配列の作成とアクセス

Making an array

• Declare an array with a type and []: int[] scores;
• Create it with new and a size: int[] scores = new int[5]; (five slots).
• Or list the values directly: int[] scores = {85, 90, 78}; (an initializer list 初始化列表).
• Every slot starts at the type's default (0 for int, null for objects).

Indexing from zero

• Array elements are numbered from 0; the last index is length - 1.
• Access a slot with scores[i]: scores[0] is the first, scores[2] the third.
• Assign with scores[0] = 100; and read with int x = scores[0];.
• Off-by-one on the last index is the most common array bug.

A one-dimensional array (a list) with its indices and bounds

The length field

• An array's size is its length field — no parentheses: scores.length.
• (Contrast String's length() method, which has parentheses.)
• length is fixed once the array is created — arrays don't resize.
• Use scores.length in loop conditions to stay in bounds.

Out of bounds

• A valid index is 0 to length - 1; anything else throws an ArrayIndexOutOfBoundsException 数组下标越界异常.
• scores[5] on a length-5 array is out of bounds (indices are 0–4).
• The loop condition i < scores.length keeps i in range.
• Never use <= with length — it steps one past the end.