为什么输出会是“100”和“500”？

Question

我很困惑为什么下面程序的输出会打印 100 和 500？任何人都可以解释这是如何发生的吗？

public class ArraysInJava

{

    public static void main(String[] args)

    {

        int[] a = new int[3]; 

        a[1] = 50; 

        Object o = a; 

        int[] b = (int[])o; 

        b[1] = 100; 

        System.out.println(a[1]); 

        ((int[])o)[1] = 500; 

        System.out.println(a[1]);

    }

}

Answer 1

让我们一行一行地做...public class ArraysInJava{    public static void main(String[] args)    {        int[] a = new int[3]; // a = {0, 0, 0} as default value for int elements is 0        a[1] = 50; // a = {0, 50, 0}        Object o = a; // o = a = {0, 50, 0} - casting affects the variable type, the referenced object remains the same (recall that objects are saved by reference)        int[] b = (int[])o; // b = o = a = {0, 50, 0}        b[1] = 100; // b = o = a = {0, 100, 0}        System.out.println(a[1]); // Prints 100        ((int[])o)[1] = 500; // b = o = a = {0, 500, 0}        System.out.println(a[1]); // Prints 500    }}所有的转换都不做任何事情，因为它只影响你在编译时可以做的事情。例如，你不能写，o[1]因为它在编译时不是数组。编辑Carlos Heuberger确实在评论中提出了一个重要观点。我认为缺少的部分是关于数据类型的。有两种主要的数据类型：原始数据类型和非原始数据类型。原始数据类型包括byte、short、long、float、double、char、boolean。当您将这些数据类型的变量传递给方法或分配给另一个变量时，您正在传递值。int a = 15;int b = a; // b now holds 15非原始数据类型（您可以称它们为对象类型）是除上述所有类型之外的所有其他类型。这包括数组（也是基本类型的数组）、枚举、类、接口和String.当您将这些数据类型的变量传递给方法，或将它们分配给另一个变量时，您传递的是对象的引用。int[] a = {1, 2};Object b = a; // "b" now holds the reference (memory address) of the "a"int[] c = b; // "c" now also points to the exact same arrayb = null; // "b" holds no memory reference now, "a" and "c" continues to hold reference of the same array

Answer 2

绘制内存模型并跟踪每个引用，如果您理解这一点，请在代码中的注释中查看我的内容，这将消除您的困惑。 a, b, and o是参考a[1]------> 50a[1]------> 50 <------------o[1]a[1], b[1]------->50 <---------------o[1]a[1], b[1] -------->100 <-----------o[1]&nbsp; (but the value 50 is overwritten with 100)a[1], b[1] -------->500<-----------o[1]&nbsp; (but the value 100 is overwritten with 500)int[] a = new int[3];&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; a[1] = 50;&nbsp; // a------> 50,&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Object o = a;&nbsp; //&nbsp; a------> 50 <------------o&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int[] b = (int[])o; // a, b------->50 <---------------o&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; b[1] = 100;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //a, b -------->100 <-----------o(but the value 50 is overwritten with 100)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; System.out.println(a[1]);&nbsp; // so prints 100&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ((int[])o)[1] = 500; //a, b -------->500<-----------o(but the value 100 is overwritten with 500)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; System.out.println(a[1]); // hence prints 500

Answer 3

如果您调试代码，您可以找到以下操作：首先 a = [0,0,0]a = [0,50,0]o = [0,50,0]b = [0,50,0]b = [0,100,0] -> o = [0,100,0] -> a = [0,100,0]打印 a[1] = 100o[1] = 500 -> o = [0,500,0] -> b = [0,500,0] -> a = [0,500,0]打印 a[1] = 500这就是 a[1] 的值是如何改变的。基本上有相同的数组 a 一直在改变。