📓3.5: Writing Methods

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 3.5

Accessor Methods (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.

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.

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:

public 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 (object) type like String orPerson 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.

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.

The toString Method

📜 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 an object’s toString method to convert a object argument into a String to be printed.

Here is the Student class again, but this time with a toString method:

  public class Student {
      private String name;
      private String email;
      private int id;

      public Student(String initName, String initEmail, int initId) {
          name = initName;
          email = initEmail;
          id = initId;
      }

      // toString() method
      public String toString() {
          return id + ": " + name + ", " + email;
      }
  }

Note that when we call System.out.println(s1) in the tester class below, it will automatically call the toString method within the Student class to get a String representation of the Student object:

  public class TesterClass {
      // main method for testing
      public static void main(String[] args) {
          Student s1 = new Student("Skyler", "skyler@sky.com", 123456);
          System.out.println(s1);
      }
  }

The toString method will return a String that is then printed out.

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Mutator Methods (Setters)

As we saw in the last section, since we typically make instance variables private, we have to define getters if we want to allow code outside the class to access the value of particular instance variables.

By the same token, if we want to allow code outside the class to change the value of an instance variable we have to provide what is formally called a mutator method but which everyone actually calls a setter. A setter is a void method with a name that starts with set and that takes a single argument of the same type as the instance variable to be set.

The effect of a setter, as you would probably expect, is to assign the provided value to the instance variable.

Just as you shouldn’t reflexively write a getter for every instance variable, you should think even harder about whether you want to write a setter. Not all instance variables are meant to be manipulated directly by code outside the class.

For example, consider the Turtle class. It provides getters getXPos and getYPos but it does not provide corresponding setters. There are, however, methods that change a Turtle\ ’s position like forward and moveTo. But they do more than just changing the values of instance variables; they also take care of drawing lines on the screen if the pen is down.

How to Define a Setter

🪄 A setter is a void method with a name that starts with set and that takes a single argument of the same type as the instance variable to be set.

public class ExampleTemplate
{
     // Instance variable declaration
     private typeOfVar varName;

     // Setter method template
     public void setVarName(typeOfVar newValue)
     {
          varName = newValue;
     }
}

The effect of a setter is to assign the new provided value to the object’s instance variable.

Here’s an example of the Student class with a setter for the name variable:

public class Student
{
     // Instance variable name
     private String name;

     /**
     * setName sets name to newName
     * @param newName
     */
     public void setName(String newName)
     {
          name = newName;
     }

     public static void main(String[] args)
     {
          // To call a set method, use objectName.setVar(newValue)
          Student s = new Student();
          s.setName("Ayanna");
     }
}

Compare the difference between setters and getters in the following figure.

  • Getters return an instance variable’s value and have the same return type as this variable and no parameters.
  • Setters have a void return type and take a new value as a parameter to change the value of the instance variable.

image

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⭐️ Summary

  • An accessor method, or 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.

  • A void method does not return a value. Its header contains the keyword void before the method name.

  • A mutator method or setter is a void method that changes the values of an instance or static variable.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.