The learning objectives for this entire module.
  • Understance objects in the real-world.
  • Understand software objects have state and behavior.
  • Understand that Java String, Scanner, and Random are reference types - not primitive types.
  • Understand the details of declaring variables, allocating objects, and using objects of type String, Scanner, and Random.
  • Understand an object's state and behavior are like a data type's set of values and set of operations.
  • Understand how to define a reference type in a Java class.
  • Understand a Java class/type consists of instance variables, constructors, and instance methods.
  • Understand how to create a tester class to test our type.
  • Understand how to declare variables of reference types.
  • Understand how to allocate objects by using new and constructors.
  • Understand how to manipulate objects by calling instance methods.
  • Understand that objects are allocated in heap memory.
  • Undertand that a reference variable has enough memory (4-bytes) allocated to hold the refence to the object allocated in heap memory.
  • Understand the naming convention for Java types.
  • Understand JavaDoc.
  • Understand how to include JavaDoc in your classes.
  • Simple Objects – An Introduction

    The Java language defines primitive types. All other types are user defined within a Java class. At this point, we have used a Java class to write our programs that have a main method and other methods. A Java class is also used to define types. The Java language comes prepackaged with many useful types defined in classes. We have already used String and Scanner types. For the most part, using String, Scanner, and other user defined types is almost exactly like using primitive types. Primitive types are defined by the Java language. The prepackaged Java types are not part of the language. Instead Java programmers write Java classes to implement the types. These types are placed in packages that are availble to you in the IDE. When your program executes, the byte code for these packages is part of the JRE.

    The mechanics of creating a Java class that can be used as a type for declaring variables is not hard; however, the concept requires some mental energy on your part.

    The terms classes and objects go hand-in-hand. You define a type in a Java class. Then you use the type to declare variables and assign objects to those variables. The following code shows a Java String variable gusty that references a String object that contains "Gusty".

    String gusty = "Gusty";

    Objects and the Wirth Pattern

    The Wirth pattern (defined earlier) defines a prgram to be algorithms and data structures.

    This module on simple objects concentrates upon the Data Structures component. We want to learn the basics of defining our own times with minimal Algorithms. Most of the algorithms will be sequential assignment statements, with an occasional conditional expression.

    Objects in the World

    We live in a world full of objects. We have cars, buses, people, clocks, desks, chairs, books, etc. Each of these objects share two characteristics

    • state - captures information about the object. For example cars have tires, steering wheel, engine, a color, seats X passengers, etc.
    • behavior - captures the actions that can be performed with the object. For example, you can drive a car, you can put gas in a car, you can add a passenger to your car, etc.

    Objects and Patterns

    You should notice that our discussion on real-world objects focuses on patterns and not a specific instance. A car is a pattern. We can have many makes and models of a car. Jerri Anne and I have a Mini Cooper, a Toyota Tacoma, and a Honda Odyssey, each of which is a specific instance of a car. This makes car the pattern for stamping out specific instances.

    Software Objects

    Software objects are similar to objects in the world. In fact, sofware objects often model objects in the world. Software objects have state and behavior. The state is captured in variables of the software object and the behavior is captured in methods of the object. The methods of a software object manipulate the variables of the software object. Many of the software object methods change the state of the object. Consider a car software object. We may have variables that contain the following.

    • Car make
    • Car model
    • Car year
    • Miles per gallon
    • Gas in tank
    • Max number of passengers
    • Odometer
    • Current number of passengers
    • Passenger in the car

    We may have car methods that perform the following.

    • Drive the car X miles.
      • This will add miles to the odometer and subtract gas from the tank
    • Pickup a passenger named P.
      • This adds the passenger to the car as long as current number of passengers is less than max number of passengers.
    • Get the passengers
      • This returns all of the passengers in the car.

    The variables and methods of an object are called instance variables and methods. Each object is an instance of a class (see next section). Each instance has variables and methods.

    Java Classes

    Java classes are the mechanism for defining types, which are used to declare variables. The class is a blueprint that can be used to stamp our objects, and variables reference objects. We already used predefined classes to declare variables that reference objects. The following are examples of our use.

    String gusty = "Gusty";
    Scanner in = new Scanner(;

    At this point, we have mimicked these programming techniques. In this module, we learn exactly what is happening, including how to define and use our own classes.

    Reference Types

    Using a Java class to define a type creates a reference type. This is because variables of a reference type reference an object. The concept of a reference type will become clearer as we continue our study.

    We now have two categories of Java types.

    • primitive types - types defined by the Java language, e.g., int, double, char, boolean
    • reference types - types defined by users within a Java class, e.g., String, Scanner

    Data Type Definition and Java Classes

    In Primitive Types we studied variables and data types. Let us review that material in the context of reference types and objects.

    A computer is a machine that stores and manipulates information under the control of a changeable program. A data type defines the characteristics of information we want to manipulate. We use data types to declare variables, which contain the information we manipulate. In Java variables must be declared to be a specific type, and the declaration must happen before you can use a variable.

    A variable has the following

    1. a name
    2. a data type
    3. a value
    4. memory locations

    A data type is a set of values and a set of operations.

    • byte b;
      • set of values is {-128, -127, ..., 0, 1, ... 127}
      • set of operations is {+, -, *, /, %}
    • Car c;
      • set of values is defined by the state
      • set of operations is defined by the behavior

    Way Forward for This Module

    In the next two sections, we return to String and Scanner - two Java provided reference types we have already used. Instead of mimicking techniques to declare variables of types String and Scanner, we disect what is actually happening. Then we discuss Random, which is another Java provided reference type that can be used to generate random numbers. Then we study how to create and use our own reference types. Finally, we study how to document our reference types with JavaDoc.

    Reference Type Naming Convention

    Reference types, by convention, begin with an uppercase letter, e.g., String and Scanner.

    Simple Objects Lab

    The overview lab for simple objects is Simple Objects.

    Tags: object class