Editing 1188: Bonding

{{comic
| number    = 1188
| date      = March 20, 2013
| title     = Bonding
| image     = bonding.png
| titletext = I'm trying to build character, but Eclipse is really confusing.
}}

==Explanation==
This is {{w|source code}} written in the {{w|Java (programming language)|Java programming language}}, which models a parent and a child playing a {{w|Catch (game)|game of catch}}, in which the parent throws a ball to their child, who catches it and throws it back, which repeats. The comic title "Bonding" refers to the {{w|Paternal bond|building of relationship}} between the parent and the child. But the computer program representation of this game is funny, using a play on the keywords <code>throw</code> and <code>catch</code> (and <code>Throwable</code>), which, in Java, do not correspond to throwing and catching a ball, but to a so-called {{w|exception handling}}, a method of signaling error conditions and responding to them.

The program, as written, will {{w|Recursion (computer science)|recursively}} call the <code>aim</code> method alternately on the parent and the child indefinitely, until the program crashes with a {{w|stack overflow}}.

Also, the terms "parent" and "child" are usually interpreted more abstractly in programming, as a generic terms used in hierarchical {{w|Data structure|data structures}}.

The title text refers to [http://www.eclipse.org/ Eclipse], which is a tool commonly used to develop software in Java. "Building character" is something that you would expect a parent to do, in order to instill in his child positive traits, such as athleticism. However, here, "build" might be a play on the term of "{{w|Software build|building}}" a program, while "character" refers to a data type. It may also refer to the common notion that programming in C++ or Java builds character due to their powerful but sometimes finicky libraries.

===Program description===
To compile this {{w|Java_(programming_language)|Java}} source code, the two [http://docs.oracle.com/javase/tutorial/java/javaOO/classdecl.html classes] would need to be in a .java file.
#The Ball class [http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html extends] [http://docs.oracle.com/javase/7/docs/api/java/lang/Throwable.html Throwable], making it possible to use an instance of Ball in [http://docs.oracle.com/javase/tutorial/essential/exceptions/handling.html exception handling].
#The P (short for Person) class contains the following members:
:*a [http://docs.oracle.com/javase/tutorial/java/javaOO/variables.html class variable] 'target' to hold another instance of P to aim an instance of Ball at.
:*a [http://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html constructor] 'P' (in Java the constructor always has the same name as the class) used to create an instance of P and initialize its state (with a target). The keyword [http://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html this] refers to the current instance of P.
:*a [http://docs.oracle.com/javase/tutorial/java/javaOO/methods.html method] 'aim' that takes an instance of Ball named 'ball' as a parameter.
:*a [http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html static] method 'main' which is called when executing this class.

The program executes in the following order:
#The static main method is called (with the command line arguments in the 'args' parameter).
#An instance of P is created without a target ({{w|Nullable_type|null}}) using the 'new' keyword and assigned to a variable named 'parent'.
#Another instance of P is created with 'parent' as target and assigned to a variable named 'child'.
#The target variable of the parent is changed to the child instance.
#An instance of Ball is created and passed as a parameter to the aim method of parent.
#The aim method first sets up a [http://docs.oracle.com/javase/tutorial/essential/exceptions/try.html try] block to handle exceptions.
#Next, the Ball instance 'ball' is [http://docs.oracle.com/javase/tutorial/essential/exceptions/throwing.html thrown] (= signal an exception situation).
#The thrown exception (= the Ball instance) is handled in the [http://docs.oracle.com/javase/tutorial/essential/exceptions/catch.html catch] block with the matching exception class.
#In the catch block the aim method of the target of the P instance is called with the Ball instance (now referred to as 'b').
#The code continues with step 6, but the current class instance ('this') has changed from parent to child (or vv.).

Notes about code quality:
#Good Java code should contain documentation in the form of {{w|Javadoc}} comments.
#It is recommended to organize Java source code into [http://docs.oracle.com/javase/tutorial/java/package/packages.html packages] to make code easier to find and use and to avoid name conflicts.
#The class interface should not expose inner logic and data. For this reason class variables are usually declared as [http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html private] (or protected) and separate public methods (called getters and setters) are created to access the data. Making target private would also require using the setTarget setter method in main to set the parent target to child.
#In most cases constructors should call their superclass constructor first. P descents from Object, which has an empty constructor, so this is not strictly required here.
#The aim method references the class variable target directly. To avoid future name conflicts it is recommended to prefix the reference to target with 'this' or use the getter from #2.

==Transcript==
 class Ball extends Throwable {}
 class P{
     P target;
     P(P target) {
         this.target = target;
     }
     void aim (Ball ball) {
         try {
             throw ball;
         }
         catch (Ball b) {
             target.aim(b);
         }
     }
     public static void main(String[] args) {
         P parent = new P(null);
         P child = new P(parent);
         parent.target = child;
         parent.aim(new Ball());
     }
 }

{{comic discussion}}
[[Category:Programming]]