5.7 Static Variables and Methods

5 min read•december 29, 2022

Milo Chang

Peter Cao

Milo Chang

Peter Cao

AP Computer Science A 💻

130 resources

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We are almost done with our classes, with only two more rounds of edits to make, one in this topic and the next in Topic 5.9. First, we will need to learn more about static variables and methods.

Introduction to Static Variables and Methods

Static variables and methods are marked by the keyword static, which means that these are properties of the entire class and not just of one particular object. For example, static variables and methods are for things that are true for every student, such as boundaries for certain letter grades which we will implement in Topic 5.9, and also the school that the student goes to, which we will implement in this Topic.

Another example of static variables and methods is the Math class we learned back in Unit 2. There are no separate "math objects," but every method is just a general math method doing a calculation.

It is important to note:

Static methods cannot access or change the values of instance variables.
Static methods do not have a this reference (which we will talk about in Topic 5.9) and cannot use instance variables or call non-static methods.
However, static methods can access or change the values of static variables.

Adding Static Variables and Methods

We will add static variables and methods to our Student class. Here is a summary of what we will add: grade boundaries for letter grades, a method to get the letter grade for a student, the school name of all students, and also a method to change this school name.

/** Represents an assignment that a student will complete
*/
public class Assignment {
  private boolean correctAnswer; // represents the answer to an assignment, either T/F

  /** Makes a new assignment with one True/False question and sets the correct answer
  */
  public Assignment(boolean answer) {
    correctAnswer = answer;
  }
  
  /** Prints details about the assignment
  */
  @Override
  public String toString() {
    return "This is an assignment with correct answer " + answer;
  }

  /** Grades an assignment, returns true if correct, false if incorrect
  */
  public boolean gradeAssignment(boolean studentAnswer) {
    return studentAnswer == correctAnswer;
  }
}

/** Represents a high school student
*/
public class Student {
  private int gradeLevel; // a grade between 9-12
  private String name; // the students name in the form "FirstName LastName"
  private int age; // the student's age, must be positive
  private Assignment assignment; // the current assignment the student is working on
  private int assignmentsComplete; // numbers of assignments completed
  private int correctAssignments; // number of correct assignments

  private static final double A_BOUNDARY = 0.9;
  private static final double B_BOUNDARY = 0.8;
  private static final double C_BOUNDARY = 0.7;
  private static final double D_BOUNDARY = 0.6;
  private static String school = "The Fiveable School";

  /** Makes a new student with grade gradeLev, name fullName, and age ageNum
  */
  public Student(int gradeLev, String fullName, int ageNum) {
    gradeLevel = gradeLev;
    name = fullName;
    age = ageNum;
    assignment = null; // There is no active assignment at the moment
    assignmentsComplete = 0; // no assignments complete yet
    correctAssignments = 0;
  }

  /** Returns the student's grade level
  */
  public int getGradeLevel() {
    return gradeLevel;
  }
  
  /** Returns the student's name
  */
  public String getName() {
    return name;
  }
  
  /** Returns the current assignment the student is working on
  */
  public Assignment returnCurrentAssignment() {
    return assignment;
  }

  /** Prints details about the student
  */
  @Override
  public String toString() {
    return name + ", a " + gradeLevel + "th grade high school student has an average grade of " + averageGrade + ".";
  }

  /** Changes the student's name
  */
  public void setName(String fullName) {
    name = fullName;
  }

  /** Changes the student's grade level
  */
  public void setGradeLevel(int gradeLev) {
    gradeLevel = gradeLev;
  }

  /** Submits an assignment
  */
  public void submitAssignment() {
    boolean grade = assignment.gradeAssignment();
    assignmentsComplete++;
    if grade {
      correctAssignments++
    }
  }  

  /** Calculates the student's grade as a decimal 
  */
  public double getGradeDecimal() {
    return (double) correctAssignments / assignmentsComplete;
  }

  /** Changes the school that the students go to
  */
  public static void setSchool(String schoolName) {
    school = schoolName;
  }
}

Using Static Methods and Variables

To use static variables and methods, we use the class name and the dot operator, since static variables and methods are associated with a class and not objects. To show you concrete examples of a few concepts, let's assume we are in a main method where the following Student objects are created:

Student alice = new Student(11, "Alice", 17);

Student bob = new Student(10, "Bob Smith", 16);

If we wanted to change the school that all objects of the Student class are associated with, we would do the following in the main method we are in:

Student.setSchool("New School");

Note that instead of doing alice.setSchool("New School"); or bob.setSchool("New School");, we used the class name (Student) and the dot operator.

Imagine for a second that we had this line under private static String school = "The Fiveable School";:

public static String welcomeMessage = "Welcome to School";

We could then use the following line in the main method we are in to print out the welcome message:

System.out.println(Student.welcomeMessage);

(We made welcomeMessage a public variable so we wouldn't have to write with a static getter method to retrieve a private message. We'll learn about scope and access in the next topic.)

Browse Study Guides By Unit

➕Unit 1 – Primitive Types

📱Unit 2 – Using Objects

🖥Unit 3 – Boolean Expressions & if Statements

🕹Unit 4 – Iteration

⚙️Unit 5 – Writing Classes

5.0Unit 5 Overview: Writing Classes

5.1Anatomy of a Class

5.2Constructors

5.3Documentation With Comments

5.4Accessor Methods

5.5Mutator Methods