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So, what is the twist?
Unlike final member variables of a class, the Java compiler and runtime system do not require you to initialize a final local variable when you declare it. Rather, you can wait and assign a value to it later. (Some authors refer to this as a blank final.) However, once you have assigned a value to a final local variable, you cannot change that value later.
The bottom line
Whether you initialize the final local variable when you declare it, or assign a value to it later, the result is the same. It behaves as a constant.The difference is that if you don't initialize it when you declare it, you cannot access it until after you assign a value to it.
F. 20, 10
Calling a parameterized constructor
This is a relatively straightforward implementation of the use of the super keyword in a subclass constructor to call a parameterized constructor in the superclass.
The interesting code in the program is highlighted in the following fragment. Note that quite a lot of code was deleted from the fragment for brevity.
class Superclass{
//...public Superclass(int x){//...
}//end constructor//...}//end Superclass
class Subclass extends Superclass{//...public Subclass(){
super(20);//...
}//end constructor//...}//end Subclass Using the super keyword
The code that is of interest is the use of super(20) as the first executable statement in the Subclass constructor to call the parameterized constructor in the superclass, passing a value of 20 as aparameter to the parameterized constructor.
Note that when the super keyword is used in this fashion in a constructor, it must be the first executable statement in the constructor.
As before, the program plays around a little with initial values for instance variables to see if you are alert, but the code that is really of interest ishighlighted in the above fragment.
D. 5, 10
The execution of constructors
The purpose of this question and the associated answer is to illustrate explicitly what happens automatically by default regarding the execution ofconstructors.
The Subclass constructor
This program defines a class named Subclass , which extends a class named Superclass . A portion of the Subclass definition, including its noarg constructor is shown in the following code fragment. (The class also defines a getter method, which was omitted here for brevity.)
class Subclass extends Superclass{
private int y = 2;public Subclass(){super();
y = 10;}//end constructor//...
}//end Subclass The super keyword
The important thing to note in the above fragment is the statement containing the keyword super .
The super keyword has several uses in Java. As you might guess from the word, all of those uses have something to do with the superclassof the class in which the keyword is used.
Invoke the superclass constructor
When the super keyword (followed by a pair of matching parentheses) appears as the first executable statement in a constructor, this is an instruction tothe runtime system to first call the constructor for the superclass, and then come back and finish executing the code in the constructor for the class towhich the constructor belongs.
Call the noarg superclass constructor
If the parentheses following the super keyword are empty, this is an instruction to call the noarg constructor for the superclass.
Invoke a parameterized superclass constructor
If the parentheses are not empty, this is an instruction to find and call a parameterized constructor in the superclass whose formal arguments match the parameters in the parentheses.
Invoke the noarg superclass constructor by default
Here is an important point that is not illustrated above. If the first executable statement in your constructor is not an instruction to call the constructor for the superclass, an instruction to call the noarg constructor for the superclass will effectively be inserted into yourconstructor code before it is compiled.
Therefore, a constructor for the superclass is always called before the code in the constructor for your new class is executed.
You can choose the superclass constructor
The superclass constructor that is called may be the noarg constructor for the superclass, or you can force it to be a parameterizedconstructor by inserting something like
super(3,x,4.5);
as the first instruction in your constructor definition.
Always have a noarg constructor ...
Now you should understand why I told you in an earlier module that the classes you define should almost always have a noarg constructor, either the default noarg version, or a noarg version of your own design.
If your classes don't have a noarg constructor, then anyone who extends your classes will be required to put code in the constructor for theirnew class to call a parameterized constructor in your class.
In this program, the super(); statement in the Subclass constructor causes the noarg constructor for the Superclass to be called. That noarg constructor is shown in the following code fragment.
class Superclass{
private int x = 1;public Superclass(){x = 5;
}//end constructor//...}//end Superclass Additional code
Beyond an exposure and explanation of the use of the super keyword to call the superclass constructor, this program plays a few games with initial valuesof instance variables just to see if you are alert to that sort of thing. However, none of that should be new to you, so I won't discuss it further here.
-end-
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