Java programming course: 3.1 Using classes and methods

In the previous lesson you saw how to use the Java APIs.

• Previous lesson: 2.4 The Java APIs
• Course contents

Using classes and methods

Methods provide the means of telling an object what it can do.

In this series of lessons you will learn:

• Methods which change an object's state
• Overloading and inheritance
• Abstract classes and methods

Methods that modify an object's state

The Animal class allows its instance variables to be set from the constructor, but only currently provides methods which return those values to client objects[1]. Although it is unlikely that an animal's name or gender could change it is still a possibility (if, for example, the entries were wrongly entered to begin with) and the age value will naturally change on an annual basis.

To this end, you will now define three additional methods within Animal that enables these values to be changed, starting with a method to change the animal's name:

public class Animal {
    
    // Declare instance variables to hold an animal's attributes
    private String name;        // the animal's name
    private String gender;      // the animal's gender ("m" or "f")
    private int age;            // the animal's age in years
    
    
    // Define constructor
    public Animal(String myName, String myGender, int myAge) {
        name = myName;
        gender = myGender;
        age = myAge;
    }
    
    
    // Define instance methods
    
    // Return the animal's name
    public String getName() {
        return name;
    }
    
    // Change the animal's name
    public void setName(String newName) {
        name = newName;
    }
    
    // Return the animal's gender
    public String getGender() {
        return gender;
    }
    
    // Return the animal's age
    public int getAge() {
        return age;
    }
    
            
    // Rest of class will go here...
    
}
 

 Note the following about the new method:

• The method is public so objects from any package can invoke it.
• The void keyword means that a value is not returned from this method. There is no need to return anything since its purpose is merely to modify an internal instance variable.
• The method is called setName() where the prefix "set" is a commonly adopted convention for naming methods that modify an object's state.

The methods to change the gender and age follow a similar pattern (from now on, surrounding code won't always be shown to save space):

// Change the animal's gender
public void setGender(String newGender) {
    gender = newGender;
}
    
// Change the animal's age
public void setAge(int newAge) {
    age = newAge;
}
 

The new methods can be invoked (for example from the VirtualZoo class) as follows:

Animal bruno = new Animal("Bruno", "m", 4);
        
// Change bruno's details
bruno.setName("Bruuuuno");
bruno.setGender("f"); // She was a girl all along!
bruno.setAge(5); // and now a year older
        
// Output bruno's details
System.out.println("Name: " + bruno.getName());
System.out.println("Gender: " + bruno.getGender());
System.out.println("Age: " + bruno.getAge());
 

In the next lesson you will learn how to overload methods:

Lesson 3.2 Method overloading