Why the Last Index of a Seven-Slot Array Is Six

Seven slots, and the highest number you may put in the brackets is six. It reads like a mistake until you see what the number means.

java

String[] days = {"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"};

System.out.println(days.length);
System.out.println(days[0]);
System.out.println(days[6]);

text

7
Mon
Sun

The index is not a count of items. It is an offset — how far from the start this slot sits. The first slot is zero away from the start, which is what makes it days[0], and the seventh is six away.

Position, index and offset in a seven-element array
Which one	Index	Distance from the start
First	0	0 slots
Second	1	1 slot
Seventh	6	6 slots

length counts and the index measures, so they are never the same number for the last element. The pair you will write for the rest of your life is 0 to length - 1.

Java
Arrays