private
and protected
won't be clear until you've studied class inheritance.This is an old revision of the document!
The ClickerCounter
class definition we used previously encapsulates the state and behavior of a clicker counter as an object should.
public class ClickerCounter { int count; void click(){ count++; } void reset(){ count = 0; } }
But the definition has issues. For example, if we change our example as follows:
public class ClickerExample { public static void main(String[] args) { 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); } }
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 and probably not what you want out from your ClickerCounter
object. In other words, the object offers no protection of its state.
This is easy 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:
public class ClickerCounter { private int count; public void click(){ count++; } public void reset(){ count = 0; } }
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':
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. 1)
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
myCounter.count = 492341;
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.
In Java programming, it's considered a best practice to make all member variables private
or protected
, except in some rare situations.
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
.
If we comment out the offending line above, we will see that the program still does not compile. The reason for this is in the statement:
System.out.println(myCounter.count);
The compiler is trying to access myCounter
's count
member, but we said that it's private so no one would clobber it. But now we can't access the value at all!
So, once you make a member variable private or protected, how do we get its value?? The answer is you can write a public method that simply returns the value of the member variable:
public class ClickerCounter { private int count; // NEW! accessor method public int getCount(){ return count; } public void click(){ count++; } public void reset(){ count = 0; } }
This is an example of an accessor: a member method that provides read-only access to a value in your object's state. Accessor names typically consist of “get” followed by name of the state value (i.e., member variable) it is modifying.
We can now change our main
method as follows:
public static void main(String[] args) { var myCounter = new ClickerCounter(); myCounter.reset(); // count is 0 System.out.println(myCounter.getCount()); myCounter.click(); myCounter.click(); System.out.println(myCounter.getCount()); }
and life if good. Accessors are sometimes called getters, but I personally try to avoid that term.
Let's now add a feature to our counter that lets the user set the maximum count: after the counter reaches the maximum, it should reset. Just like a physical clicker counter that only counts up to, say, 9,999.
We'll need another member variable to store this upper bound, and we'll have to modify the click()
method to check it.
private int maxCount; // new member variable ... // modified click() method public void click(){ if (count < maxCount) { count++; } else { count = 0; } }
Following best-practices, I made maxCount
private. But now how do we set it? The answer is we write a public method that sets the value. Even better, the public method can check to make sure the value it's being set to is value. Such a method is called a mutator: an instance method that mutate the value of a state value. Mutator names typically consist of “set” followed by with the name of the state value (member variable) being modified.
// NEW! mutator for maxCount public void setMaxCount(int newMax) { if (newMax > 0) { maxCount = newMax; } else { System.out.println("Invalid new maximum count: " + newMax); } }
Mutators should checks that new values are valid. This is another example of what's needed to make an object self-governing.
Mutators are sometimes called setters, but I personally try to avoid that term.
Putting it all together, we now have:
/** * Encapsulate a tally counter/clicker-counter */ public class ClickerCounter { // Member variables private int count; private int maxCount; // Accessors and mutators public int getCount() { return count; } public void setMaxCount(int newMax) { if (newMax > 0) { maxCount = newMax; } else { System.out.println("Invalid maximum count: " + newMax); } } // Interface methods public void click() { if (count < maxCount) { count++; } else { count = 0; } } public void reset() { count = 0; } }
public class ClickerExample { public static void main(String[] args) { var myCounter = new ClickerCounter(); // instantiate a ClickerCounter myCounter.setMaxCount(100); myCounter.reset(); // count is 0 for (int i = 0; i < 99; i++){ myCounter.click(); } System.out.println(myCounter.getCount()); // 99 myCounter.click(); System.out.println(myCounter.getCount()); // 100 myCounter.click(); System.out.println(myCounter.getCount()); // 0 } }
Many new programmers feel the need to write both accessors and mutators for all instance variables. Don't. To reduce the potential for introducing bugs, you should only write methods that will actually be used. If you don't need a accessor or mutator, don't write it.
Copyright © 2011-2018 Mithat Konar. All rights reserved.
private
and protected
won't be clear until you've studied class inheritance.