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Engineering LibreTexts

2.4: Getters and Setters

  • Page ID
    15280
  • Recall that the instance variables of Time are private. We can access them from within the Time class, but if we try to access them from another class, the compiler generates an error.

    For example, here’s a new class called TimeClient, because a class that uses objects defined in another class is called a client:

    public class TimeClient {
    
        public static void main(String[] args) {
            Time time = new Time(11, 59, 59.9);
            System.out.println(time.hour);      // compiler error
        }
    }
    

    If you try to compile this code, you will get a message like hour has private access in Time. There are three ways to solve this problem:

    • We could make the instance variables public.
    • We could provide methods to access the instance variables.
    • We could decide that it’s not a problem, and refuse to let other classes access the instance variables.

    The first choice is appealing because it’s simple. But the problem is that when Class A accesses the instance variables of Class B directly, A becomes “dependent” on B. If anything in B changes later, it is likely that A will have to change, too.

    But if A only uses methods to interact with B, A and B are “independent”, which means that we can make changes in B without affecting A (as long as we don’t change the method signatures).

    So if we decide that TimeClient should be able to read the instance variables of Time, we can provide methods to do it:

    public int getHour() {
        return this.hour;
    }
    
    public int getMinute() {
        return this.minute;
    }
    
    public int getSecond() {
        return this.second;
    }
    

    Methods like these are formally called “accessors”, but more commonly referred to as getters. By convention, the method that gets a variable named something is called getSomething.

    If we decide that TimeClient should also be able to modify the instance variables of Time, we can provide methods to do that, too:

    public void setHour(int hour) {
        this.hour = hour;
    }
    
    public void setMinute(int minute) {
        this.minute = minute;
    }
    
    public void setSecond(int second) {
        this.second = second;
    }
    

    These methods are formally called “mutators”, but more commonly known as setters. The naming convention is similar; the method that sets something is usually called setSomething.

    Writing getters and setters can get boring, but many IDEs can generate them for you based on the instance variables.

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