OCP Practice Exams Sef-Assessment Test 2 Q7

[henry joe]

In the code fragment below :

public class Game {
static String s = "-";
String s2 = "s2";
Game(String args){s+=args;}
}


public class Go extends Game{
	
	static String s2 = "s4 ";
	Go(){
		
		super(s2); //calls the games constructor
	}
	{s+= "i ";} 
	
	
	
	public static void main(String[] args){
		new Go();
		System.out.println(s);	
	}
	static{
		s+="sb ";
	}
}

I have the following questions:

1. Is this line {s+= "i ";} a static block in the code?

2. Why must the s2 String variable in the Go class be static ? I noticed when I remove the static keyword, it complains in the super() method.

3.I was expecting it to print out "-s4 sb i" but instead it printed out "-sb s4 i" why is this so? Could anyone explain to me?

Thanks

[Jeanne Boyarsky]

To be more explicit about the source: K&B, practice exams, self assessment test #2.

1) No, it is not a static block. You can tell because the keyword static does not appear immediately before it.

2) This code snippet shows the flow of initializers (which run near when the instance variables are set up) and constructors:

class Sub extends Super {
  { System.out.println("sub instance"); }
  public Sub() {
    System.out.println("sub constructor");
  }
}
	
class Super  {
  { System.out.println("super instance"); }
  public Super() {
    System.out.println("super constructor");
  }
}

The output of a main method that says new Sub() is:
super instance
super constructor
sub instance
sub constructor

This shows why removing static is a problem. The subclass' instance variable does not exist when the superclass is run. That happens later after the superclass constructor completes

3) static initializers don't run in the same order as instance initializers do because they do not depend on an instance of the class existing.

[henry joe]

Jeanne Boyarsky wrote:

1) No, it is not a static block. You can tell because the keyword static does not appear immediately before it.
.

So, what does the block mean in this code, why is the syntax written this way

 {s+= "i ";} 

instead of this

s+= "i ";

?

[Henry Wong]

henry joe wrote:
So, what does the block mean in this code, why is the syntax written this way

 {s+= "i ";} 

Well, it's an instance initializer -- which is executed (in textual order) along with initializations of instance variables.

henry joe wrote:
instead of this

s+= "i ";

?

Try it. Does it compile? Can you figure out what it doesn't?

Henry

[henry joe]

Henry Wong wrote:

henry joe wrote:
So, what does the block mean in this code, why is the syntax written this way

 {s+= "i ";} 

Well, it's an instance initializer -- which is executed (in textual order) along with initializations of instance variables.


Please, I am just so curious about this instance initializer. I would, finally, like to know why it was used in the code fragment? I never come across such syntax until now. So, what is the use?

Thank

[Henry Wong]

henry joe wrote:
Please, I am just so curious about this instance initializer. I would, finally, like to know why it was used in the code fragment? I never come across such syntax until now. So, what is the use?

Thank

The purpose of an instance initializer, is to initialize instance members.

http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html

Henry