Why am I not able to initialize Generic Parameter and Array?

Question

I am trying to initialize Generic Parameter and Array

like

import java.util.ArrayList;
import java.util.List;

public class GenericExample<T> {

/*
 * Complier error -->
 * 
 * Cannot instantiate the type T
 */
private T t = new T();

/*
 * Complier error -->
 * 
 * Cannot create a generic array of T
 */
private T[] tArray = new T[10];

/*
 * No Complier error.
 */
private List<T> list = new ArrayList<T>();

}

There is no error when initialize list with Generic type as follows:

      /*
 * No Complier error.
 */
private List<T> list = new ArrayList<T>();

However, Complier errors appear when I am using the following initialization.

/*
 * Complier error -->
 * 
 * Cannot instantiate the type T
 */
private T t = new T();

        /*
 * Complier error -->
 * 
 * Cannot create a generic array of T
 */
private T[] tArray = new T[10];

Can someone help me out with the following 2 questions:

Q1: Why List<T> list = new ArrayList<T>(); does not encounter Complier error?

Q2: Why Complier error is thrown when using private T t = new T(); and private T[] tArray = new T[10]; ?

Vivin Paliath · Accepted Answer · 2013-11-15 15:40:25Z

2

This is due to type erasure. The JVM has no idea what T is during runtime, since generic information is not retained for use during runtime (Java doesn't have reified generics). So when you have T t = new T(); and private T[] tArray = new T[10];, the JVM has no idea what T is.

List<T> list = new ArrayList<T>();

This works because you actually end up the following after compilation:

List list = new ArrayList();

Explicit casts to the appropriate type are then used when retrieving. That is, if you have:

List<String> list = new ArrayList<String>();
list.add("hello");
String hello = list.get(0);

During runtime what the JVM actually sees is:

List list = new ArrayList();
list.add("hello");
String hello = (String) list.get(0);

answered Nov 15, 2013 at 15:40

Vivin Paliath

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Comments

Arash Saidi · Accepted Answer · 2013-11-15 15:41:18Z

0

In Java you cannot initialize a generic object if the generic has to be stored in memory (like in your first case and in the case of an object).

There are workarounds to this, you can cast your generics to the Object class. I can't remember how you do it in terms of syntax, but search for generic casting of arrays.

answered Nov 15, 2013 at 15:41

Arash Saidi

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