1. Workshop for Programming And Systems Management Teachers
Chapter 4
Writing Recipes (Methods)
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Learning Goals
Understand at a conceptual and practical level
How to invoke methods in Java
Both class and object
How to write methods in Java
How to return a value from a method
How to specify that a method doesn’t return a value
How to pass variables to a method
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3. Invoking Methods in Java
Use .methodName(parameters) to send a message which will map to a method
Examples
FileChooser.pickAFile()
pickAFile is a message which maps to the pickAFile class method in the class FileChooser
“Hi”.toLowerCase()
toLowerCase is a message which maps to the toLowerCase object method in the class String
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Naming a Recipe
Writing a method is giving a name to a recipe (a series of Java statements) in a class file
When you want to execute the series of statements again
You use the method name
As the name of the message
You can also pass parameters
To be used as variables in the method
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Naming a Method
When we name variables we use
Type name; or Type name = expression;
When we name methods we use
visibility Type name (parameterList) {body}
The order is important
We call this the syntax of the declaration
Example method declarations
public void show()
private double computeTax(double total)
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Writing a Method
There are several visibility keywords
The keyword “public” means it can be invoked from inside any class definition
The keyword “private” means it can be invoked from within the current class definition
The “Type” is the type of value returned
Or the keyword “void” if no value is returned
The “parameterList” is a list of type and name pairs separated by commas
There are other visibility keywords but these are the two you should be using.
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7. Declaring a Class Method
A class method adds the keyword “static”
public static String pickAFile()
Class methods can be invoked using the class name
FileChooser.pickAFile
Use a class method when you are not operating on data in the current object
The method may create an object
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Method Body
The statements that make up the method are placed in a block
between opening and closing curly braces
public Type name (parameterList) {
statement 1;
statement 2; . }
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9. Returning from a Method
Use the keyword “return” to return a value or object from a method
return x;
return person;
The type of the thing being returned must agree with the declared type
public int getNum() {
return “3”; // would cause a compile error }
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Class Definitions
In Object-oriented programming we break the tasks to be done up and assign them to classes
The class that is responsible for the task
Method definitions go inside a class definition
public class Name {
field declarations
constructors
method declarations }
The order of things declared in a class definition doesn’t matter. Some people declare the methods first and then the constructors and fields. Some prefer the fields first, then the constructors, and then the methods. You an actually mix definitions up like declare a field then a method and then another field but it is good practice to group the fields, constructors, and methods.
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11. Java Class Definitions
In Java each class is defined in its own file
With the same name as the class
With .java as the extension
The FileChooser class definition would be defined in the file
FileChooser.java
The Picture class definition would be in
Picture.java
The compiler won’t compile a file with a class name that differs from the file name it is defined in.
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12. Writing your First Method
How about a method that lets you pick the file name, creates the picture object, and shows it? It could also return the picture object for later use.
public static Picture pickAndShow() {
String fileName = FileChooser.pickAFile();
Picture picture = new Picture(fileName);
picture.show();
return picture; }
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Method Exercise
Open DrJava
Open the file Picture.java
Type the method before the last ‘}’ in the file
Compile the file
Fix any errors and compile again
Invoke the method in the interactions pane
Files Pane
Definitions Pane
The files are in c:\intro-prog-java\bookClasses\
Interactions Pane
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14. Method to Pick And Play a Sound
Write a method to pick and play a sound
First pick a file name
Create the sound object
Tell the sound object to play
Return the sound object
How do you do each of these things?
What class does this method need to be declared in?
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15. Pick and Play Sound Method Exercise
Write the following method
In the Sound.java file
Before the last curly brace
Then compile it
And invoke it by
Sound.pickAndPlay();
public static Sound pickAndPlay() {
String fileName = FileChooser.pickAFile();
Sound sound = new Sound(fileName);
sound.play();
return sound; }
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16. Names are Important to People
Go back and change the names you use for variables in the method
Does it change what the method does?
Use names that are understandable
To you
To others
Use names that aren’t too long
thisIsAVeryLongVariableName is a pain to type
It doesn’t matter to the computer what names you use. Good names make your code easier for people to understand what you are doing.
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17. Method to Show A Picture
Write a method to show a particular picture
specify the file name
create the picture object
using the file name
ask the picture object to show itself
return the picture object
How do you do each of those things?
What class should the method be in?
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18. Showing A Particular Picture
public static Picture showPicture() {
String myFile = "FILENAME";
Picture myPicture = new Picture(myFile);
myPicture.show();
return myPicture; }
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19. Slashes and Backslashes
We have seen file names with backslashes ‘\’ in them
As a result of FileChooser.pickAFile()
However we can’t just use backslashes in string objects
They have a special meaning “\t” is tab “\n” is newline
There are two ways to fix this problem
Use a slash instead ‘/’
Use double backslashes ‘\\’
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20. Using the Interaction Pane
If you aren’t sure about how to do something you want to do in a method
Try it out in the Interactions pane
Use System.out.println(expression) to see the result
Copy it to the Definitions pane
Select what you want to copy
Control-C to copy it
Control-V to paste it
You can also copy text from the definitions pane to the interactions pane.
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21. Passing a Parameter to a Method
We wouldn’t want to create a new method for every picture we want to show
That would be a huge number of methods
Instead let’s take the file name as a parameter (input variable)
Making the method more general (reusable)
Parameters specify their type and name
Just like a variable declaration
They are used like variables in the method
We also don’t want to have to edit the program and recompile to change the file name for each file we want to create a picture from.
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22. Showing a Picture Given the Name
Write a method to show a picture when the method is passed the file name to use.
Need to pass in a file name
Need to create the picture object using the file name
Need to tell the picture object to show itself
Need to return the picture object
How do we do those things?
What class should this method be defined in?
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showNamed Method
Add the method to the Picture.java file
Compile the Picture.java file
Try the method in the interactions pane
Picture.showNamed(“c:/intro-prog-java/mediasources/katie.jpg”);
public static Picture showNamed(String fileName) {
Picture myPicture = new Picture(fileName);
myPicture.show();
return myPicture; }
Try showNamed() with different file names. You can use FileChooser.pickAFile() to get the file names to use.
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24. How Does showNamed Work?
When you type
Picture.showNamed(“c:/intro-prog-java/mediasources/katie.jpg”);
The compiler checks that the Picture class has a class method showNamed that takes a string object.
At run time the method is executed and the input variable fileName refers to the input string object during execution of the method
The name is only in use during the method
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Scope
Parameters or variables declared in a method have method scope
The names are known inside the method
They are not known outside the method
Can use the same variable names in the interactions pane
Or in other methods
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playNamed Exercise
Try to do a similar method in the Sound.java class to play a sound
It should take a file name as input
It should create the sound object
It should tell the sound object to play
It should return the created sound object
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Show a Passed Picture
How would you write a method that takes a picture object as an input variable?
What type would the input variable be?
What should you name it?
How would you get the picture object to show?
Does it need to return anything?
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showPicture Method
public static void showPicture(Picture myPicture) {
myPicture.show(); }
How is this different from just calling picture.show()?
Which one is a class method?
Which one is an object method?
Which way is better?
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Summary
Methods are declared inside of class definitions
Methods declarations have the following syntax
visibility Type name (parameterList) { code}
Class methods have the keyword static
Methods that do not return any value use the keyword void
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