Object[] array = { ... };
System.out.println(array.length);

Object[][] array = { ... };
int count = Stream.of(array).mapToInt(m -> m.length).sum();
System.out.println(count);

public static int count(Object[][] array) {
    int result = 0;
    for (Object[] m : array) {
        result += m.length;
    }
    return result;
}

String s = "...";
System.out.println(s.length());

Set<Integer> s = new TreeSet<Integer>();
s.add(...);
System.out.println(s.size());

public class ExampleArrayLenght {
  public static void main(String[] args) {
    String[] arrays = {"one", "two", "three"};
    int arrayLength = arrays.length;
    System.out.println("Длинна массива = " + arrayLength + " элемента.");
  }
}

int[] array = {1, -4, 3, 5};
int length = array.length;

public int countArr(List<Integer> arr){
    return (int) arr.stream().count();
}