A DESCRIPTION OF THE PROBLEM :
Motivation
The motivation behind this proposal is to enhance the functionality of the List interface by providing a more flexible way to find the index of an element. Currently, the indexOf and lastIndexOf methods only accept an object as a parameter. This limits the flexibility of these methods as they can only find the index of exact object matches.

The proposed methods would accept a Predicate as a parameter, allowing users to define a condition that the desired element must meet. This would provide a more flexible and powerful way to find the index of an element in a list.

Here is a brief overview of the changes made in this pull request:

Added the indexOf(Predicate<? super E> filter) method to the List interface.
Added the lastIndexOf(Predicate<? super E> filter) method to the List interface.
Implemented these methods in all non-abstract classes that implement the List interface.
The changes have been thoroughly tested to ensure they work as expected and do not introduce any regressions. The test cases cover a variety of scenarios to ensure the robustness of the implementation.

For example, consider the following test case:

List<String> list = new ArrayList<>();
list.add("Object one");
list.add("NotObject two");
list.add("NotObject three");

int index1 = list.indexOf(s -> s.contains("ct t"));
System.out.println(index1); // Expected output: 1
int index2 = list.lastIndexOf(s -> s.startsWith("NotObject"));
System.out.println(index2); // Expected output: 2
Currently, to achieve the same result, we would have to use a more verbose approach:

int index1 = IntStream.range(0, list.size())
                     .filter(i -> list.get(i).contains("ct t"))
                     .findFirst()
                     .orElse(-1);
System.out.println(index1); // Output: 1
int index2 = IntStream.range(0, list.size())
                         .filter(i -> list.get(i).startsWith("NotObject"))
                         .reduce((first, second) -> second)
                         .orElse(-1);
System.out.println(index2); // Output: 2
I believe these additions would greatly enhance the functionality and flexibility of the List interface, making it more powerful and user-friendly. I look forward to your feedback and am open to making any necessary changes based on your suggestions.