The ClickerCounter class definition we used previously encapsulates the state and behavior of a clicker counter as an object should.

ClickerCounter.java

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:

ClickerExample.java

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:

ClickerCounter.java

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.

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:

ClickerCounter.java

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:

ClickerCounter

/**
 * 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;
    }
}

ClickerExample.java

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
    }    
}

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