How does java autoboxing work Given that Yasser Ibrahim @Jobilize

Author: Yasser Ibrahim

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Question 126 / 297: Given that Long and Integer extend Number, what is the result of compiling and running the following code?

public class Tester {

	public static void main(String[] args) {
		
		 Number x = 12; // Line 5
		 Number y = (Long) x; // Line 6
		 System.out.print(x+""+y); // Line 7

	}

}

A 1212

B Compilation error at line 5

C Compilation error at line 6

D Compilation error at line 7

E An exception is thrown at run time

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Explanation:

x will refer to an Integer object because 12 is by default of type int and will be wrapped to an Integer object. At compile time the casting works fine since Long and Number in the same type tree but at run time it will fail and a classCastException will be thrown because java.lang.Integer cannot be cast to java.lang.Long.

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