Organized memo in the head (Java --Array)

My Java environment


Array declaration

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];
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.

testArr[0] = new TestBean();

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