Time estimate: 45 min.

6.4. Accessors / Getters

Since the instance variables in a class are usually marked as private to the class, if you want code outside the class to be able to access the value of an instance variable, you need to write what is formally called an accessor methods but which everyone actually just calls a getter. A getter is a public method that takes no arguments and returns the value of the private instance variable. (We discussed using getters in section 2.5.)

If you used a language like App Inventor in an AP CSP class, you may have used setter and getter blocks. In App Inventor, you cannot make your own classes, but you can declare UI objects like Button1, Button2 from the Button class and use their get/set methods for any property like below. (We’ll talk about setters in Java in the next section.)

../_images/AppInvSetGet.png

Figure 1: App Inventor Set and Get blocks for object Button1

You don’t need to write a getter for every instance variable in a class but if you want code outside the class to be able to get the value of one of your instance variables, you’ll need to write a getter that looks like the following.

class ExampleTemplate
{

    // Instance variable declaration
    private typeOfVar varName;

    // Accessor (getter) method template
    public typeOfVar getVarName()
    {
        return varName;
    }
}

Notice that the getter’s return type is the same as the type of the instance variable and all the body of the getter does is return the value of the variable using a return statement. (We’ll talk more about the return statement in section 5.6 but for now just notice that it is followed by an expression whose value must be the same type as the return type in the method’s header. In a getter that will definitely be true as long as the type of the instance variable and the return type of the getter are the same.)

Here’s an example of an accessor method called getName for the Student class which also demonstrates how to call getName using a Student object:

class Student
{

  //Instance variable name
  private String name;

  /** getName() example
   *  @return name */
  public String getName()
  {
     return name;
  }

  public static void main(String[] args)
  {
     // To call a get method, use objectName.getVarName()
     Student s = new Student();
     System.out.println("Name: " + s.getName() );
  }

Note, that getters only return the value of the variable. In other words, the code that called the getter and which receives that value has no ability to change the object’s instance variable; they just get a copy of the value. However if the instance variable is a reference type like String or Person the value that is copied is the value of the reference. That means the caller receives a new copy of the reference that points to the same object as is stored in the instance variable. In the next section, when we talk about mutation, you’ll see how that means that the caller might be able to change the object even though it can’t change the reference.

Note

Some common errors when writing and using getters are:

  • Forgetting a return type like int before the method name.

  • Forgetting to use the return keyword to return a value at the end of the method.

  • Forgetting to do something with the value returned from a method, like assigning it to a variable or printing it out.

Try the following code. Note that this active code window has 2 classes! The main method is in a separate Tester or Driver class. It does not have access to the private instance variables in the other Student class. Note that when you use multiple classes in an IDE, you usually put them in separate files, and you give the files the same name as the public class in them. In active code and IDEs, you can put 2 classes in 1 file, as demonstrated here, but only 1 of them can be public and have a main method in it. You can also view the fixed code in the Java visualizer.

coding exercise Coding Exercise

Try the following code. Note that it has a bug! It tries to access the private instance variable email from outside the class Student. Change the main method in Tester class so that it uses the appropriate public accessor method (get method) to access the email value instead.

6.4.1. toString

While not strictly speaking a getter, another important method that returns a value is the toString method. This method is called automatically by Java in a number of situations when it needs to convert an object to a String. Most notably the methods System.out.print and System.out.println use it to convert a object argument into a String to be printed and when objects are added to Strings with + and += their String representation comes from calling their toString method.

Here is the Student class again, but this time with a toString method. Note that when we call System.out.println(s1) it will automatically call the toString method to get a String representation of the Student object. The toString method will return a String that is then printed out. Watch how the control moves to the toString method and then comes back to main in the Java visualizer by using the Show CodeLens button.

See the toString() method in action.

6.4.2. groupwork Programming Challenge : Class Pet

Animal Clinic

You’ve been hired to create a software system for the Awesome Animal Clinic! They would like to keep track of their animal patients. Here are some attributes of the pets that they would like to track:

  • Name

  • Age

  • Weight

  • Type (dog, cat, lizard, etc.)

  • Breed

  1. Create a class that keeps track of the attributes above for pet records at the animal clinic. Decide what instance variables are needed and their data types. Make sure you use int, double, and String data types. Make the instance variables private.

  2. Create a constructor with many parameters to initialize all the instance variables.

  3. Create getters for each of the instance variables.

  4. Create a toString method that returns all the information in a Pet.

  5. In the main method below, create 2 Pet objects with different values and call the constructor, accessor methods, and toString methods to test all your code.

  6. Make sure you use good commenting!

Create a Pet class that keeps track of the name, age, weight, type of animal, and breed for records at an animal clinic.

6.4.3. Summary

  • A getter allows other objects to obtain the value of instance variables or static variables.

  • A non-void method returns a single value. Its header includes the return type in place of the keyword void.

  • A getter is a non-void method that returns the value of an instance variable. Its return type matches the type of the instance variable.

  • Methods “return by value” where a copy of the value is returned. When the value is a primitive type, the value is copied. When the value is a reference to an object, the reference is copied, not the object.

  • The return keyword is used to return the flow of control to the point immediately following where the method or constructor was called.

  • The toString method is an overridden method that is included in classes to provide a description of a specific object. It generally includes what values are stored in the instance data of the object.

  • If System.out.print or System.out.println is passed an object, that object’s toString method is called, and the returned String is printed.

  • An object’s toString method is also used to get the String representation used when concatenating the object to a String with the + operator.

6.4.4. AP Practice

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