Lab: PiggyBank. Defining objects & classes

Transcription

1 Lab: PiggyBank Defining objects & classes

2 Review: Basic class structure public class ClassName { Fields Constructors Methods } Three major components of a class: Fields store data for the object to use Constructors allow the object to be set up properly when first created Methods implement the behavior of the object

3 Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Kölling Fields Fields store values for an object. They are also known as instance variables. Fields define the state of an object. public class Square { private int x; private int y; private int size; private Color fillcolor; } // Further details omitted. visibility modifier type variable name private int size;

4 Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Kölling Constructors public Square() { x = 0; y = 0; size = 0; fillcolor = Color.blue; } Constructors initialize an object. They have the same name as their class. They store initial values into the fields. They often receive external parameter values for this.

5 Objects First with Java - A Practical Introduction using BlueJ, David J. Barnes, Michael Kölling Methods method header/signature visibility modifier return type method name /** * Gets the size of the square. */ public int getsize() { return size; } start and end of method body (block) parameter list (empty) return statement

6 Create a new project! File > New > Java Project Give it a name (like PiggyBank_netid) Right click on your new project & go to New > Class Give it a name (PiggyBank) Click the check box to create a main method Click finish

7 Consider the following partial PiggyBank class definition: public class PiggyBank { private int numpennies; private int numnickels; private int numdimes; private int numquarters; } 1. Write a constructor that takes no parameters and initializes all of the fields so the new PiggyBank represents an empty PiggyBank. 2. Write the insertpenny, insertnickel, insertdime, and insertquarter methods. The method signatures are similar in that there is no return value and no parameters. 3. Write the insertpennies, insertnickels, insertdimes, and insertquarters methods. The method signatures are similar in that there is no return value and one parameter.

8 Testing your PiggyBank with main 4. Write a main method that creates a new, empty PiggyBank stored in local variable mypiggybank. 5. Write a print method that prints out how many quarters, dimes, nickels, and pennies are in the piggy bank, and call this method from main. 6. Insert 67 into your mypiggybank you just declared above. You do not need to write a new method, just add the statements to main that would insert the correct amount. There are different ways of doing this; just make sure you insert the correct amount. Print out the contents to verify your code is correct.

9 Extending your PiggyBank s functionality 7. Write a method that returns the total amount of money in the piggy bank. The amount should be expressed as an integer that represents the total number of cents ( ) in the piggy bank (do not assume the bank only has 67 in it). The method should be called getamount. 8. Modify the print method to print the total by calling getamount() 9. Write a method insertamount that takes a number of cents as a parameter and adds the appropriate number of quarters, dimes, nickels, and pennies into the piggy bank. You should use the largest coins possible to add the amount. (Hint: you may need to use %) 10. Insert 67 more into your mypiggybank object using the insertamount method you just created. Print out the contents to verify your code is correct. 11. When submitting, your main method should have 3 calls to print (one when empty, one when has 67, and one when has 134 ), and it should have all the other calls as stated in the previous steps.

10 Homework Finish lab Work on CodingBat exercises Read again Chapter 4