关于Java中数组赋值的有趣事实

先决条件： Java中的数组

在处理数组时，我们必须完成 3 个任务，即声明、创建、初始化或赋值。
数组声明：

int[] arr;

创建数组：

// Here we create an array of size 3
int[] arr = new int[3];

数组的初始化：

arr[0] = 1;
arr[1] = 2;
arr[3] = 3;

int intArray[];    // declaring array
intArray = new int[20];  // allocating memory to array

将元素分配给数组时的一些重要事实：

1. 对于原始数据类型：对于原始类型数组，作为数组元素，我们可以提供可以隐式提升为声明类型数组的任何类型。除此之外，如果我们尝试使用任何其他数据类型，那么我们将收到编译时错误，说明可能会丢失精度。

   // Java program to illustrate the concept of array
   // element assignments on int type array
   public class Test {
   public static void main(String[] args)
       {
           int[] arr = new int[3];
           arr[0] = 1;
           arr[1] = 'a';
           byte b = 10;
           arr[2] = b;
           System.out.println(arr[0] + arr[1] + arr[2]);
       }
   }
   

   输出：

   108
   

   // Java program to illustrate the concept of array
   // element assignments on int type array
   public class Test {
   public static void main(String[] args)
       {
           int[] arr = new int[3];
     
           // Assigning long value to int type.
           arr[0] = 10l;
           arr[1] = 'a';
           byte b = 10;
           arr[2] = b;
     
           System.out.println(arr[0] + arr[1] + arr[2]);
       }
   }
   

   输出：

   possible loss of precision.
   

   // Java program to illustrate the concept of array
   // element assignments on char type array
   public class Test {
   public static void main(String[] args)
       {
           char[][] arr = new char[2][2];
     
           // Assigning long value to int type.
           arr[0][0] = 10l;
           arr[0][1] = 'a';
           char b = 10;
           arr[1][0] = b;
     
           // Assigning double value to char type
           arr[1][1] = 10.6;
       }
   }
   

   输出：

   error: incompatible types: possible lossy conversion from long to char
   error: incompatible types: possible lossy conversion from double to char
   

2. 对象类型数组：如果我们正在创建对象类型数组，那么该数组的元素可以是声明的类型对象，也可以是子类对象。

   // Java program to illustrate the concept of array
   // element assignments on Number type array
   public class Test {
   public static void main(String[] args)
       {
           Number[] num = new Number[2];
           num[0] = new Integer(10);
           num[1] = new Double(20.5);
           System.out.println(num[0]);
           System.out.println(num[1]);
       }
   }
   

   输出：

   10
   20.5
   

   // Java program to illustrate the concept of array
   // element assignments on Number type array
   public class Test {
   public static void main(String[] args)
       {
           Number[] num = new Number[3];
           num[0] = new Integer(10);
           num[1] = new Double(20.5);
     
           // Here String is not the child class of Number class.
           num[2] = new String(“GFG”);
       }
   }
   

   输出：

   Compile-time error(incompatible types)
   

   // Java program to illustrate the concept of array
   // element assignments on Number type array
   public class Test {
   public static void main(String[] args)
       {
           Number[][] arr = new Number[2][2];
           arr[0][0] = 10l;
     
           // Assigning char to Number type array
           arr[0][1] = 'a';
           byte b = 10;
           arr[1][0] = b;
     
           // Assigning String to Number type array
           arr[1][1] = "GEEKS";
       }
   }
   

   输出：

   error: incompatible types: char cannot be converted to Number
   error: incompatible types: String cannot be converted to Number
   

3. 接口类型数组：对于接口类型数组，我们可以指定元素作为它的实现类对象。

   // Java program to illustrate the concept of array
   // element assignments on Interface type array
   public class Test {
   public static void main(String[] args)
       {
           Runnable[] run = new Runnable[2];
     
           // Thread class implements Runnable interface.
           run[0] = new Thread();
           run[1] = new Thread();
       }
   }
   

   // Java program to illustrate the concept of array
   // element assignments on Interface type array
   public class Test {
   public static void main(String[] args)
       {
           Runnable[] run = new Runnable[2];
           run[0] = new Thread();
     
           // String class does not implements Runnable interface.
           run[1] = new String(“GFG”);
       }
   }
   

   输出：

   Compile-time error(Incompatible types)
   

   说明：在上面的程序中，我们给出了导致编译时错误的 String 类的元素。因为我们知道 String 没有实现 Runnable 接口。

参考： https://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html