4.5. Methods

4.5.1. Calling Methods on Objects

A method is a function that belongs to a class. In Java, all procedures must be part of a class. Let’s revisit our HelloWorld class.

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public class HelloWorld {

    private String message = "Hello World";

    void sayHello() {
        System.out.println(message);
    }

}

There is one method in this class, sayHello(). In order to call this method, we must have an object created from the HelloWorld class template. In other words, we must have an instance of HelloWorld.

Here’s how you call methods on an object.

HelloWorld hello = new HelloWorld();
hello.sayHello();

It is not possible to call sayHello() without having a HelloWorld object. This begins to make more sense when you note that the message field is used within sayHello(), and unless we are calling sayHello() on an instantiated object, there will be no message field available to print.

Note

As mentioned before, class members should have the most restrictive level of access possible. This goes for methods as well as fields. For example, if you create a utility method that should only be used within your class, then it should be private. You can think of private methods as those that are not useful outside of the class, but that can contribute internally to helping the class behave as desired or expected.

4.5.2. Instance Methods

So far, we’ve only looked at examples of methods that are relatively specialized: constructors, getters, and setters. Every class you create will have these methods. What will make your classes different from each other, and thus fulfill the purpose of creating each class, are the specific behaviors that are unique to your classes.

Let’s say we want to add a method in our Student class that reports the GPA of a student. This method is an instance method since it will belong to each Student object created, and will use the data of each such object.

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public class Student {

    private static int nextStudentId = 1;
    private String name;
    private int studentId;
    private int numberOfCredits;
    private double gpa;

    public Student(String name, int studentId, int numberOfCredits, double gpa) {
        this.name = name;
        this.studentId = studentId;
        this.numberOfCredits = numberOfCredits;
        this.gpa = gpa;
    }

    public Student(String name, int studentId) {
        this(name, studentId, 0, 0);
    }

    public Student(String name) {
        this(name, nextStudentId);
        nextStudentId++;
    }

    public String studentInfo() {
        return (this.name + " has a GPA of: " + this.gpa);
    }

    /* getters and setters omitted */

}

We will make use of instance methods more in the next chapter, however, we wanted to share more about them now in our first conversation about classes. Sometimes when we create a class, we will need that class to have more behaviors than using only constructors, setters, and getters can provide. When we do want to add additional behaviors to our classes, we can use instance methods!

4.5.3. Check Your Understanding

Question

Fill in the blanks with the appropriate terms.

A _____________ gives a class property a field. A _____________ gives a programmer access to the value of a private class property. A _____________ creates a new instance of a class with values for the fields. A _____________ is a method that belongs to each instance of a class.