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I am going through official TreeMap documentation. I see subMap() prototype as:

public SortedMap<K,V> subMap(K fromKey, K toKey) is equivalent to subMap(fromKey, true, toKey, false).

I have seen everywhere, Java does not include the last value of a given range by default? Why was this decision made that default inclusion value for toKey will be false (and not true)?

Mark Rotteveel
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GAURAV KABRA
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  • I think it's for consistency, substring behaves the same, so does sublist. – criztovyl Jul 23 '22 at 08:44
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    my **guess** (this should have been asked to the developers of Java): because *often* in programming languages all indices are zero-based (e.g. arrays), so it is in Java - doing so allow doing `subMap(0, map.size())` using `size()` directly without having to subtract `1` – user16320675 Jul 23 '22 at 08:51
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    I like this author's way of thinking about it: https://blog.nelhage.com/2015/08/indices-point-between-elements/ – slothrop Jul 23 '22 at 08:53
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    You might also want to check this answer https://stackoverflow.com/questions/26631078/why-is-substring-method-substringstart-indexinclusive-end-index-exclusive#26631968 – criztovyl Jul 23 '22 at 09:03

5 Answers5

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This has been "answered" for String#substring here:

https://stackoverflow.com/a/26631968/1571268

The question about the "Why" may be considered as philosophical or academic, and provoke answers along the line of "That's just the way it is".

[...]

However, regardless of which choice is made, and regardless how it is justified: It is important to be consistent throughout the whole API.

criztovyl
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this is general in almost all programming languages I guess! like I see in 'python' and 'C'.

This is kind of a rule of sets(collections) in mathematics. you can imagine this as below:

fromKey <= x < toKey

eg:

0 <= x < 10

anyway, if you want to include toKey itself in any range, you should do:

toKey + 1

or

toKey ++
kamran mt21
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Indices in most programming languages start at 0. Thus, the last element of any linear collection is size - 1.

Now, any range is defined as start (inclusive) - end (exclusive), since it makes the code more readable.

Example

Let's say you have "Java" and you want to split it at the "v".

With this definition, it is very easy:

String java = "Java"; // length: 4
int index = java.indexOf("v"); // 2

String start = java.substring(0, index); // from 0 (inclusive) to 2 (exclusive)
String end = java.substring(index); // from 2 (inclusive) to length (exclusive, implicit)

System.out.println(start + ", " + end); // Ja, va
Mark Rotteveel
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Cactusroot
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    I have changed "all programming languages" to "most programming languages", because there are languages where indices start at 1, and even languages where you can configure the start index. – Mark Rotteveel Jul 23 '22 at 09:23
  • Ah yes, like the dialects of BASIC with `OPTION BASE 0` and `OPTION BASE 1`! A good example of a contemporary 1-based language is R. – slothrop Jul 23 '22 at 12:37
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If you are going to specify ranges by specifying end-points it's generally best to specify the range as B<=x<E. At the very least it means you can specify the empty range by B=E.

But when it comes to 'infinite' value spaces it becomes essential.

Suppose I want to specify every string beginning with C? In the 'end exclusive' model that's B="C" and E="D". In the 'end inclusive' model that's B="C" and E="CZZZZZZZZZZZZZZ..." where E is some string we've decided is longer than any string we're going to practically encounter.

It also makes it practical to define non-overlapping coverages as [B,E),[E,F) and so on.

NB: Mathematical convention is that [ indicates >= and ) indicates <

Some people argue it's a matter of taste. But in practice you can create a partition from ordered values A,B,C,D,E... as [A,B),[B,C),[C,D) without any fiddling around trying to identify (the potentially non-existent or unconstructable) value immediately before B.

It's somewhere between messy and impossible to construct the partition [A,B-e1],[B,C-e2]. Just duck the issue and use half-inclusive intervals! It normally makes sense to use inclusive-exclusive ranges but the approach works the other way.

Persixty
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in java almost every method to get a sub part of a data structure it goes form the fromkey (included) to the tokey (not included) so what you have to do is do tokey +1 instead of just tokey. As for example, if we do map.subMap(a,b); we will get the values from indexes a to b-1. Hope this helps.

Natanel
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