So far we have learned about several different data types. For example, int, double, String, and boolean. We can use these different data types to store things like a student's name, their grade level, their gpa, or whether or not they have a scholarship. But what if we wanted to do this for thousands of students? Before we answer this questions, check out the video below,
Classes in java allow us to model other data types that are not built into the the programming language and create as many subsequent objects of the class as needed. For example, we could create a data type called Student which could then store the name, gradeLevel, and GPA of different students. Below, is an illustration of two Student data types: student1 and student2. For each Student data type we have also defined the name, gradeLevel, GPA, and hasScholarship variables.
| Student student1 | Student student2 | |||||
|---|---|---|---|---|---|---|
| data type | name | value | data type | name | value | |
| String | name | Bart | String | name | Kyle | |
| int | gradeLevel | 10 | int | gradeLevel | 11 | |
| double | GPA | 2.6 | double | GPA | 3.5 | |
| boolean | hasScholarship | false | boolean | hasScholarship | true | |
In the above example Student is referred to as a class. student1 and student2 are referred to as Student objects, or instances of the Student class.
A class is used to model a data type.
An instance of a class is referred to as an object.
All classes in java are created using the class key word. For example,
public class Student{
}
To properly describe the students above we need the following instance variables: name, gradeLevel, GPA, and hasScholarship. Instance variables are variables that can be accessed by any memeber of the class. For this reason they are declared at the top of the class.
The instance variables necessary to describe our students have been added to our class below.
public class Student{
public String name;
public int gradeLevel;
public double GPA;
public boolean hasScholarship;
}
In the above code snippet, the key word public is an access level modifier that allows other classes to interact with this class and the variables inside it. For now, all classes and variables will be public.
But, before we can start creating student data types, we need to add one more thing to our class, a constructor,
public class Student{
public String name;
public int gradeLevel;
public double GPA;
public boolean hasScholarship;
public Student(){
}
}
A constructor is required for all classes. In fact, if you do not specify one, java will include one for you! But, the above code is all that is needed to start creating Student data types.
Instantiating is a fancy word for creating. If you instantiate a class you are creating an object (or data type) of that class. In a previous example we defined a Student data type called student1 and another Student data type called student2. A Student object for each of these students can be created by instantiating the Student class.
To create Student objects from the Student class we use the new key word as illustrated below.
public class StudentMaker{
public static void main(String args[]){
Student student1 = new Student();
Student student2 = new Student();
}
}
The code above created two instances of the Student class called student1 and Student2. This is just another way of saying we created two student objects, student1 and student2.
Notice, that we created our Student objects in the main method of a different class called StudentMaker. We were able to do this because the student class was given the public access modifier. Below are the two classes side by side,
| Student | StudentMaker |
|---|---|
|
public class Student{ |
public class StudentMaker{ |
Now, that we have created our students, we can start defining their characteristics, like their name, gradeLevel, GPA, etc. To access variables associated with our student object we use the dot notation,
| Student | StudentMaker |
|---|---|
|
public class Student{ |
public class StudentMaker{ |
A parameter is a value you can pass to a method (or a constructor). For example, rather than defining the students name, gradeLevel, GPA, etc. after the Student object has been created, this can be done all at once. Before we can do so however, we must modify the constructor in the Student class to allow for parameters.
public class Student{
public String name;
public int gradeLevel;
public double GPA;
public boolean hasScholarship;
public Student(
String n, int g, double gpa, boolean s ){
}
}
Once the values are received by the constructor, they can be assigned to the variables name, gradeLevel, GPA, and hasScholarship as follows,
public class Student{
public String name;
public int gradeLevel;
public double GPA;
public boolean hasScholarship;
public Student(
String n, int gl, double gpa, boolean s ){
name = n;
gradeLevel = gl;
GPA = gpa;
hasScholarship = s;
}
}
Now, that we have modified the constructor, we must also modify our code to create our Student instances. Below, we create two Student instances, student1 and student2,
public class StudentMaker{
public static void main(String args[]){
Student student1 = new Student("Bart", 10, 2.6, false);
Student student2 = new Student("Kyle", 11, 3.5, true);
}
}