Organized memo in the head (Java --Array)

My Java environment

java11

Array

Can only handle the same type or compatible types
The number of elements cannot be changed later when the instance is created.

Array declaration

The number of elements in the array must be of type ʻint`.
The number of elements in the first dimension cannot be omitted.
In the case of a multidimensional array, it is not necessary to write the parentheses [] together.
In a multidimensional array, it is not necessary to match the number of elements in the second and subsequent arrays.

Declaration example


int arr[];
int[] arr1;
int[][] arr2;
int arr3[][];
int[] arr4[];
int[][] arr5[];

arr = new int[2];
arr1 = new int[2];
arr2 = new int[2][1];
arr3 = new int[1][2];
arr4 = new int[2][];
arr5 = new int[2][][];

Array instance generation

For primitive types


int arr[] = new int[2];

In the above, ʻarr [0]becomes0. Because the initial value of ʻint is 0.

Array with `0` elements

int array[] = new int[0];

Even if the number of elements is 0, the compilation will pass, but ʻarray [0]` does not exist. I get an exception when I try to access it.
When executed without arguments of the main method, the array with the number of elements 0 is set to ʻargs`.

Array initialization

Use {...} if you want to initialize at the same time as creating an array instance. {...} can only be used at the same time as the variable declaration.


int arr[] = {1, 2, 3};

If it is initialized, the number of elements cannot be specified.


int arr[] = new int[]{1, 2}; // OK
int arr[] = new int[2]{1, 2}; // NG

In the case of a multidimensional array, the number of dimensions of the variable and the number of dimensions of the reference destination must be the same.


int arr[][] = new int[][]{}; // OK
int arr[][] = new int[]{}; // NG

The following is also OK.


int arr1[][] = {};

For reference type


TestBean testArr[] = new TestBean[2];

In the above, testArr [0] becomes null. Declaring an array only creates an array instance, not each element of the array. It needs to be initialized as follows.