There is a very important method indexOf which is part of the List interface. The description in Java documentation states:
“Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.”
Example:
import java.util.Arrays;
import java.util.List;
public class MainListIndexOfTest {
public static void main(String[] args) {
List<Integer> list = List.of(1, 2, 3, 2, 4, 5);
System.out.println(list);
int index = list.indexOf(2);
// It will print index 1, although there is second occurrence of '2'
System.out.println("First index of '2' is: " + index);
// There is also the opposite connection with get method.
// it will return the object on a certain index in the List
Integer elementFromList = list.get(index);
System.out.println("Element from the list: " + elementFromList);
// In case when we have array we can create List and use it again
String[] arr = new String[] {"A", "B", "C", "D"};
List<String> list1FromArr = Arrays.asList(arr);
int indexOfC = list1FromArr.indexOf("C");
System.out.println("Index of 'C' is: " + indexOfC);
}
}
Output:
[1, 2, 3, 2, 4, 5]
First index of '2' is: 1
Element from the list: 2
Index of 'C' is: 2