Re: static initialization of arrays

Daniel Pitts <>
15 Oct 2007 21:13:39 -0700
On Oct 15, 3:33 pm, Johan <> wrote:

Hi guys,

I started to write some Java today, see the code below. What I want to
do is produce a static array V from several static arrays A, B, ...,
but I don't want V to have any duplicate elements. I was wondering if
there is a better way of coding this up?


import java.util.*;

class StringSet extends TreeSet<String>
    public void addAll( String[] strings )
        for (String s: strings )
            add( s );


No need for a special StringSet class...

class Main
    private static final String[] A = { "A1", "A2", "B1" };
    private static final String[] B = { "B1", "B2" };

    private static final StringSet S =
        new StringSet ()
                addAll( A );
                addAll( B );

    private static final String[] V = S.toArray(new String[S.size()]);



You really should use List instead of String[], but I'll show you both

import java.util.*;

public class Main {
  public static final String[] A = { "A1", "A2", "B1" };
  public static final String[] B = { "B1", "B2" };
  public static final String[] V;
  static {
     Set<String> stringSet = new
     V = stringSet.toArray(new String[stringSet.size()]);

The problem with that approach, is that A and B and V are still
V[0] = "B1" will have an effect you don't want.

import java.util.*;

public class MainWithCollection {
  public static final List<String> A =
        Collections.unmodifiableList(Arrays.asList("A1", "A2", "B1"));
  public static final List<String> B =
        Collections.unmodifiableList(Arrays.asList("A1", "B1", "B2"));
  public static final List<String> V;
  static {
     Set<String> stringSet = new LinkedHashSet<String>(A);
     ArrayList<String> stringList = new ArrayList<String>(stringSet);
     V = Collections.unmodifiableList(stringList);

Also, is there a way to
guarantee that arrays A, B, ... are constructed before S and V without
relying on the order in which they are declared?

If they are in the same class, then you have to declare them in the
order you expect them to be initialized. The compiler won't let you
refer to them before they are. You can use a static initializer (such
as I did) to explicitly initialize them in a particular order.

Now, if you have many arrays/lists to add together (more than just A
and B), you might consider two things: One, loading this data from an
external source (data file, for instance), and at the very least, use
a for-loop to iterate over them:

public class Main {
   public static final Object[] a1 = {1, 2, 3};
   public static final Object[] a2 = {1, 2, 3};
   public static final Object[] a3 = {1, 2, 3};
   public static final Object[] a4 = {1, 2, 3};

   static {
       for (Object[] arr : new Object[][] {a1, a2, a3, a4}) {

Or similarly with the List version.

The more I think about this, the more I think you should separate out
these strings into an external file, and not have them be public nor
static. What is it that you're trying to achieve?

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