Analysis of the definition and usage examples of Java arrays

One-dimensional array

The array in Java language is a reference data type; does not belong to Basic data type; The parent class of array is Object. An array is actually a container that can hold multiple elements at the same time. (An array is a collection of data)
Array: literally means "a set of data"

Arrays can store data of "basic data types" and data of "reference data types".
Because the array is a reference type, the array object is in the heap memory. (Arrays are stored in the heap) If the "java object" is stored in the array, what is actually stored is the "reference (memory address)
" of the object, which cannot be stored directly in the array. java object (store its address).

Once the array is created, it is stipulated in java that the length is immutable. (The length of the array is immutable) Classification of arrays: one-dimensional array, two-dimensional array, three-dimensional array, multi-dimensional array... (One-dimensional arrays are more common, two-dimensional arrays are occasionally used!)

All The array object has a length attribute (java comes with it), which is used to get the number of elements in the array.
Arrays in java require that the types of elements in the array are consistent. For example, an int type array can only store int type, and a Person type array can only store Person type. For example: when shopping in a supermarket, you can only put apples in your shopping bag, not apples and oranges at the same time. (The types of elements stored in the array are uniform) When the array is stored in memory, the memory addresses of the elements in the array (each element stored is arranged next to each other regularly) are continuous. Memory addresses are consecutive. This is the characteristic (feature) of arrays storing elements. Array is actually a simple data structure.
All arrays use the "memory address of the first small box" as the memory address of the entire array object. (The memory address of the first element in the array is used as the memory address of the entire array object.)
Each element in the array has a subscript, and the subscript starts from 0 and increases by 1. The subscript of the last element is: length - 1; the subscript is very important, because when we "access" the elements in the array, we need to use the subscript.

ArrayWhat are the advantages and disadvantages of this data structure?
Advantages: It is extremely efficient when querying/finding/retrieving elements on a certain subscript. It can be said to be the data structure with the highest query efficiency. Why is the retrieval efficiency so high? First:
The memory address of each element is continuous in space storage. Second:
Each element is of the same type, so it takes up the same space. Third:
Know the memory address of the first element, the size of the space occupied by each element, and the subscript, so you can calculate the element above a certain subscript through a mathematical expression memory address. Locate elements directly through memory addresses, so the array retrieval efficiency is the highest. When 100 elements are stored in an array, or 1 million elements are stored, the efficiency is the same in terms of element query/retrieval, because the elements in the array are not searched one by one, but are calculated through mathematical expressions. (Calculate a memory address and locate it directly.)
Disadvantages: First: In order to ensure that the memory address of each element in the array is continuous, the elements are randomly deleted or added to the array. Sometimes, the efficiency is low, because randomly adding and deleting elements will involve the operation of uniform forward or backward displacement of subsequent elements.                   Second: Arrays cannot store large amounts of data, why? Because it is difficult to find a particularly large continuous memory space in the memory space. There is no efficiency impact on the addition or deletion of the last element in the array.

How to declare/define a one-dimensional array?

Syntax format:

int[] array1;
double[] array2;
boolean[] array3;
String[] array4;
Object[] array5;

How to initialize a one-dimensional array?

It includes two methods: static initialization of one-dimensional array and dynamic initialization of one-dimensional array.

        (1)
Static initialization syntax format:
        int[] array = {100, 2100, 300, 55};        (2)
Dynamic initialization syntax format:
int[] array = new int[5];

这里的5表示数组的元素个数。
            初始化一个5个长度的int类型数组，每个元素默认值0
            再例如：String[] names = new String[6];

初始化6个长度的String类型数组，每个元素默认值null。

什么时候使用静态数组初始化？什么时候使用动态数组初始化？

（1）创键数组的时候，确定数组中存储哪些具体的元素时，采用静态初始化方式

（2）创键数组的时候，不确定将来存储哪些数据，可以采用动态初始化的方式，预先分配内存空间

package com.bjpowernode.javase.array;

public class ArrayTest01 {
    public static void main(String[] args) {
    //1.静态初始化
       int[] a1 = {1,3,5,7,9};
        //所有的数组对象都有length属性，而不是方法！
        System.out.println("数组元素的个数是："+a1.length);
        //取第一个元素
        System.out.println(a1[0]);
        //取最后一个元素
        System.out.println(a1[a1.length-1]);
        //改数据
        a1[a1.length-1] = 0;
        //遍历数据
        for(int i=0;i< a1.length;i++){
            System.out.println(a1[i]);
        }
        //数据下标越界异常，例如：访问下面为6的数据元素
        //System.out.println(a1[6]);// ArrayIndexOutOfBoundsException


    //2.动态初始化
        int[] a2 = new int[5]; //默认值是0
        for(int i=0;i< a2.length;i++){
            System.out.println(a2[i]);
        }
        //初始化一个Object类型的数组，
          //1.采用静态初始化方式
        Object o1 = new Object();
        Object o2 = new Object();
        Object o3 = new Object();
        Object[] object = {o1,o2,o3};
        //上面就等价于：Object[] object = {new Object(),new Object(),new Object()};
        for(int i=0;i<object.length;i++){
            System.out.println(object[i]);// 默认调用toString方法
        }
          //2.采用动态初始化的方式
        Object[] obj = new Object[3];
        for(int i=0;i<obj.length;i++){
            System.out.println(obj[i]);// null null null
        }

