I am learning using binary search in Java. In an integer list it returns expected index but in char or a String list (code example) it returns a negative index, which is not expected (index = -4).
List<String> str = new ArrayList<>();
str.add("Hey");
str.add("Hi");
str.add("Bye");
int index = Collections.binarySearch(str,"Hi");
System.out.println(index);
The pre-condition of a binary search is: the underlying data must be sorted.
So, sort that list first.
And then you want to check that the index returned by that method is between 0 and the size() of your list. It is a bit naive to expect that any string could be found in your list and return a non-zero index.
Beyond that; the real answer here: don't just blindly use a built-in functionality. Read its javadoc first to understand what this method is doing, and for example: what values it will return to you!
Your List must be sorted in order for binary search to work.
The natural ordering of Strings is lexicographical order, so "Bye" should come before "Hey" and "Hi".
Try
List<String> str = new ArrayList<>();
str.add("Bye");
str.add("Hey");
str.add("Hi");
int index = Collections.binarySearch(str,"Hi");
System.out.println(index);
Collections#sort.