Is there an "UNIVERSAL" way to initialize an array of objects? (Java)

Question

I use an init method per classes.

Spam[] spam1 = new Spam[13];
Spam[] spam2 = new Spam[7];
Spam[] spam3 = new Spam[5];

initSpamArray(spam1);
initSpamArray(spam2);
initSpamArray(spam3);

void initSpamArray (Object[] a) {
    for (int i = 0, len = a.length; i < len; i++) {
        a[i] = new Spam();
    }
}


Ham[] ham1 = new Ham[13];
Ham[] ham2 = new ham[7];
Ham[] ham3 = new Ham[5];

initHamArray(ham1);
initHamArray(ham2);
initHamArray(ham3);

void initHamArray (Object[] a) {
    for (int i = 0, len = a.length; i < len; i++) {
        a[i] = new Ham();
    }
}

Is it possible to define a kind of "universal" method to init any kind of object?

Like at least:

void initObjArray (Object[] a, <s.g. which suitable to transfer Class>) {
    for (int i = 0, len = a.length; i < len; i++) {
        a[i] = new <s.g. which suitable to transfer Class>();
    }
}

I tried to Google a lot and to play with java reflection also (Object.getClass(); Constructor.newInstance(); Class.newInstance() ). However I have not been successful.

Well usually you don't just want to call a parameterless constructor... — Jon Skeet
– Jon Skeet, Commented Apr 10, 2014 at 20:09
"I have not been successful" is not a good problem description. What did you try and where did it go wrong? Aside from that: is it really a problem? How often do you use a parameterless constructor of a self defined type? — Jeroen Vannevel
– Jeroen Vannevel, Commented Apr 10, 2014 at 20:11
Are you trying to follow Sagan's recipe for apple pie? I can't get the crust right either. — Elliott Frisch
– Elliott Frisch, Commented Apr 10, 2014 at 20:23
Thank you for your answers. I see your points and probably parameterless constructor not make so much sense. This thing just came into my mind when I was doing some experiments for better understanding. Because this all thing came from exercising I did a lot of overwritings and I could not find my previous achievements but I would try to reconstruct the things with reflection. — osiixy
– osiixy, Commented Apr 10, 2014 at 20:36

Mike Samuel · Accepted Answer · 2014-04-10 20:39:02Z

1

Using a Supplier to specify the way to create an instance:

public static <T> T[] fullArray(Class<T> componentType, int n,
                                Supplier<? extends T> constructor) {
  // This introduces no new type-unsafety.
  // Array.newInstance has to return Object since it can take a primitive
  // component type and !(new int[0] instanceof Object[]), but we know that
  // the result is a reference type since type parameters like T can only
  // bind to reference types.
  @SuppressWarnings("unchecked")
  T[] array = (T[]) Array.newInstance(componentType, n);
  for (int i = 0; i < n; ++i) {
    array[i] = constructor.get();
  }
  return array;
}

Instead of

Foo[] foos = new Foo[42];
for (int i = 0; i < foos.length; ++i) {
  foos[i] = new Foo();
}

you could do

Foo[] foos = fullArray(
    Foo.class, 42,
    new Supplier<Foo>() { public Foo get() { return new Foo(); } });

or in Java 8

Foo[] foos = fullArray(Foo.class, 42, () -> new Foo());

edited Apr 10, 2014 at 20:39

answered Apr 10, 2014 at 20:24

Mike Samuel

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1 Comment

Giovanni Botta Over a year ago

When using Java 8, the Supplier interface is now part of the language. Which means a lambda can be used.

robert_difalco · Accepted Answer · 2014-04-10 20:52:35Z

1

If I understand the question correctly I think you want this (note: handle or propagate the exceptions any way you wish to):

public static <T> T[] newDefaultArray( Class<T> type, int N ) {
    T[] array = (T[])Array.newInstance(type, N);
    for ( int at = 0; at != N; ++at ) {
        try {
            array[at] = type.newInstance();
        }
        catch ( Exception e ) {
           throw new RuntimeException( e );
        }
    }

    return array;
}

Then you can use it like this (very simple):

   User[] users = newDefaultArray( User.class, 100 );

The class will need to have a default constructor.

edited Apr 10, 2014 at 20:52

answered Apr 10, 2014 at 20:16

robert_difalco

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1 Comment

Mike Samuel Over a year ago

Class.newIntance has a specification bug in that it silently converts checked exceptions to unchecked.

Gregor Koukkoullis · Accepted Answer · 2014-04-10 20:43:04Z

Here is a possible solution. To prevent using Class.newInstance() as Mike suggest in his comment, I changed the implementation. It would also support a constructor with possible parameters

public <T> void fillArray(T[] a, ArrayFillFunction<? extends T> f)
{
    for (int i = 0, len = a.length; i < len; i++)
    {
        a[i] = f.fill(i);
    }
}

public interface ArrayFillFunction<T> {
    T fill(int idx);
}

Your call would look like this:

Ham[] ham1 = new Ham[13];
fillArray(ham1, new ArrayFillFunction<Ham>()
{
    public Ham fill(int idx)
    {
        return new Ham();
    }
});

Or with custom constructor parameters:

Ham[] ham1 = new Ham[13];
fillArray(ham1, new ArrayFillFunction<Ham>()
{
    public Ham fill(int idx)
    {
        return new Ham(getParameterForIndex(idx));
    }
});

With Java 8 you can do this even more elegantly using Lambdas:

Ham[] ham1 = new Ham[13];
fillArray(ham1, idx -> new Ham());
fillArray(ham1, idx -> new Ham(getParameterForIndex(idx)));

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