        //初始化一个String类型的数组
          //1.静态初始化
        String[] str1 = {"abc","bcd","cde"};
        for (int i = 0; i < str1.length; i++) {
            System.out.println(str1[i]);
        }
        //2.动态初始化
        String[] str2 = new String[3];
        for (int i = 0; i < str2.length; i++) {
            System.out.println(str2[i]);
        }

    }

}

动态存储内存图

方法的参数是数组

当传递的是一个数组，方法也用数组的形式进行接收；这个数组可以是静态的，也可以是动态创建的；并且我们把方法写成写成静态的，这样不需要new对象就可以调用！

例1：

package com.bjpowernode.javase.array;

public class ArrayTest02 {
    //也可以采用C++的风格,写成String args[]
    public static void main(String args[]) {
        System.out.println("HelloWorld");
        // 1.方法的参数传数组---静态初始化方式
        int[] a = {1,2,3,4,5};
        printArray(a);
        // 2.方法的参数传数组---动态初始化方式
        int[] arr = new int[5];
        printArray(arr);
        //   直接一步完成
        printArray(new int[3]);

    }
    //静态方法进行打印
    public static void printArray(int[] arr){
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }
    }

}

例2：（掌握）

（1）一种特殊情况传递静态数组；如果直接传递一个静态数组的话，语法必须这样写！

（2）我们先看一个例子：int[] arr = {1,2,3};我们传递数组的参数时候，一般就是传递数组名arr，例如：printArray(arr)；但是另一种方法就是传过去，去掉数组名arr剩余的组成部分：

int[]{1,2,3}，但是要加上new关键字，例如： printArray(new int[]{1,2,3});

package com.bjpowernode.javase.array;

public class ArrayTest03 {
    public static void main(String[] args) {
     //----------1.动态初始化一位数组（两种传参方式）
        //第一种传参方式
        int[] a1 = new int[5];//默认是5个0
        printArray(a1);
        System.out.println("-------------");
        //第二种传参方式
        printArray(new int[3]);
        System.out.println("-------------");
     //----------2.静态初始化一位数组（两种传参方式）
        //第一种传参方式
        int[] a2 = {1,2,3};
        printArray(a2);
        System.out.println("-------------");
        //第二种传参方式----直接传递一个静态数组
        printArray(new int[]{4,5,6});

    }
    //调用的静态方法----静态方法比较方便，不需要new对象
    public static void printArray(int[] arr){
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }
    }
}

main方法中的String数组

（1）对于main(String[] args)；分析一下：谁负责调用main方法（JVM）
     JVM调用main方法的时候，会自动传一个String数组过来，长度为0。

例1：

package com.bjpowernode.javase.array;

public class ArrayTest04 {
    // 这个方法程序员负责写出来，JVM负责调用。JVM调用的时候一定会传一个String数组过来。
    public static void main(String[] args) {
        // JVM默认传递过来的这个数组对象的长度？默认是0
        // 通过测试得出：args不是null。
        System.out.println("JVM给传递过来的String数组参数，它这个数组的长度是？"
        + args.length); //0

        // 以下这一行代码表示的含义：数组对象创建了，但是数组中没有任何数据。就等价于：
        String[] strs = new String[0]; //动态的方式
        //String[] strs = {}; // 静态初始化数组，里面没东西。
        printLength(strs); //调用printLength静态方法

/*
     既然传过来的“String[] args”数组里什么都没有；那么这个数组什么时候里面会有值呢？
     其实这个数组是留给用户的，用户可以在控制台上输入参数，这个参数自动会被转换为“String[] args”
     例如这样运行程序：java ArrayTest04 abc def xyz；相当于在编译时进行传参
     那么这个时候JVM会自动将“abc def xyz”通过空格的方式进行分离，分离完成之后，自动放到“String[] args”数组当中。
     所以main方法上面的String[] args数组主要是用来接收用户输入参数的。
     把abc def xyz 转换成字符串数组：{"abc","def","xyz"}
*/
        // 遍历数组
        for (int i = 0; i < args.length; i++) {
            System.out.println(args[i]);
        }
        //既然是编译时进行传参，对于编译运行一体的IDEA怎么使用呢？
        //Run--->EditConfiguration--->Program Arguments里面进行传参，然后在从后重新运行

    }

    public static void printLength(String[] args){
        System.out.println(args.length); // 0
    }
}

例2：

（1）main方法上面的“String[] args”有什么用？

可以用来模拟一个登陆系统！请看下面这个有趣的例题：

package com.bjpowernode.javase.array;
/*
   模拟一个系统，假设这个系统要使用，必须输入用户名和密码。
*/
public class ArrayTest05 {
    public static void main(String[] args) {
        //先判断长度，是不是两个字符串长度，不是2直接终止程序
        if(args.length != 2){
            System.out.println("请输入用户名和密码");
            return;
        }

