Relationships Between Classes

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From the course by Duke University

Java Programming: Principles of Software Design

599 ratings

Duke University

Java Programming: Principles of Software Design

599 ratings

Course 4 of 5 in the Specialization Java Programming and Software Engineering Fundamentals

Solve real world problems with Java using multiple classes. Learn how to create programming solutions that scale using Java interfaces. Recognize that software engineering is more than writing code - it also involves logical thinking and design. By the end of this course you will have written a program that analyzes and sorts earthquake data, and developed a predictive text generator. After completing this course, you will be able to: 1. Use sorting appropriately in solving problems; 2. Develop classes that implement the Comparable interface; 3. Use timing data to analyze empirical performance; 4. Break problems into multiple classes, each with their own methods; 5. Determine if a class from the Java API can be used in solving a particular problem; 6. Implement programming solutions using multiple approaches and recognize tradeoffs; 7. Use object-oriented concepts including interfaces and abstract classes when developing programs; 8. Appropriately hide implementation decisions so they are not visible in public methods; and 9. Recognize the limitations of algorithms and Java programs in solving problems. 10. Recognize standard Java classes and idioms including exception-handling, static methods, java.net, and java.io packages.

From the lesson

Earthquakes: Programming and Interfaces

In this module, we will introduce a data set containing details about earthquakes around the world. You will learn how to pull this data into a program, search through the data, and filter the data based on desired criteria. By the end of this module, you will be able to (1) write programs that include multiple classes and ArrayLists of class types, (2) find the maximum value in an ArrayList, (3) use a Filter interface to search through data, (4) implement interfaces with method signatures, and (5) combine several filters together.

Introduction7:12

Relationships Between Classes4:47

Licensing and APIs3:08

Coding a Magnitude Filter10:32

Coding the Closest Quakes12:12

Meet the Instructors

Robert Duvall
Lecturer
Computer Science
Owen Astrachan
Professor of the Practice
Computer Science
Andrew D. Hilton
Assistant Professor of the Practice
Electrical and Computer Engineering
Susan H. Rodger
Professor of the Practice
Computer Science

Hi in this lesson we're going to talk about a few concepts related

to how classes are used together in object oriented programming in general, and

specifically when using Java and

the earthquake programs and classes you'll see.

The QuakeEntry Class created in QuakeEntry.Java.

Encapsulates the basic characteristics of one earthquake.

Where the quake occurred, the quake's magnitude and depth, and

a title for the quake.

This makes QuakeEntry what's called a POJO, or Plain Old Java Object.

This means a QuakeEntry object is not much more than the characteristics of

the earthquake.

Creating a QuakeEntry object requires providing all of these characteristics.

For example, it doesn't make sense to talk about an earthquake

without knowing where it occurred or how strong it is.

It's not possible to creat a QuakeEntry object without supplying

all the parameters of the constructor.

Which makes this different than what some consider a Plain Old Java Object or POJO.

As we'll see QuakeEntry objects are created when data for

earthquakes is red and parched.

Creating a QuakeEntry object requires supplying all the information for a quake.

The latitude, the longitude, the magnitude, the tile and the depth.

A QuakeEntry object represents the data for

an earthquake that has occurred somewhere in the world.

So it's immutable and doesn't change once constructed.

Accessing the state of an QuakeEntry object is done using

getter methods as you see here.

Get location returns the location of a quake.

Get depth returns the depth of the quake and so on.

A QuakeEntry object also has a reasonable .toString method

to help with printing information about an earthquake.

An earthquake occurs at a specific location, and

so we'll use a location class for that.

Location classes have many uses outside of the QuakeEntry class we're studying here.

Smartphones use locations.

Web browsers use locations and many other applications do as well.

We have a choice between creating a location class that's simple

that we could use and study for this course or we could use

an industrial strength class that would be tested and useful in other contexts.

We've adopted the location class from the Android platform.

This gives us the best of several options.

We can be confident that the class is well tested and

the licensing of the source code allows us to use and adapt the code as we need to.

We'll see more about this later.

Location objects are created from a latitude and a longitude.

These specify one location on Earth.

Typically these values might come from your phone

which get data from GPS satellites or

your web browser which can determine location based on your IP address.

The Android location class has a dot distance to method

to determine the distance between any two locations A and B.

This will allow you to find earthquakes close to where you live.

This makes the location class more than a Plain Old Java Object or POJO.

It has state, the latitude, and the longitude, but

it also has behavior, a method to determine the distance between locations.

Classes in Java can have both Has-A, and Uses-A relationships.

The earthquake partial class creates QuakeEntry objects when data is read

and parsed.

This BlueJ class diagram shows how the parser uses QuakeEntry objects by

creating them.

A QuakeEntry object also creates a location object as seen in the diagram.

The QuakeEntry class creates a location object by passing the latitude and

longitude to the location object.

This creates a new location object using this data.

The location object created is stored in instance field myLocation.

When one class Has-A or contains another class, we call this a Has-A relationship.

The location class implements comparable, we'll discuss this at another lesson.

As you can see the BlueJ diagram,

the earthquake client class also uses a location object.

Locations are used to determine which earthquakes are closest to where you live,

or to where I live.

We'll see this in the coding demo.

The location .distanceTo method helps in writing this code.

A QuakeEntry object is also used by the EarthQuakeClient class.

For example, this allows client programs to find all quakes of high magnitude

using the getMagnitude method of the QuakeEntry class.

Happy programming.

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