Interesting facts about Array assignment in Java

Prerequisite : Arrays in Java

While working with arrays we have to do 3 tasks namely declaration, creation, initialization or Assignment.
Declaration of array :

int[] arr;

Creation of array :

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

Initialization of array :

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

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

Some important facts while assigning elements to the array:

1. For primitive data types : In case of primitive type arrays, as array elements we can provide any type which can be implicitly promoted to the declared type array. Apart from that, if we are trying to use any other data-types then we will get compile-time error saying possible loss of precision.

   // 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]);
       }
   }

   Output:

   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]);
       }
   }

   Output:

   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;
       }
   }

   Output:

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

2. Object type Arrays : If we are creating object type arrays then the elements of that arrays can be either declared type objects or it can be child class object.

   // 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]);
       }
   }

   Output:

   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”);
       }
   }

   Output:

   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";
       }
   }

   Output:

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

3. Interface type array : For interface type array, we can assign elements as its implementation class objects.

   // 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”);
       }
   }

   Output:

   Compile-time error(Incompatible types)
   

   Explanation: In the above program, we are giving elements of String class that’s cause compile time error. Because we know that String does not implements Runnable interface.

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