Question about generic method

Question

I am trying to make the following method to a generic method and got struck.

public static MyObject[] getObjectFromList(List<MyObject> inputList) {
        if (inputList != null && inputList.size() > 0) {
            return inputList.toArray(new MyObject[inputList.size()]);
        } else {
            inputList = new ArrayList<MyObject>();
            MyObject obj = new MyObject();
            inputList.add(obj);
            return inputList.toArray(new MyObject[inputList.size()]);
        }
    }

and have the other method which is similar but it's MyObject2

public static MyObject2[] getObjectFromList(List<MyObject2> inputList) {
            if (inputList != null && inputList.size() > 0) {
                return inputList.toArray(new MyObject2[inputList.size()]);
            } else {
                inputList = new ArrayList<MyObject2>();
                MyObject2 obj = new MyObject2();
                inputList.add(obj);
                return inputList.toArray(new MyObject2[inputList.size()]);
            }
        }

How do I make this as a single method which takes any object(myobject,myobject2) list and returns an array ?

Answer 1

You can't do it without changing method signature due to type erasure. If you can pass a Class object identifying the desired type you can do something like this:

public static <T> T[] getObjectFromList(List<? extends T> inputList, Class<T> c) {
    if (inputList == null || inputList.isEmpty()) {
        inputList = new ArrayList<T>();
        T obj = null;
        try {
            obj = c.newInstance();
        } (Exception ex) {
            throw new RuntimeException("Instantiation failed", ex);
        }
        inputList.add(obj);
    }             
    return inputList.toArray(Array.newInstance(c, inputList.size()));            
}

Answer 2

There is a method for that already - List.toArray() .

However, generics in Java do not allow for creating array of generic types (generics are lost at runtime, so E[] e = new E[5] is not possible . So if you want to have an array of a specific type, rather than Object[] (which t he above method returns), use:

MyObject[] array = list.toArray(new MyObject[list.size()]);

Having "at least one object in the list" is a requirement I would very much question. It does not sound right. If you are really certain you want it, axtavt has provided the correct method.

Answer 3

Try this

  public static void main(String[] args) {
    List<Integer> list = new ArrayList<Integer>();
    list.add(1);
    list.add(2);
    list.add(3);
    System.out.println(list);
    Integer[] array = listToArray(list);
    for(Integer i : array) {
      System.out.println(i);
    }
  }

  @SuppressWarnings("unchecked")
  public static <T> T[] listToArray(List<T> list) {
    return listToArray(list, (Class<T>) list.get(0).getClass());
  }

  @SuppressWarnings("unchecked")
  public static <T> T[] listToArray(List<T> list, Class<T> clazz) {
    return list.toArray((T[]) Array.newInstance(clazz, 0));
  }

Answer 4

Give the method a generic type parameter:

public <T> T[] getObjectFromList(List<T> c);

Or with objects of type MyObjectInterface:

public <T extends MyObjectInterface> T[] getObjectFromList(List<T> c);