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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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Sar Maroof is a professional software development teacher, gives master classes and publishes technical articles. He is also an expert software developer and worked for several big as well as small companies and later as a freelancer. |
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