Java quiz 03: Static and instance variables

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It is important for every Java programmers to understand the difference between class(static) variables and instance variables. Therefore I wrote this quiz, which clarifies those differences.

Java quiz 3: The difference between static and instance variables

Level: Normal

What is written to the standard output as the result of executing the following code?

public class MyClass
{
  int x;
  static int y;

  MyClass(int i)
  {
    x += i;
    y += i;
  }
  public static void main(String[] args)
  {
    new MyClass(2);
    MyClass mc = new MyClass(3);
    System.out.print(mc.x + "," + mc.y);
  }
}

Author: Sar Maroof

Answer explanation

By creating the first object, we pass the parameter value 2 to the constructor.
The statement x += i; increments the value of x by 2. the variable x is not initialized.
Therefore, x = 0 + 2 = 2.
The statement y += i; increments the value of y by 2. The initial value of y is 0, therefore y = 0 + 2 = 2.
By creating the second object mc, we pass the variable 3 as parameter to the constructor.
The statement x += i; increments the value of x by 3. The variable x is an instance variable, therefore
its value for the object mc is equal to 3.
The statement y += i; increments the value of y by 3. The variable y is a static (class) variable and its last
value was 2. Therefore the value of y becomes 2 + 3 = 5 and that value is the same for all the objects of the class.The correct answer is c. 3, 5.

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