Java Improvement Chapter (18)-----Array One: Understanding JAVA Arrays

Oh, it understood. The river is neither as shallow as Uncle Cow said, nor as deep as the little squirrel said. You can only know if you try it yourself! Hearsay can always only show the phenomenon. Only by trying it yourself can you know the depth of it! ! ! ! !

1. What is an array

Array? What is an array? In my mind, arrays should be like this: create and assemble them through the new keyword, access its elements by using integer index values, and its size is immutable!

           But this is only the most superficial thing of the array! Deeper? That's it: an array is a simple composite data type. It is a collection of ordered data. Each data in it has the same data type. We uniquely use the array name plus a subscript value that will not go out of bounds. Determine the elements in the array.

          There is something deeper, that is, the array is a special object! ! (I don’t understand this LZ very well, and I haven’t read JVM, so my insights are limited). The following references: http://www.php.cn/, http://www.php.cn/

##           Regardless of whether arrays are used in other languages What, in java it is an object. A rather special object.

public class Test {
    public static void main(String[] args) {
        int[] array = new int[10];
        System.out.println("array的父类是：" + array.getClass().getSuperclass());
        System.out.println("array的类名是：" + array.getClass().getName());
    }
}
-------Output:
array的父类是：class java.lang.Object
array的类名是：[I

As can be seen from the above example, the array is a direct subclass of Object, which belongs to " "First Class Object", but it is very different from ordinary Java objects. It can be seen from its class name: [I, what is this? ? I didn't find this class in the JDK. It is said that this "[I" is not a legal identifier. How to define it as a category? So I think the geniuses at SUM must have done special processing on the bottom layer of the array.

          Let’s look at the following example:

public class Test {
    public static void main(String[] args) {
        int[] array_00 = new int[10];
        System.out.println("一维数组：" + array_00.getClass().getName());
        int[][] array_01 = new int[10][10];
        System.out.println("二维数组：" + array_01.getClass().getName());
        
        int[][][] array_02 = new int[10][10][10];
        System.out.println("三维数组：" + array_02.getClass().getName());
    }
}
-----------------Output:
一维数组：[I
二维数组：[[I
三维数组：[[[I

       Through this example we know: [ represents the dimension of the array, one [ represents one dimension, and two [ represents two dimensions. It can simply be said that the class name of an array consists of several '['s and the internal name of the array element type. If it’s not clear, let’s look at it again:

public class Test {
    public static void main(String[] args) {
        System.out.println("Object[]:" + Object[].class);
        System.out.println("Object[][]:" + Object[][].class);
        System.err.println("Object[][][]:" + Object[][][].class);
        System.out.println("Object:" + Object.class);
    }
}
---------Output:
Object[]:class [Ljava.lang.Object;
Object[][]:class [[Ljava.lang.Object;
Object[][][]:class [[[Ljava.lang.Object;
Object:class java.lang.Object

              From this example we can see the "true face of Mount Lu" of the array . At the same time, it can also be seen that arrays are different from ordinary Java classes. Ordinary Java classes use fully qualified path names + class names as their only identifiers, while arrays use several [+L+array element class full names. Define the path + class to uniquely identify it. This difference may explain to some extent that there is a big difference in the implementation of arrays and ordinary Java classes. Perhaps this difference can be used to make the JVM distinguish between arrays and ordinary Java classes.

## No matter what this [i Dongdong, who is declared from what is the stuff? Can confirm: this is determined at runtime). Take a look at the following first:

##

public class Test {
    public static void main(String[] args) {
        int[] array = new int[10];
        Class clazz = array.getClass();   
        System.out.println(clazz.getDeclaredFields().length);   
        System.out.println(clazz.getDeclaredMethods().length);   
        System.out.println(clazz.getDeclaredConstructors().length);   
        System.out.println(clazz.getDeclaredAnnotations().length);   
        System.out.println(clazz.getDeclaredClasses().length);   
    }
}
----------------Output：
0
0
0
0
0

                                                                                                                                              There are no member variables, member methods, constructors, Annotations or even the length member variable. It is a completely empty class. Length is not declared, so why doesn't the compiler report an error when we array.length? Indeed, the length of an array is a very special member variable. We know that the array is the direct class of Object, but Object does not have the member variable length, so length should be the member variable of the array, but from the above example, we find that the array does not have any member variables at all. Isn't it contradictory?

public class Main {
    public static void main(String[] args) {
        int a[] = new int[2];
        int i = a.length;
    }
}

Open the class file and get the bytecode of the main method:

0 iconst_2                   //将int型常量2压入操作数栈  
    1 newarray 10 (int)          //将2弹出操作数栈，作为长度，创建一个元素类型为int, 维度为1的数组，并将数组的引用压入操作数栈  
    3 astore_1                   //将数组的引用从操作数栈中弹出，保存在索引为1的局部变量(即a)中  
    4 aload_1                    //将索引为1的局部变量(即a)压入操作数栈  
    5 arraylength                //从操作数栈弹出数组引用(即a)，并获取其长度(JVM负责实现如何获取)，并将长度压入操作数栈  
    6 istore_2                   //将数组长度从操作数栈弹出，保存在索引为2的局部变量(即i)中  
    7 return                     //main方法返回

       在这个字节码中我们还是没有看到length这个成员变量，但是看到了这个:arraylength ,这条指令是用来获取数组的长度的，所以说JVM对数组的长度做了特殊的处理，它是通过arraylength这条指令来实现的。

       二、数组的使用方法

       通过上面算是对数组是什么有了一个初步的认识，下面将简单介绍数组的使用方法。

       数组的使用方法无非就是四个步骤：声明数组、分配空间、赋值、处理。

       声明数组：就是告诉计算机数组的类型是什么。有两种形式：int[] array、int array[]。

       分配空间：告诉计算机需要给该数组分配多少连续的空间，记住是连续的。array = new int[10];

       赋值：赋值就是在已经分配的空间里面放入数据。array[0] = 1 、array[1] = 2……其实分配空间和赋值是一起进行的，也就是完成数组的初始化。有如下三种形式：

int a[] = new int[2];    //默认为0,如果是引用数据类型就为null
        int b[] = new int[] {1,2,3,4,5};    
        int c[] = {1,2,3,4,5};

       处理：就是对数组元素进行操作。通过数组名+有效的下标来确认数据。

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