java - why from index is inclusive but end index is exclusive?

Question

In Java API methods like:

• String.substring(int beginIndex, int endIndex)
• String.subSequence(int beginIndex, int endIndex)
• List.subList(int fromIndex, int toIndex)

Why is the beginning index inclusive but the end index exclusive? Why shouldn't they have been designed both inclusive?

Answer 1

Because:

• Java is based on C, and C does it this way
• It makes the code cleaner: If you want to capture to end of object, pass object.length (however the object implements this, eg size() etc) into the toIndex parameter - no need to add/subtract 1

For example:

String lastThree = str.substring(str.length() - 3, str.length());

This way, it is very obvious what is happening in the code (a good thing).

EDIT An example of a C function that behaves like this is strncat from string.h:

char *strncat(char *dest, const char *src, size_t n);

The size_t parameter's value corresponds to the java endPosition parameter in that they are both the length of the object, but counting from 0 if they are the index, it would be one byte beyond the end of the object.