Methods

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.

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!

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