如何编写通用方法在数组中插入元素？

Question

I have an input array [3, 5, 12, 8] and I want an output array (the input must not be affeccted) identical to the input, but with the element 7 inserted between 5 and 12, so at index 2 of the input array.

Here is what I have so far. I commented out code that doesn't event compile, and added a couple of questions that arose while trying this or that way:

public static <O>ArrayList<O> addToSet(O[] in,O add,int newIndex){
//    O obj = (O) new Object(); //this doesnt work
//    ParameterizedType obj = (ParameterizedType) getClass().getGenericSuperClass(); // this is not even recognized
    ArrayList<O> out = multipleOfSameSet(obj, in.length);
    if (newIndex > in.length){
        out = new ArrayList<>(newIndex+1); // also noticed that initializing an ArrayList 
        //like this throws an IndexOutOfBoundsException when i try to run out.get(),
        // could someone explain why??  
        out.set(newIndex, add);
    }
    int j = 0;
    int i = 0;
    while(j<in.length+1){
        if (j==newIndex){
            out.set(j, add);
        } else if(i<in.length){
            out.set(j, in[i]);
            i++;
        }
        j++;
    }
    return out;
}

The array component type could be String, Integer or even a JPanel.

Answer 1

Here is the generic version of the code

@SuppressWarnings("unchecked")
public <T> T[] insertInCopy(T[] src, T obj, int i) throws Exception {
    T[] dst = (T[]) Array.newInstance(src.getClass().getComponentType(), src.length + 1);
    System.arraycopy(src, 0, dst, 0, i);
    dst[i] = obj;
    System.arraycopy(src, i, dst, i + 1, src.length - i);
    return dst;
}

but you may want to specialize the method for dealing with primitive types. I mean, generics and arrays don't mix well - so you'll have troubles with int and will need to use wrapper types:

@Test
public void testInsertInTheMiddle() throws Exception {
    Integer[] in = {3, 5, 12, 8};
    Integer[] out = target.insertInCopy(in, 7, 2);
    assertEquals(out, new Integer[] {3, 5, 7, 12, 8});
}

Answer 2

You can do this way.

static <T> void fromArrayToCollection(T[] a, Collection<T> c) {
    for (T o : a) {
        c.add(o);
    }
}