List is an ordered collection implementation under Java's Collections Framework, where each elemant is assigned a zero-based integer position corresponding to its placement in the sequence. To fetch the position of a specific element by value, you can use either built-in List methods or implement custom lookup logic depending on use case requirements.

Built-in indexOf() Method

The List interface provides a native indexOf() method that accepts a target object as input and returns the index of the first occurrence of the matching element in the sequence. If no matching element is found, the method returns -1.

import java.util.ArrayList;
import java.util.List;

public class ListIndexDemo {
    public static void main(String[] args) {
        List<Double> scoreRecords = new ArrayList<>();
        scoreRecords.add(76.5);
        scoreRecords.add(91.0);
        scoreRecords.add(82.3);
        
        int targetPosition = scoreRecords.indexOf(91.0);
        System.out.println("Position of score 91.0: " + targetPosition);
    }
}

When executed, the above code outputs Position of score 91.0: 1, as elements are zero-indexed.

Custom Index Lookup Implementation

For use cases that require custom matching logic (such as case-insensitive string matching or null-safe lookups), you can implement a generic lookup functon to traverse the list and return the matching element's index.

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;

public class CustomIndexLookup {
    public static <T> int findElementPosition(List<T> inputList, T targetElement) {
        for (int pos = 0; pos < inputList.size(); pos++) {
            if (Objects.equals(inputList.get(pos), targetElement)) {
                return pos;
            }
        }
        return -1;
    }

    public static void main(String[] args) {
        List<String> fruitInventory = new ArrayList<>();
        fruitInventory.add("Orange");
        fruitInventory.add("Mango");
        fruitInventory.add("Grape");
        
        int mangoPosition = findElementPosition(fruitInventory, "Mango");
        System.out.println("Position of Mango in inventory: " + mangoPosition);
    }
}

The Objects.equals() call in the above implementation ensures null safety, avoiding NullPointerExceptions when either the target element or list elements are null. The function returns -1 for non-existent elements, matching the behavior of the built-in indexOf() method.