4.3. Encapsulation¶

4.3.2.1. Fields¶

We previously defined a field as a variable, or piece of data, that belongs to a class. For our Student class, let’s think about the data that is typically associated with a student (in the sense of a high school or college student). There are a lot of possibilities, but here are the most important:

1. Name
2. Student ID
3. Number of credits
4. GPA

In order to declare these fields within our class, we’ll need to determine the best data type for each. A field may be of any primitive or object type. In this case, the following types will work best:

1. Name: String
2. Student ID: int
3. Number of credits: int
4. GPA: double

Let’s put these inside of a class. While they may be declared anywhere within a class, fields should always be declared at the top of the class. When we’re ready to add methods, we’ll add them below the fields.

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public class Student {

    String name;
    int studentId;
    int numberOfCredits;
    double gpa;

}

Like variables within a method, fields may be initialized when they are declared. For example, we could provide default values for numberOfCredits and gpa (default values for name and studentId don’t make sense since they should be different for each student).

int numberOfCredits = 0;
double gpa = 0.0;

Fields are also referred to as instance variables, since they belong to an instance of a class.

4.3.2.2. Getters and Setters¶

As declared, our four fields are package-private, which means that they can be read or changed by any code within the same package. As a rule-of-thumb, fields should always be private unless you have a very, very, very good reason to not make them so. So, let’s make our fields private.

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public class Student {

    private String name;
    private int studentId;
    private int numberOfCredits = 0;
    private double gpa = 0.0;

}

In order to provide access to private fields, getter and setter methods are used. Getters and setters do what you might guess: get and set a given field. If we make the getter and/or setter method for a given property public, then others will be able to access or modify the field in that way.

Here is a getter/setter pair for name (you can imagine how the others would be written).

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public String getName() {
    return name;
}

public void setName(String aName) {
    name = aName;
}

An astute question to ask at this point would be, “Why make the fields private if you’re just going to allow people to get and set them anyway!?” Great question. There are lots of reasons to use getters and setters to control access. Here are just a few:

1. Sometimes you’ll want to implement behavior that happens every time a field is accessed (get) or changed (set). Even if you can’t think of such a reason when writing your class, you might later have the need to add such behavior. If you don’t use getters and setters, you’ll have to do a lot more refactoring if you ever decide to add such behaviors.
2. You can perform validation within a setter. For example, we might want to ensure that a student’s name contains only certain characters, or that their student ID is positive.
3. You can use different access modifiers on getters and setters for the same field, based on desired usage. For example, you might want to allow anyone to be able to read the value of a field, but only classes within the same package to modify it. You could do this with a public getter and a package-private setter, but not as a field without getters and setters, which could only be public to everyone or package-private to everyone.

As an example of reason 2, let’s take a short detour to look at a Temperature class. A valid temperature can only be so low (“absolute zero”), so we wouldn’t want to allow somebody to set an invalid value. In setFahrenheit we print out if an invalid value is provided.

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public class Temperature {

    private double fahrenheit;

    public double getFahrenheit() {
        return fahrenheit;
    }

    public void setFahrenheit(double aFahrenheit) {

        double absoluteZeroFahrenheit = -459.67;

        if (aFahrenheit < absoluteZeroFahrenheit) {
            System.out.println("Value is below absolute zero");
        }

        fahrenheit = aFahrenheit;
    }
}

4.3.2.3. Properties¶

A property in Java is a characteristic that users can set. Our Student class had properties name, studentId, numberOfCredits, and gpa, while our Temperature class had only one property, fahrenheit.

Most often, properties will be fields that have public setters, though they need not have a corresponding field. Let’s look at an example of a property that doesn’t directly correspond to a field. If we wanted to add a celsius property to the Temperature class above, we might do it as follows:

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public double getCelsius() {
    return (fahrenheit - 32) * 5.0 / 9.0;
}

public void setCelsius(double celsius) {
    double fahrenheit = celsius * 9.0 / 5.0 + 32;
    setFahrenheit(fahrenheit);
}

Since there’s a link between fahrenheit and celsius, we want to make sure that when one is updated, so is the other. In this case, we only store one field value (fahrenheit) and make the appropriate calculation when getting or setting the celsius property.

Using properties, getters/setters, and fields, we can encapsulate the information we need in our student class.