How to get elements of an array that is not null

问题

I am doing gift (String-type) storage, using arrays, with the maximum 500 million. I want to get the number of used elements in an array like how many gifts are currently in stock, (I.E. I stored 253538 gifts, but I don't know that. Is there a command in Java that can know that there is only 253538 Slots in the array that contain an element). But I don't know how to. Here's the snippet of code I want to use:

static String[] Gifts = new String[500000000];
static int i = 0;
String command, Gift;
while (true) {
    //Gift Array Console
    String command = scan.next();
    if (command == "addgift") {
        String Gift = scan.next();
        Gifts[i] = Gift;
        i++;
    }
}

回答1:

You can iterate through the array and count the initialized array elements.

int counter = 0;
for (int i = 0; i < arrayName.length; i ++)
    if (arrayName[i] != null)
        counter ++;

Also it would be even better if you used ArrayList<String> so you could use size()

ArrayList<String> arrayName = new ArrayList<String>(20);
System.out.println(arrayName.size());

It will print 0, as there are no elements added to the ArrayList.

回答2:

You can use Arrays.stream to iterate over this array of strings, then use filter to select nonNull elements and count them:

String[] arr = {"aaa", null, "bbb", null, "ccc", null};

long count = Arrays.stream(arr).filter(Objects::nonNull).count();

System.out.println(count); // 3

Or if you want to find the index of the first null element to insert some value there:

int index = IntStream.range(0, arr.length)
        .filter(i -> arr[i] == null)
        .findFirst()
        .getAsInt();

arr[index] = "ddd";

System.out.println(index); // 1
System.out.println(Arrays.toString(arr));
// [aaa, ddd, bbb, null, ccc, null]

^{See also: How to find duplicate elements in array in effective way?}

来源：https://stackoverflow.com/questions/65328677/how-to-get-elements-of-an-array-that-is-not-null

标签

java

arrays

user-input