How to add new element into an array

Question

I need to port code from blackberry to android and facing small problem: Example: the bb code is:

public class MyClass{
   private MyObject[] _myObject;

   public void addElement(MyObject o){
      if (_myObject == null){
        _myObject = new MyObject[0];
      }
      Arrays.add(_myObject, o);
   }
}

unfortunately android does not have Arrays.add() which is part of net.rim.device.api.util.Arrays (static void add(Object[] array, Object object))

Is there any replacement for android to dynamically extend and append in to simple array so I don't change the rest of my code.

I tried to write my own utility but it does not work:

public class Arrays {
  public static void add(Object[] array, Object object){
    ArrayList<Object> lst = new ArrayList<Object>();
    for (Object o : array){
      lst.add(o);
    }
    lst.add(object);
    array = lst.toArray();
  }
}

.. after I call

public void addElement(MyObject o){
      if (_myObject == null){
        _myObject = new MyObject[0];
      }
      Arrays.add(_myObject, o);
   }

the _myObject still contain 0 elements.

Solution 1

Yes, because the _myObject reference is passed by value. You'd need to use:

public static Object[] add(Object[] array, Object object){
  ArrayList<Object> lst = new ArrayList<Object>();
  for (Object o : array){
    lst.add(o);
  }
  lst.add(object);
  return lst.toArray();
}

...

_myObject = Arrays.add(_myObject, o);

However, it would be better to just use an ArrayList<E> to start with...

There are two important things to understand here:

Java always uses pass-by-value

The value which is passed is either a reference (a way of getting to an object, or null) or a primitive value. That means if you change the value of the parameter, that doesn't get seen by the caller. If you change the value of something within the object the parameter value refers to, that's a different matter:

void doSomething(Person person) {
  person.setName("New name"); // This will be visible to the caller
  person = new Person(); // This won't
}

Arrays are fixed-size in Java

You can't "add" a value to an array. Once you've created it, the size is fixed. If you want a variable-size collection (which is a very common requirement) you should use an implementation of List<E> such as ArrayList<E>.

Solution 2

• commons-lang has ArrayUtils.add(..)
• guava has ObjectArrays.concat(..).

Here's the code of ObjectArrays.concat(object, array):

public static <T> T[] concat(@Nullable T element, T[] array) {
    T[] result = newArray(array, array.length + 1);
    result[0] = element;
    System.arraycopy(array, 0, result, 1, array.length);
    return result;
  }

The apache-commons code is a bit longer.

Comments

[kompotFX]

thanks for quick answer. Please, take a look to this spec: blackberry.com/developers/docs/3.7api/net/rim/device/api/uti‌​l/… There is void add(Object[] array, Object object); How do rim developers achieve that it works for them? I mean they just call add(object_array, new_element); and object_array extended with new element?

[Jon Skeet]

@kompotFX: They don't. It's entirely unclear what that badly-documented method does, but I can guarantee you that if you print out array.length, then call add(array, "hello"), then print array.length again, it'll have the same value. It's possible that it looks for an element in the array which has a null value, and set the value of that element to the one that was passed in. That's not the same thing as adding though.

[kompotFX]

it is very strange, because actually the sample of code I need to port works on blackberry.

[Jon Skeet]

@kompotFX: Well the code you've given at the top would always leave _myObject as a length-0 array. That's assuming RIM hasn't broken the way that Java works, of course, but I doubt that they have. If you can show a short but complete program showing RIM's code working, that would be useful.

[kompotFX]

so, you want to say that RIM developers are wrong with this: <blackberry.com/developers/docs/3.7api/net/rim/device/api/ut‌​il/…, java.lang.Object)> ?

[Jon Skeet]

@kompotFX: Almost certainly, unless their "version" of Java is fundamentally different to everyone else's. Again, some sample code showing how that code "works" would be useful.

[kompotFX]

unfortunately it is a part of huge project and I am sure 100% that this application and currently this method works on blackberry device.

[Jon Skeet]

@kompotFX: Just because you've got something working as part of a big project doesn't mean you'd have to post the project. Just a short but complete bit of code which shows it working. Something which creates an array of length 5 (say), passes a reference to that array to the Arrays.add method, and then prints out the length again afterwards...

[kompotFX]

check real example here: docs.blackberry.com/en/developers/deliverables/11942/… Look for method private String[] getEncodings()

[Jon Skeet]

@kompotFX: Right, that seems unambiguous. Under normal Java, there is no way that can work. So either their code really is broken, or their implementation of Java is completely wacky.

[Jon Skeet]

@kompotFX: Just looking at this it looks like it's gone in later versions of the API... so maybe they've stopped the craziness over time.