How to create generic object in base class for derived class

Language Design
1

I have a base class ,it need create generic object for derived class, when i code it in java ,i can generate the object from nonargs constructor as init,i use IDEA to convert the Java code to Kotlin code, i think lateinit var model :T is much more suitable to private var model: T? = null,but when i use lateinit the IDEA will tell me there is an error :hint

can I make the Kotlin code more Kotlin?

my Java code:

public class CommonAction<T> extends ActionSupport implements ModelDriven<T> {
	private T model;

	public T getModel() {
		return model;
	}

	public CommonAction() {
		ParameterizedType genericSuperclass = (ParameterizedType) this.getClass().getGenericSuperclass();
		Type[] actualTypeArguments = genericSuperclass.getActualTypeArguments();
		Class<T> entityClass = (Class<T>) actualTypeArguments[0];
		try {
			model = entityClass.newInstance();
		} catch (InstantiationException e) {
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			e.printStackTrace();
		}
	}

the Kotlin code:

open class CommonAction<T> : ActionSupport(), ModelDriven<T>, ServletResponseAware {

    private  var model: T? = null
    override fun getModel(): T? = model

    init {
        val genericSuperclass = this.javaClass.genericSuperclass as ParameterizedType
      
        val actualTypeArguments = genericSuperclass.actualTypeArguments
        
        val entityClass = actualTypeArguments[0] as Class<*>       
       
         model = entityClass.newInstance() as T
    }
2

lateinit is a workaround that lets you not initialize a non-nullable value. Itā€™s like saying: ā€œThis will never be null, but I donā€™t have the initial value yet. Trust me, I will assign the value before it will be used. You can Null Pointer Exception me if Idonā€™t.ā€
But if the value is nullable, there is no point in using lateinit. Not being initialized is perfectly valid state for that value already,

3

thanks for answer,this is my point too, I donā€™t want see the value is nullable declaration,the object model will never be null,but i have no idea what can I do with the lateinit for my code,

4

You need to declare a constraint for the type parameter: open class CommonAction<T : Any>

1 Like
5

It works! Thank you! I have no idea what to do with the hint ā€œā€˜lateinitā€™ modifier is not allowed on properties of a type with nullable upper boundā€ before.