• A free Java Development Environment designed for beginners, used by millions worldwide
• Created by University of Kent
• Supported by Oracle
• "One of my favourite IDEs out there is BlueJ"
  • James Gosling, creator of Java.

Download Bluej

• objects
  • represent things from the real world, or from some problem domain
  • (example: “the yellow bike with basket”)
• classes
  • represent all objects of a kind (example: “bike”)

• Java objects model objects from a problem domain.
• Objects are created from classes.
  • The class describes the kind of object;
  • the objects represent individual instantiations of the class.
• We can communicate with objects by invoking methods on them.
  • Objects usually do something if we invoke a method.
• Methods can have parameters to provide additional information for a task.
• The header of a method is called its signature.
  It provides information needed to invoke that method.
• Parameters have types.
  • The type defines what kinds of values a parameter can take.

• start names of classes with capital letters
• and names of objects with lowercase letters
• This helps to distinguish what we are talking about.

List of Java datatypes

• Start BlueJ and open the example named figures
  • Create some objects from class Circle
  • Invoke the changeColor method on one of your circle objects and enter the string "red". This should change the color of the circle. Try other colors.
  • See what happens when you specify a color that is not known.
  • Invoke the changeColor method, and write the color into the parameter field without the quotes. What happens?
  • Create several circle objects on the object bench. You can do so by selecting new Circle() from the pop-up menu of the Circle class. Make them visible, then move them around on the screen using the “move” methods. Make one big and yellow; make another one small and green. Try the other shapes too: create a few triangles, squares, and persons. Change their positions, sizes, and colors.

• Multiple instances.
  • Many similar objects can be created from a single class.
• Objects have state.
  • The state is represented by storing values in fields (attributes).
• The class defines what fields an object has, but each object stores its own set of values (the state of the object).

• Make sure you have several objects on the object bench, and then inspect each of them in turn.
• Try changing the state of an object (for example, by calling the moveLeft method) while the object inspector is open.
• You should see the values in the object inspector change.

• Select Show Terminal from the View menu.
• Then select Record method calls from the terminal’s Options menu.
• Now create a few objects, call some of their methods, and observe the output in the terminal window.

Something like this

Person person1 = new Person();
person1.makeVisible();
person1.moveRight();

• Select Show Code Pad from the View menu.
• This pane is the Code Pad.
• You can type Java code here.

try

Person myPerson = new Person();
myPerson.makeVisible();
myPerson.moveRight();

Tip You can recall previously used commands in the Code Pad by using the up arrow.

• Method calling
  • Objects can communicate by calling each other’s methods.
• The source code of a class determines for each of the objects of that class.
  • the structure (fields)
  • and behavior (methods)

In Bluej the pop-up menu of a class has an option labeled Open Editor.
This will open a text editor displaying the source code of the class.

• About compilation
  • If we use a “higher-level” programming language such as Java.
    A computer cannot execute Java source code directly.
  • The compiler translates the Java code into machine code.
  • If we change the source code, we must run the compiler before we can use the class again to create an object.

• Use Project House
• In the source code of class Picture, find the part that actually draws the picture. Change it so that the sun will be blue rather than yellow.
• Add a second sun to the picture.
• Add a sunset to the single-sun version of Picture. That is, make the sun go down slowly.
  • Remember: The circle has a method slowMoveVertical that you can use to do this
• Exercise If you added your sunset to the end of the draw method (so that the sun goes down automatically when the picture is drawn), change this now.
  • We now want the sunset in a separate method, so that we can call draw and see the picture with the sun up, and then call sunset (a separate method!) to make the sun go down
• Exercise Make a person walk up to the house after the sunset

Use lab-classes project and look at code

• Result.
  • Methods may return information about an object via a return value.
  • (The word void indicates that a method does not return any result)

• Objects can be passed as parameters to methods of other objects

• object
  • Java objects model objects from a problem domain.
• class
  • Objects are created from classes. The class describes the kind of object; the objects represent individual instantiations of the class.
• method
  • We can communicate with objects by invoking methods on them. Objects usually do something if we invoke a method.
• parameter
  • Methods can have parameters to provide additional information for a task.
• signature
  • The header of a method is called its signature. It provides information needed to invoke that method.

• type
  • Parameters have types. The type defines what kinds of values a parameter can take.
• multiple instances
  • Many similar objects can be created from a single class.
• state
  • Objects have state. The state is represented by storing values in fields.
• method calling
  • Objects can communicate by calling each other’s methods.
• source code
  • The source code of a class determines the structure and behavior (the fields and methods) of each of the objects of that class.
• result
  • Methods may return information about an object via a return value.