Can't use binary search with Object Arraylist?

Question

I tried using a binary search with an Arraylist and it gave me this message:

The method binarySearch(List>, T) in the type Collections is not applicable for the arguments (ArrayList, String)

Here's the code:

ArrayList <Object> a = new ArrayList <Object> ();
String date = JOptionPane.showInputDialog(null, "Please enter the date.")
int index = Collections.binarySearch(a, date);

The binary search should return the position of a specific date from an array (a) of several dates. What am I doing wrong?

Show us the declaration of a, please.

Louis Wasserman
– Louis Wasserman

2015-06-09 21:56:31 +00:00
Commented Jun 9, 2015 at 21:56 — Louis Wasserman
– Louis Wasserman, Commented Jun 9, 2015 at 21:56

Turing85 · Accepted Answer · 2015-06-09 22:22:11Z

3

To perform a binary search, your Collection must be sorted. In order to sort something, this something needs to have a well-defined order. That is, what the Comparable<T> interface is for. Object does not implement Comparable<Object> and therefore a List of Object cannot be sorted (ther are no criteria to sort them). Please look at the API for mor details.

edited Jun 9, 2015 at 22:22

answered Jun 9, 2015 at 21:59

Turing85

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2 Comments

Vctrm6 Over a year ago

k thanks. The problem is, my array is going to contain both doubles and strings, so how would I go about searching the array for a specific string among doubles and strings?

Turing85 Over a year ago

@Vctrm6 you mean your ArrayList, right? I think your algorithm is broken by design. Either store the doubles as Strings in your ArrayList<String> and re-parse them into doubles (dirty work-around) or overthink your design, so you do not need to store Strings and doubles in the same list.

Louis Wasserman · Accepted Answer · 2015-06-09 22:01:01Z

2

a should be a List<String>, not a List<Object>. Collections.binarySearch expects the list to contain the same type you are searching for, and the lowest type in common -- Object -- is not Comparable, so it cannot be used for a binary search.

answered Jun 9, 2015 at 22:01

Louis Wasserman

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3 Comments

Vctrm6 Over a year ago

Ok thanks. The problem is, my array is going to contain both doubles and strings, so how would I go about searching the array for a specific string among doubles and strings?

Louis Wasserman Over a year ago

You can't use binarySearch, then. You could use indexOf, and accept that it'll take O(n). (But frankly, mixing two different types in the same list like that is a huge design smell that says you're doing something deeply wrong.)

Turing85 Over a year ago

@Vctrm6 you could have posted this as comment under your question instead of asking both of us the same. Anyway, you got the same basic answer: bad design.

user4910279 · Accepted Answer · 2015-06-09 22:22:52Z

0

Object does not implement Comaparable<Object>. You must provide Comparator<Object>.

    ArrayList <Object> a = new ArrayList<>();
    // add elements to a.
    Comparator<Object> comparator = (x, y) -> x.toString().compareTo(y.toString());
    Collections.sort(a, comparator);
    String date = JOptionPane.showInputDialog(null, "Please enter the date.")
    int index = Collections.binarySearch(a, date, comparator);

comparatoer in this code is only an example.

answered Jun 9, 2015 at 22:22

user4910279

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