Array Length

Array Length

1. What .length Is (beginner)

.length = box count (fixed at new). Field (no brackets!) on arrays. Last valid index is always length-1. Beginners: arrays use .length, Strings use .length(), ArrayList uses .size() — three different forms!

2. Code — Loop the Safe Way

int[] marks = {80, 90, 70};
System.out.println(marks.length); // 3 (count, not last index!)
for (int i = 0; i < marks.length; i++) { // < (not <=!) — safe pattern
    System.out.println(marks[i]);
}
// 2-D: rows then cols!
int[][] m = {{1,2,3},{4,5,6}};
System.out.println(m.length);    // 2 (rows)
System.out.println(m[0].length); // 3 (cols of row 0)

3. Trap Table (exam favourite!)

Code	Correct?	Why
i < arr.length	Yes	0 to length-1
i <= arr.length	NO → crash!	arr[length] out of bounds!
arr.length()	NO → error!	arrays: field, no ()
"hi".length	NO → error!	String: method with ()
list.length	NO → error!	ArrayList: .size()

4. length Can't Change

new int[3] stays 3 forever (final field). Need growable? ArrayList (next topics).

5. Exam Points

• Loop with < length + 2-D lengths — 5 marks, very frequent.
• .length vs .length() vs .size() — 3-5 marks, favourite trap.