How can I instantiate a class instance in java with generics?

Question

How can I instantiate a class instance in java with generics?

I am trying to read data from XML file and then instantiate an object so that I can add the other properties of the object (read from the XML file) into the object. I thought the easiest way (so that I don't have to read the class method names and find the setters) would be to instantiate the object with all the values in the constructor.

I'm trying to do something like this so I need to have something like: T obj = new Object() but get the objects class

 private static final boolean string_field = true;
 private static <T> T getObject(Element e, String[] fieldNames, boolean[] fieldTypes) {
 Object[] values = new Object[fieldNames.length];
 for (int i=0; i<fieldNames.length; i++) {
 values[i] = (fieldTypes[i]==string_field)? getStringValue(e, fieldNames[i])
 : getIntegerValue(e, fieldNames[i]);
 }
 return new T(values);
 }

Thanks for your advice.

EDIT:

This is my updated code (untested):

 public static <T> List<T> populateObjectList(Document xmlDoc, String tagName,
 Class clazz, String[] fieldNames, Class[] fieldTypes) {
 List<T> objList = new ArrayList<T>();
 NodeList nl = xmlDoc.getElementsByTagName(tagName);
 if (nl!=null && nl.getLength()>0) {
 for (int i=0; i<nl.getLength(); i++) {
 Element e = (Element) nl.item(i);
 T t;
 try {
 t = getObject(e, clazz, fieldNames, fieldTypes);
 objList.add(t);
 } catch (InstantiationException ex) {
 } catch (IllegalAccessException ex) {
 } catch (IllegalArgumentException ex) {
 } catch (InvocationTargetException ex) {
 } catch (NoSuchMethodException ex) {
 }
 }
 }
 return objList;
 }
 private static <T> T getObject(Element e, Class clazz, String[] fieldNames, Class[] fieldTypes)
 throws InstantiationException, IllegalAccessException, IllegalArgumentException,
 InvocationTargetException, NoSuchMethodException {
 Object[] initargs = new Object[fieldNames.length];
 for (int i=0; i<fieldNames.length; i++) {
 initargs[i] = (fieldTypes[i].getName().equals("int"))?
 getIntegerValue(e, fieldNames[i])
 : getStringValue(e, fieldNames[i]);
 }
 return (T) clazz.getConstructor(fieldTypes).newInstance(initargs);
 }

I posted this so that some of you would understand what I'm trying to do.

Thanks again everyone for their advice.

• Steven: You might be having problems with type erasure...?
• Ozzy: I know this code is wrong, I used it to explain what I'm intending to achieve. I want to instantiate an object of type T with parameter values in the values array.
• Daniel Pryden: @user1031312: How do you know what type T is supposed to be? Is it determined somehow based on the Element? If so, perhaps a Map<String, Class<?>> mapping element names to classes is what you want; then you can construct the object as per Taymon's answer. Alternatively, perhaps your getObject() method should take a Class<T> as an argument.
• Daniel Pryden: Also, I could be wrong, but it looks a bit like you might be trying to re-implement XStream. Could you possibly just use XStream instead?
• Ozzy: To Daniel: I was planning on using XML to define all of the GUIs in my program. I wanted a helper method to simplify the process; which could parse the XML file and populate a list of T objects; to reuse that code without having to specifically write it for each Object I plan on parsing. I'll have a look into XStream though, thanks.

Answer 1

You cannot instantiate a generic type. Java generics have an annoying feature called type erasure, which basically means that the JVM doesn't know what type was used for a generic class or method at runtime. This is for backwards compatibility with pre-generic versions of generic classes, such as collections.

What you can do is add a Class<T> clazz to your parameter list, then replace the end of the method with this:

Class<?>[] paramTypes = new Class[values.length];
for (int i = 0; i < paramTypes.length; i++) {
 paramTypes[i] = values[i].getClass();
}
return clazz.getConstructor(paramTypes).newInstance(values);

This will throw an exception if the class doesn't have a constructor that takes the correct type and number of arguments in the correct order.

CAVEAT: This only works if the constructor's parameter types are exactly the same as the object types. For instance, if values consists of { "Hello", new Integer(2) }, then this will only find a constructor with signature SomeClass(String, Integer), not one with signature SomeClass(Object, Object).

• Ozzy: Thanks, could you provide an example or point me in the right direction in how to invoke the constructor using reflection?
• Maarten Blokker: it is possible to get the type's class at runtime, its just needlessly complicated.
• Ozzy: Thanks, thats exactly what I did now. Actually I wrote clazz.getConstructors()[0].newInstance(values). I'll have to make sure I use the first constructor correctly. I don't understand what your Object[].class does?
• Taymon: When calling getConstructor, you have to specify which constructor you want, and the JVM can only tell constructors apart by their signatures. So you pass an (implicit) array of Class objects corresponding to the constructor's argument types, and it gets you that constructor. Here, it looks like you want a constructor that takes a single Object[] as an argument. Object[].class is a class literal; it represents the Class object that corresponds to type Object[].
• Ozzy: Oh, I understand now. But actually, I was just using an Object[] array to store the values!! I'm just a beginner, forgive me. It is not actually an Object[] parameter, I thought if I had say 4 parameter constructor (int, String, int, String) I could create an Object[] array with: new Integer(2), "String1", new Integer(4), "String2"; and use that as the parameter argument to create a new instance

Answer 2

How would you even know that T contained a constructor that took a single Object[] argument? The way to fix this is to pass in an abstract factory.

interface ThingFactory { // Choose an appropriate name.
 T create(Object[] values);
}
private static <T> T getObject(
 ThingFactory<T> factory, Element e, String[] fieldNames, boolean[] fieldTypes
) { 
 ...
 return factory.create(values);
}

Now T might not even have a rather strange Object[] constructor. T might be an abstract type. The factory might return a different type dependent upon the arguments. Perhaps sometimes these products are immutable objects that can be cached.

• Ozzy: Ohh, I thought the object array acted like a varargs for the parameters list, because in the IDE the parameter comes up as "Object... initargs" and the javadoc says: Uses the constructor represented by this Constructor object to create and initialize a new instance of the constructor's declaring class, with the specified initialization parameters.
• Tom Hawtin - tackline: @user1031312 I wouldn't use reflection unless you actually need it, which you almost certainly don't.
• Ozzy: This would speed up my process a lot thats why I'm doing it, I may change it in the future when I have some more time.

Answer 3

You can't do new T() because at runtime, T is effectively just Object. The compiler uses generics for type-checking (e.g. you can't put an Integer into a List<String>), but then throws the generics information away. There's one compiled copy of the method for all types of arguments, so there's no way for it to know which type you expect it to instantiate.

Read about type erasure for more information.

Answer 4

I dont really see your problem, is it not possible to use obj.getClass ?

• Ozzy: Could you give me an example of what you mean? I tried passing the Class as a parameter before but that got me into a different tangle. I did this getObject(..., Class clazz) then tried to get field names and field types but I got stuck there so I thought it would be easier to pass the field names which I want to set the values of from the XML file
• Maarten Blokker: Never mind, i see what your trying to do now, i think its better to just return a collection of objects here, or just use two different methods, one returning a collection of ints, and one returning a collection of strings. Another way to go would be a wrapper object perhaps