无法将 int 添加到整数的 ArrayList

Question

I'm supposed to create a new array, which contains the first element of arrays a and b (excpet if one is empty, I must skip it). The problem is that when I try to add the int element to the ArrayList I get the error: "incompatible types: java.util.ArrayList cannot be converted to int[]" This is so annoying.. Here's the code:

public int[] front11(int[] a, int[] b) {
ArrayList<Integer> list = new ArrayList<Integer>();
if (a.length>0 & b.length>0){
    list.add(a[0]);
    list.add(b[0]);
}
if (b.length==0 & a.length>0){
    list.add(a[0]);
}
if (b.length>0 & a.length==0){
    list.add(b[0]);
}
return list;

}

Answer 1

list is not an int[] and you don't need a List here. Nor do you need an instance, so I'd make it static and I would do it inline (and I would also guard against null ). Like,

public static int[] front11(int[] a, int[] b) {
    int alen = (a != null) ? a.length : 0, blen = (b != null) ? b.length : 0;
    if (alen > 0 && blen > 0) {
        return new int[] { a[0], b[0] };
    } else if (alen > 0) {
        return new int[] { a[0] };
    } else if (blen > 0) {
        return new int[] { b[0] };
    } else {
        return new int[0];
    }
}

Answer 2

An ArrayList is not an array, no matter the name of the class. It's named that way because it uses an array internally to manage the list.

Using an ArrayList is also overkill here. You have 4 scenarios, so just code them and create the returning array as needed:

public static int[] front11(int[] a, int[] b) {
    if (a.length > 0 && b.length > 0)
        return new int[] { a[0], b[0] };
    if (a.length > 0)
        return new int[] { a[0] };
    if (b.length > 0)
        return new int[] { b[0] };
    return new int[0];
}

Answer 3

Convert int[] return type into ArrayList<Integer>

    public ArrayList<Integer> front11(int[] a, int[] b) {
ArrayList<Integer> list = new ArrayList<Integer>();
if (a.length>0 & b.length>0){
    list.add(a[0]);
    list.add(b[0]);
}
if (b.length==0 & a.length>0){
    list.add(a[0]);
}
if (b.length>0 & a.length==0){
    list.add(b[0]);
}
return list;
}