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Java Array length

Theory

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Java Array length

Array length is about Java arrays. An array is a fixed-size row of the same type. Index from 0. Length is a.length (a field, not a method). Last valid index is length - 1.

Create Array length: int[] a = new int[5]; or int[] a = {10, 20, 30};. A 2D array is an array of arrays: int[][] m = {{1,2},{3,4}}; m[1][0] is 3. Jagged means rows can have different lengths.

Array length size cannot grow. If you need grow/shrink, use ArrayList. Out of range → ArrayIndexOutOfBoundsException.

Let's take this on the board with one tiny Main class — no extra files. Java Array length — prints length 3, middle value 20, then all values. Start from main, go line by line, and stop at each print. That output is the proof for Array length.

Diagram

index →  0    1    2
  array → [10,  20,  30]
           ↑         ↑
         first   length-1

Exam tip

Contrast array vs ArrayList in one line.

Example

// Array demo
public class Main {
  public static void main(String[] args) {
    int[] nums = {10, 20, 30};
    System.out.println(nums.length);
    System.out.println(nums[1]);
    for (int n : nums) System.out.print(n + " ");
  }
}

Java Array length — prints length 3, middle value 20, then all values.

Short notes

DefArray length — array = fixed-size, same type, index from 0.
RuleArray length — a.length is a field. Last index = length - 1.
RememberArray length — 2D = array of arrays. Jagged = rows of different length.
TrapArray length — a[a.length] crashes. Size cannot grow — use ArrayList.

// Array demo public class Main { public static void main(String[] args) { int[] nums = {10, 20, 30}; System.out.println(nums.length); System.out.println(nums[1]); for (int n : nums) System.out.print(n + " "); } }

Java Array length

Example

Short notes

Questions

Java Array length

Example

Short notes

Questions