字符串列表始终为null

Question

I'm trying to initialize a list of strings as private static final but I always get the value of l as null, and if I declare it as shown in the second snippet of code it works.

What I'm trying to do is add the elements of a list I declare to a trie to match some pattern later...

Any thoughts on why the null value in the first example and how can I correct it? Thanks

public class Myclass {
    public static final Myclass INSTANCE = new Myclass();

    private static final List<String> l = Arrays.asList("ofo", "oof", "foo");

    private Trie trie;

    private Myclass() {
        trie = buildTrie();
    }

    private Trie buildTrie() {
        TrieBuilder builder = Trie.builder();
        Iterator<String> iterator = l.iterator();
        while (iterator.hasNext()) {
            builder.addKeyword(iterator.next());
        }
        return builder.build();
    }
}

public class Myclass {
    public static final Myclass INSTANCE = new Myclass();
    private Trie trie;

    private Myclass() {
        List<String> l = Arrays.asList("ofo", "oof", "foo");
        trie = buildTrie();
    }

    private Trie buildTrie() {
        TrieBuilder builder = Trie.builder();
        Iterator<String> iterator = l.iterator();
        while (iterator.hasNext()) {
            builder.addKeyword(iterator.next());
        }
        return builder.build();
    }
}

Answer 1

public class Myclass {
    public static final Myclass INSTANCE = new Myclass();

    private static final List<String> l = Arrays.asList("ofo", "oof", "foo");

Static initialization is done in the order of declaration. This means that INSTANCE is initialized before l , so the constructor of Myclass reads the uninitialized value of l .

Reverse the order of the declarations:

public class Myclass {
    private static final List<String> l = Arrays.asList("ofo", "oof", "foo");

    public static final Myclass INSTANCE = new Myclass();

Or, if you don't actually require l otherwise, consider passing it as a constructor parameter.