Questions

• Reading?
• Quiz?
• Homework?

Concept - About Static Variables

• Defined outside any method - with the instance variables
• Belong to class as a whole rather than to each instance (object) of the class.
• Almost never a good idea
• Many companies like Google forbid the use a static variables
• Static CONSTANTS are a good idea.
• Example: public static final int INCHES_PER_FOOT = 12;
• Can you think of a static constant you have used from the Java library?
• Access a static constant using the name of the class
  • no object required
• If INCHES_PER_FOOT is defined in a class called Measurement, what code would you use to access it in your class?

•  Measurement.INCHES_PER_FOOT

 

Concept - Why static constants

Why would someone use a static constant?

• Avoid magic numbers
• Make it available to other classes (if it is public)
  • Math.PI
  • Color.RED
• Do not have to create an object of the class to access the constant

Activity - Static constant

Record your participation in Piazza clicker question Lesson19 Q1

• You are writing a class called Invoice. Define a public static constant for the tax rate with a value of 8.5%. You do not need Bluej here. Just think how you would define the constant

  public class Invoice
  {
      //define static constant here
      ...
  }

• Now I want to use your constant in a calculation in the main method of my class. What should be coded at the ???

  public class Store
  {  
      public static void main(String[] args)
      {
          ...
          //how do you access the tax rate from the Invoice class?
          double total = cost + cost * ???; 
          ...
      }
  }

Concept - Static methods

A static method:

• Is not invoked on a particular object of the class it is defined in
• Does not (can not) access the instance variables of the class
• Invoked on the class not an object of the class

      Math.pow(...)
      Math.round(...)
      Character.isLetter(...)
      Character.toLowerCase(...)

• All methods in the Math class are static
  • in fact, you can not create a Math object
  • Look at the Math class in the API
• A class with only static methods is called a utility class
  • It should not have a constructor or instance variables

Activity - Character class

Record your participation in Piazza clicker question Lesson19 Q2

What about the Character class?

• Are all if its methods static?
• Look in the API and see. (in google search bar, type java api character)

1. Yes
2. No
3. I did not check

Activity - Static vs instance method

Record your participation in Piazza clicker question Lesson19 Q3

Non-static methods are called instance methods.

Pick a library class we have used and name an instance method it contains

1. I found one
2. I didn't know how to look
3. I didn't try

 

Concept - Writing static methods

• You use the keyword static to indiate the method is static
  • That it belongs to the class and not to an instance.
  • That it can not use instance variables
  • It takes all the data it needs as parameters
• Example: A static method that takes a Rectangle as a parameter and returns its area.

  public static double rectangleArea(Rectangle rec)
    {
         return rec.getWidth() * rec.getHeight();
    }
    

• Now you know what the word static does in public static void main(String[] args)

 

Activity - Writing a static method

Record your participation in Piazza clicker question Lesson19 Q4

Here is a project containing a Circle class which represents a circle in 2d space.

• Open it is Bluej and look at it.
• Add a static method to the Circle class which takes a double as a parameter and returns the area of a circle with that radius. Call it area()
• Notice there are now two methods called area():
  • an instance method that returns the area of this Circle object (using the instance variable)
  • a static method that returns the area of a circle with the given radius. (does not use the instance variable)
• Add code to CircleTester to print the area of a circle with radius 10 using the static method
• Notice how the two methods are called in the tester.

 

Activity - More practice writing static methods

Record your participation in Piazza clicker question Lesson19 Q5

Now in the same project, add a class called Util

• Write a static method called sum that takes an int array as a parameter and returns the sum of the elements.

Activity - More practice writing static methods

Record your participation in Piazza clicker question Lesson19 Q6

Add a static method to Util that takes an ArrayList of Strings as a parameter and returns the longest word or the empty String if the ArrayList is empty. Call the method longest()

Activity - More practice writing static methods

Record your participation in Piazza clicker question Lesson19 Q7

Add a tester class to the project which tests your methods. Here is a starter.

import java.util.ArrayList;
public class UtilTester
{
    public static void main(String[] args)
    {
        int[] numbers = {2, 7, 4, 9, 11};
        
        ArrayList<String> strings =new ArrayList<>();
        strings.add("int");
        strings.add("final");
        strings.add("static");
        strings.add("interface");
        strings.add("final");
        
         ArrayList list = new ArrayList();      
         
        //put your test code below

        //test sum
               
        //test longest
        
        //test empty ArrayList        
    }
}