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One 、 brief introduction

describe ：

• An array is The same type of data Of Orderly aggregate .
• among , Each piece of data is called an array Elements , Each array element can pass through a Subscript To visit them .

Basic characteristics ：

1. The length of the array is determined . Once the array is created , Its size can't be changed .
2. Array Elements must be of the same type , Mixing type... Is not allowed .
3. Array The element can be any data type , Including basic types and reference types .
4. Array variables are of reference type , Arrays can also be thought of as objects , Each element in the array is equivalent to the member variable of the object . An array itself is an object ,Java The object in the middle is in the heap , So arrays hold either basic data types or other object types , The array object itself is in the heap .

Two 、 Declaration array

grammar ：

/* The first one is , Bracket after data type */elementType[] arrayVarName/* The second kind , Bracket after variable name */elementType arrayVarName[]

describe ：

• First, you must declare variables of array type , In order to freely use array objects in the program .
• Java There are two ways to declare array variables in , A kind of The brackets are in ' data type ' after , Another kind of bracket is in ' Variable name ' after .

Be careful ：

• When declaring array variables , At the beginning ' data type '（ Without brackets ） Represents the array of Elements Data type of .
• Declare the second type of array variable The brackets are in ' Variable name ' After the way , come from C/C++ The style of language , It's for convenience C/C++ Programmers are learning Java When you can get started faster , Not recommended .
• When an array is declared, it is actually created , No objects are instantiated .
• The length of an array cannot be specified when it is declared （ The number of elements in the array ）, Only when you create an array object ,JVM To allocate space , Now it's about length .

Example ：

/* Recommended */int[] intArr1;/* Not recommended */int intArr2[];

3、 ... and 、 Create array

grammar ：

1. dynamic initialization ：

   new elementType[arrayLength]

2. initiate static ：

   {firstElementValue, secondElementValue, thirdElementValue...}

describe ：

• There are two ways to create an array ： dynamic initialization and initiate static .
• When you create an array , You must specify the length of the array , Dynamic initialization is specified in brackets , In static initialization, the array length is the number of elements in braces .
• Use dynamic initialization When creating an array object , The length of the array will be specified first , And initialize the element by default .
• Use initiate static When creating an array object , Not only will the array length be specified and the elements will be initialized by default , It also assigns values to all elements .

Be careful ：

• An array is a reference type , Its elements are equivalent to instance variables of a class , therefore Once an array is allocated space , Each of them Elements It's also true

  Example variables are the same way that Implicit initialization .

Example ：

/*  dynamic initialization  */new int[5];/*  initiate static  */{1, 2, 3, 4, 5};

Four 、 Using arrays

Ⅰ、 Declare and create

grammar ：

/* Declare array variables and dynamically initialize */elementType[] arrayVarName = new elementType[arrayLength];/* Declare array variables and statically initialize */elementType[] arrayVarName = {firstElementValue, secondElementValue, thirdElementValue...};

describe ：

• The declaration and creation of array variables can be done in one statement , We usually operate on an array object many times in this way .

Example ：

/* Declare array variables and dynamically initialize */int[] intArr1 = new int[5];/* Declare array variables and statically initialize */int[] intArr2 = {1, 2, 3, 4, 5};

Ⅱ、 The length of the array

grammar ：

arrayObject.length

describe ：

• The length of the array Sure