        //取出用户名和密码
        String username = args[0];
        String password = args[1];
        // 假设用户名是admin，密码是123的时候表示登录成功。其它一律失败。
        // 判断两个字符串是否相等，需要使用equals方法。
        // if(username.equals("admin") && password.equals("123")){ //这样有可能空指针异常
        // 下面这种编写方式，也可以避免空该指针异常!
        if("admin".equals(username) && "123".equals(password)){ 
            System.out.println("恭喜你，登录成功");
            System.out.println("您可以继续使用该系统");
        }else{
            System.out.println("账户或密码错误，请重新输入");
        }
    }
}

数组中存储引用数据类型（重点）

（1）一维数组的深入：数组中存储的类型为：引用数据类型；对于数组来说，实际上只能存储java对象的“内存地址”。数组中存储的每个元素是“引用”。下面这个例题重点理解！

（2）数组要求数组中元素的类型统一；但是也可以存储它的子类型！

package com.bjpowernode.javase.array;
public class ArrayTest06 {
    public static void main(String[] args) {
        //1.静态创建一个Animal类型的数组
        Animal a1 = new Animal();
        Animal a2 = new Animal();
        Animal[] animals = {a1,a2};

        //对Animal数组进行遍历
        for (int i = 0; i < animals.length; i++) {
            //方法1
            /*Animal a = animals[i];
            a.move();*/
            //方法2
            animals[i].move();
        }

        //2.动态初始化一个长度为2的animal类型的数组
        Animal[] ans = new Animal[2];
        ans[0] = new Animal();
        //ans[1] = new Product(); //err，Product和Animals没有任何关系
        //Animal数组中只能存放Animal类型，不能存放Product类型

        //3.Animal数组中可以存放Cat类型的数据，因为Cat是Animal一个子类
       ans[1] = new Cat();
        for (int j = 0; j < ans.length; j++) {
            ans[j].move();
        }

        //4.创建一个Animal类型的数据，数组当中存储Cat和Bird
          //4.1静态创建
        Cat cat = new Cat();
        Bird bird = new Bird();
        Animal[] anim = {cat,bird};
        for (int i = 0; i < anim.length; i++) {
            //直接调用子类和父类都有的move()方法
            //anim[i].move();
            
            //这里想要调用子类Bird里面特有的方法，需要向下转型
            if(anim[i] instanceof Bird){
                Bird b = (Bird)anim[i]; //向下转型
                b.move();
                b.sing(); //调用子类特有的方法
            }else{
                anim[i].move();
            }

        }


    }
}
//动物类
class Animal{
    public void move(){
        System.out.println("Animals move.....");
    }
}
//商品类
class Product{

}

//有一个猫类继承动物类
class Cat extends Animal{
    public void move(){
        System.out.println("Cat move.....");
    }
}

//有一个鸟类继承动物类
class Bird extends Animal{
    public void move(){
        System.out.println("Bird move.....");
    }

    //鸟特有的方法
    public void sing(){
        System.out.println("鸟儿在歌唱！");
    }
}

数组扩容和拷贝

在Java开发中，数组长度一旦确定不可变，那么数组满了，需要扩容怎么办？
（1）java中对数组的扩容是：先创建一个大容量的数组，然后将小容量数组中的元素一个个拷贝到大数组当中，小容量会被释放。
（2）结论：数组扩容效率较低。因为涉及到拷贝的问题。所以在以后的开发中请注意：尽可能少的进行数组的拷贝。可以在创建数组对象的时候预估计以下多长合适，最好预估准确，这样可以减少数组的扩容次数。提高效率。

（3）利用System.arraycopy进行拷贝，总共5个参数；System.arraycopy(源头数组，下标，目的地数组，下标，要拷贝的个数)

package com.bjpowernode.javase.array;
public class ArrayTest07 {
    public static void main(String[] args) {
        //java中的数组是怎样拷贝的呢？System.arraycopy(5个参数)
        //System.arraycopy(源,下标,目的地,下标,个数)

        //拷贝源---把3、5、7拷贝过去
        int[] src = {1,3,5,7,9};
        //拷贝目的地---拷贝到下标为5的地方
        int[] dest = new int[20];
        //调用拷贝函数
        System.arraycopy(src,1,dest,5,3);
        //打印验证
        for (int i = 0; i < dest.length; i++) {
            System.out.println(dest[i]+" ");
        }

        //拷贝引用数据类型
        String[] str = {"hello","world"};
        String[] strs = new String[10];
        System.arraycopy(str,0,strs,3,2);
        for (int i = 0; i < strs.length; i++) {
            System.out.println(strs[i]);
        }
        System.out.println("--------------");
        
        //采用动态开辟的时候拷贝的是地址
        Object[] objs = {new Object(),new Object(),new Object()};
        Object[] objects = new Object[5];
        System.arraycopy(objs,0,objects,0,3);
        for (int i = 0; i < objects.length; i++) {
            System.out.println(objects[i]);

        }

    }
}

内存图

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