Definition of a one-dimensional array and a two-dimensional array in Java

In Java, arrays are regarded as an object.

When defining an array, there are two defining methods: int [] A and int A []; the second is the way C/C ++ defines the array, we recommend that you use the first definition method for Java.

General principle: Any object must be initialized before being called!

1. Definition of one-dimensional array

// Define a one-dimensional array containing three elements

// Method 1, first new object, and then initialize each element

Int [] A = new int [3];

A [0] = 1;

A [1] = 2;

A [2] = 3;

Method 1: Do not write as follows:

Int [] A = new int [3];

A = {1, 2, 3 };

The reason is that new int [3] is used to create an object. The object has been initialized and assigned an initial value of 0. You can use the following code to verify the object:

Int [] d = new int [3];

For (INT I = 0; I <D. length; I ++ ){

System. Out. println (d [I]);

}

Input result: 0 0 0

If a = {1, 2, 3}; is used to assign values, we know that constants cannot be assigned and cannot be modified.

// Method 2: directly assign the initial value to create the object

Int [] B = {1, 2, 3 };

// Method 3. initialize the new object directly.

Int [] C = new int [] {1, 2, 3 };

Method 3: Do not write in this way: int [] C = new int [3] {1, 2, 3 };

That is, This method cannot specify dimensions. Cause: If array Initialization is specified, dimension expressions cannot be defined.

Note: If you use new to define an array, you must specify its dimension. In this case, the definition is incorrect: int [] d = new int [];

If you cannot determine the number of elements, you can define it as: int [] e = {};

2. Two-dimensional array Definition

Similar to a one-dimensional array

// Define a two-dimensional array with three rows and five columns

// Method 1, first new object, and then initialize each element

Int [] [] A = new int [3] [5];

A [0] [0] = 1;

A [0] [1] = 2;

A [0] [2] = 3;

 

// Method 2: directly assign the initial value to create the object

Int [] [] B = {, 1}, {, 2}, {, 3 }};

 

// Method 3. initialize the new object directly.

Int [] [] A = new int [] []

{, 1}, {, 2}, {, 3, 3 }};

To define a two-dimensional array, you must specify the number of rows, and the number of columns can be specified.

The definition is correct: int [] [] d = new int [3] [];

This definition is incorrect: int [] [] d = new int [] [4]; int [] [] d = new int [] [];

You can also define irregular Arrays:

Arr = new int [2] [];

Arr [0] = new int [3];

Arr [1] = new int [5];

3. Length of the array

Length is an attribute of an array (not a method !), For a one-dimensional array int [] B = {, 3}; B. length is 3,

For a two-dimensional array, first study its configuration details: int [] [] arr = new int [2] [3];

In the above program snippet
2
Line
3
A two-dimensional array object of a column, because the data type is
Int
So the default element of the array element is
0
.

Actually
Arr [0]
,
Arr [1]
Is two one-dimensional array objects, each of which has a length
3
, And
Arr
Type is
Int [] []
, The content value is
Arr [0]
And
Arr [1]
. Its Relationship 1
.

Figure
1
Configuration relationship of two-dimensional arrays

Slave chart
1
,
Arr
Reference
Int [] []
Type object, including
Arr [0]
And
Arr [1]
, And
Arr [0]
And
Arr [1]
Reference a one-dimensional array object.

Therefore
Arr. Length
And
Arr [0]. Length
The length is different. Arr. Length indicates the number of rows in the array, and ARR [I]. Length indicates the number of elements contained in the specified row. In this example, arr. Length = 2, arr [0]. Length = 3.

4. Initial Value of the array

Quantity Data Class Type	Chu   Start   Value	Quantity Data Class Type	Chu   Start   Value
Byte	0	Float	0.0f
Short	0	Double	0.0d
Int	0	Char	/U0000
Long	0l	Boolean	False