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--- java:defining_classes_in_java [2020/09/12 02:49] – [Our first Java class] mithat
+++ java:defining_classes_in_java [2020/09/14 01:38] (current) – [Our first Java class] mithat
@@ Line 1: / Line 1: @@
 ======  Defining Classes in Java ======
 We are going to write a simple class for implementing a counter similar to one of these:((Picture from: "Totty Clicker - Gadgets at Play.com (UK)." Play.com (UK): DVDs, Music CDs, MP3s, Video Games, Books, Electronics & Gadgets - Free Delivery. [[http://www.play.com/Gadgets/Gadgets/4-/11566684/Totty-Clicker/Product.html?ptsl=1&ob=Price&fb=0#]] (accessed January 25, 2011).
@@ Line 43: / Line 43: @@
 </code>
-==== Where does the definition go? ====
+I have deliberately not used any comments in the definition so you can more easily see the code.
+In the above, ''count'' is an attribute/instance variable that keeps track of the state of the object. and the ''click'' and ''reset'' methods implement the behavior.
-In Java, class definitions need to go in their own files, and the files need to be called the name of the class with the ''.java'' extension.((There are certain kinds of classes that don't need to be in their own file, but that's something we can't discuss yet.)) I have deliberately not used any comments in the definition so you can more easily see the code.
+In actuality, this is a pretty crappy class definition for reasons we'll see later. But for now it's enough to get us started.
-In the above, ''count'' is an attribute/instance variable that keeps track of the state of the object. the ''click'' and ''reset'' methods implement the behavior. In actuality, this is a pretty crappy class definition for reasons we'll see later. But for now it's enough to get us started
 ==== Instantiation and use ====
-The above is just a class definition. It doesn't actually make an object we can use.
+The above is just a class definition. It doesn't actually make any objects you can use. The code below shows you how to make, or **instantiate** a ''ClickerCounter'' object and then call the object's methods to change the state of the object.
-The program below shows you how to make, or **instantiate** a ''ClickerCounter'' and then call the object's methods to change the state of the object.
+<code java>
+var myCounter = new ClickerCounter();  // instantiate a ClickerCounter
-<file java ClickerExample.java>
+myCounter.reset();  // count is 0
-public class ClickerExample {
+myCounter.click();  // count is 1
+System.out.println(myCounter.count);
+myCounter.click();  // count is 2
+myCounter.click();  // count is 3
+System.out.println(myCounter.count);
+myCounter.reset();  // count is 0
+System.out.println(myCounter.count);
+</code>
+===== Where does the code go? =====
+==== The easy-for-beginners way ====
+If there's only one custom class in your program, an easy way to write your program is to incorporate the class definition and the program's ''main()'' method into the same class definition.
+<file java ClickerCounter.java>
+public class ClickerCounter {
+    // Class definition (instance members)
+    int count;
+    void click(){
+        count++;
+    }
+    void reset(){
+        count = 0;
+    }
+    // Program's main method that uses the class
     public static void main(String[] args) {
         var myCounter = new ClickerCounter();  // instantiate a ClickerCounter
@@ Line 74: / Line 101: @@
 </file>
+This isn't generally recommended though because very often you will write more than one custom class in your program. If this is the case, the approach above won't work because Java's rules say any file can only contain at most one public class definition.((You //can// have additional non-public class definitions in a file, but that's something we can't discuss yet.))
-===== Access specifiers =====
+==== The better way ====
-The ''ClickerCounter'' class definition above effectively encapsulates the state and behavior of a clicker counter. But it has issues. For example, if we change our example as follows:
-<file java ClickerExample.java>
+A better way to manage your programs' custom classes is to put each custom class into its own file. When you do this, the files need to be the name of the class with the ''.java'' extension.
-public class ClickerExample {
-    public static void main(String[] args) {
+The following is how to break up the class definition and the "main" stuff. Here is the file that has only the class definition:
-        var myCounter = new ClickerCounter();
-        myCounter.reset();
-        myCounter.count = 492341;  // <- LOOK HERE!
-        System.out.println(myCounter.count);
-        myCounter.click();
-        myCounter.click();
-        System.out.println(myCounter.count);
-    }
-}
-</file>
-the object will happily oblige the user's wish to set the count to some arbitrary value. That's not something you can typically do with a clicker counter. In other words, the object offers no **protection** of its state. This is easy enough to solve with Java's access modifiers. Let's look at a modified version of our class definition that gives us some protection using access modifiers:
 <file java ClickerCounter.java>
-public class ClickerCounter {
+public class Counter {
+    // Class definition
+    int count;
-    private int count;
+    void click(){
-    public void click(){
         count++;
     }
-    public void reset(){
+    void reset(){
         count = 0;
     }
 }
 </file>
-Access modifiers are placed before the start of a member. Java offers four levels of protection, and they just so happen to all start with the letter 'p':
+And here is the file that has the "main" stuff:
-  * ''package'': this is what you get by default (i.e., when you don't specify anything, as in our first definition of the class). It means anything within the same package as the class definition is able to access the member.
-  * ''public'': anyone anywhere can access the member. This is a good choice for methods that are part of the class' interface.
-  * ''private'': only those of the same class can access the member.
-  * ''protected'': only those of the same _or a derived_ class can access the member. ((The difference between ''private'' and ''protected'' won't be clear until you've studied class inheritance.))
-With the changes made above, if we try to run ''ClickerExample'' we will see that the code will refuse to compile. When the compiler hits the statement
+<file java ClickerExample.java>
+public class ClickerExample {
-<code java>
+    public static void main(String[] args) {
-myCounter.count = 492341;
+        var myCounter = new ClickerCounter();  // instantiate a ClickerCounter
-</code>
+        myCounter.reset();  // count is 0
-it cannot access the ''count'' member of ''myCounter'' because we said it's private. Remember, we want our objects to be **self-governing**, and this helps make sure this is the case.
+        myCounter.click();  // count is 1
+        System.out.println(myCounter.count);
-<WRAP center round tip 80%>
+        myCounter.click();  // count is 2
-In Java programming, it's considered a best practice to make all member variables ''private'' or ''protected'', except in some rare situations.
+        myCounter.click();  // count is 3
+        System.out.println(myCounter.count);
+        myCounter.reset();  // count is 0
+        System.out.println(myCounter.count);
+    }
+}
+</file>
-Methods can be at whatever access level is appropriate for their use. If it is part of the class' interface, it should be ''public''. If it's for use only internally by the object, it will typically be ''private'' or ''protected''.
+This is the best way to manage your files, and the way I will use in the remainder of these tutorials. However, depending on how you are expected to submit homework, you may be required to use the "easy for beginners" way to write you code.
-</WRAP>
 Copyright © 2020 Mithat Konar. All rights reserved.