of us find, we are sometimes only recipients of unmanaged content and have to do the best we can to use it properly.

Cheers.

Villatoro

Answer 10

So, this is an old post, however I think I can contribute something to it.

You can always do something like this:

package com.dyna.test;  

import java.io.File;  
import java.lang.reflect.Constructor;  

public class DynamicClass{  

  @SuppressWarnings("unchecked")  
  public Object castDynamicClass(String className, String value){  
    Class<?> dynamicClass;  

    try  
    {  
      //We get the actual .class object associated with the specified name  
      dynamicClass = Class.forName(className);  



    /* We get the constructor that received only 
     a String as a parameter, since the value to be used is a String, but we could
easily change this to be "dynamic" as well, getting the Constructor signature from
the same datasource we get the values from */ 


      Constructor<?> cons =  
        (Constructor<?>) dynamicClass.getConstructor(new Class<?>[]{String.class});  

      /*We generate our object, without knowing until runtime 
 what type it will be, and we place it in an Object as 
 any Java object extends the Object class) */  
      Object object = (Object) cons.newInstance(new Object[]{value});  

      return object;  
    }  
    catch (Exception e)  
    {  
      e.printStackTrace();  
    }  
    return null;  
  }  

  public static void main(String[] args)  
  {   
    DynamicClass dynaClass = new DynamicClass();  

    /* 
 We specify the type of class that should be used to represent 
 the value "3.0", in this case a Double. Both these parameters 
 you can get from a file, or a network stream for example. */  
    System.out.println(dynaClass.castDynamicClass("java.lang.Double", "3.0"));  

    /* 
We specify a different value and type, and it will work as 
 expected, printing 3.0 in the above case and the test path in the one below, as the Double.toString() and 
 File.toString() would do. */  
    System.out.println(dynaClass.castDynamicClass("java.io.File", "C:\\testpath"));  
  }  

Of course, this is not really dynamic casting, as in other languages (Python for example), because java is a statically typed lang. However, this can solve some fringe cases where you actually need to load some data in different ways, depending on some identifier. Also, the part where you get a constructor with a String parameter could be probably made more flexible, by having that parameter passed from the same data source. I.e. from a file, you get the constructor signature you want to use, and the list of values to be used, that way you pair up, say, the first parameter is a String, with the first object, casting it as a String, next object is an Integer, etc, but somehwere along the execution of your program, you get now a File object first, then a Double, etc.

In this way, you can account for those cases, and make a somewhat "dynamic" casting on-the-fly.

Hope this helps anyone as this keeps turning up in Google searches.

Caryopsis

Answer 11

Try this for Dynamic Casting. It will work!!!

    String something = "1234";
    String theType = "java.lang.Integer";
    Class<?> theClass = Class.forName(theType);
    Constructor<?> cons = theClass.getConstructor(String.class);
    Object ob =  cons.newInstance(something);
    System.out.println(ob.equals(1234));

Occupational

Answer 12

I recently felt like I had to do this too, but then found another way which possibly makes my code look neater, and uses better OOP.

I have many sibling classes that each implement a certain method doSomething(). In order to access that method, I would have to have an instance of that class first, but I created a superclass for all my sibling classes and now I can access the method from the superclass.

Below I show two ways alternative ways to "dynamic casting".

// Method 1.
mFragment = getFragmentManager().findFragmentByTag(MyHelper.getName(mUnitNum));
switch (mUnitNum) {
case 0:
    ((MyFragment0) mFragment).sortNames(sortOptionNum);
    break;
case 1:
    ((MyFragment1) mFragment).sortNames(sortOptionNum);
    break;
case 2:
    ((MyFragment2) mFragment).sortNames(sortOptionNum);
    break;
}

and my currently used method,

// Method 2.
mSuperFragment = (MySuperFragment) getFragmentManager().findFragmentByTag(MyHelper.getName(mUnitNum));
mSuperFragment.sortNames(sortOptionNum);

Plectrum

You shouldn't need to dynamically cast ever. Your formula of having a superclass with the method doSomething() and subclasses that implement the method doSomething() is one of the main purposes of OOP, polymorphism. Object foo = new Integer("1"); foo.toString() returns 1. Even though it is assigned to Object, it is an Integer, and therefore behaves as such. Running the method toString will run the Integer implementation. Polymorphism. – Guib

Answer 13

Just thought I would post something that I found quite useful and could be possible for someone who experiences similar needs.

The following method was a method I wrote for my JavaFX application to avoid having to cast and also avoid writing if object x instance of object b statements every time the controller was returned.

public <U> Optional<U> getController(Class<U> castKlazz){
    try {
        return Optional.of(fxmlLoader.<U>getController());
    }catch (Exception e){
        e.printStackTrace();
    }
    return Optional.empty();
}

The method declaration for obtaining the controller was

public <T> T getController()

By using type U passed into my method via the class object, it could be forwarded to the method get controller to tell it what type of object to return. An optional object is returned in case the wrong class is supplied and an exception occurs in which case an empty optional will be returned which we can check for.

This is what the final call to the method looked like (if present of the optional object returned takes a Consumer

getController(LoadController.class).ifPresent(controller->controller.onNotifyComplete());

Beauty