Question

I wrote a data model, into one class I need to merge 3 arrays. These arrays has a fixed size (of course) and initialized to null. I use this code :

public static <T> T[] merge(T[]... arrays) {
    int size = 0;
    for (T[] array : arrays) {
        size += array.length;
    }

    T[] merged = (T[]) Array.newInstance(arrays[0][0].getClass(), size);

    int start = 0;
    for (T[] array : arrays) {
        System.arraycopy(array, 0,
           merged, start, array.length);
        start += array.length;
    }
    return (T[]) merged;
}

This line is not correct :

T[] merged = (T[]) Array.newInstance(arrays[0][0].getClass(), size);

I need to precise which class I use, but I get a null pointer !

Answer 1

I would leverage the Collections library

public static <T> T[] merge(T[]... arrays) {    
    List<T> list = new LinkedList<T>();
    for(T[] array : arrays) {
        for(T t : array) {
            list.add(t);
        }
    }
    return (T[])(list.toArray());    
}

Test program:

import java.util.*;
class Merger {
    // break out total length logic. Will probably be inlined, but this seems like
    // functionality that deserves its own method.
    public static <T> int totalLength(T[]... arrays) {
        int length = 0;
        for(T[] arr : arrays) length += arr.length;
        return length;
    }

    public static <T> T[] merge(T[]... arrays) {
        int length = totalLength(arrays);
        if(length == 0) return (T[])new Object[0];
        List<T> list = new ArrayList<T>(length);
        for(T[] array : arrays) {
            for(T t : array) {
                list.add(t);
            }
        }
        return (T[])(list.toArray(arrays[0])); 
    }

    public static void main(String[] args) {
        String[] first = new String[] { "This", "is", "a", "test" };
        String[] second = new String[] { "of", "the", "merger" };
        String[] third = new String[] { "and", "it", "works!" };

        String[] merged = merge(first,second,third);
        for(String s : merged) System.out.println(s);

        // validate that it works for merging all 0 arrays:
        merge();
    }
}