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Classes

Course video 14 of 66

In this module, you will learn to write and run your first Java programs, including one program that prints “Hello!” in various countries’ languages and another where you will analyze the perimeters and other information of shapes. To accomplish these tasks, you will learn the basics of Java syntax and how to design stepwise solutions with programs. By the end of this module, you will be able to: (1) Download and run BlueJ, the Java programming environment for this course; (2) Access the documentation for the Java libraries specially designed for this course; (3) Edit, compile, and run a Java program; (4) Construct methods, variables, if else statements, and for each loops in Java; and (5) Use Iterables (like DirectoryResource) to run a program that iterates over multiples lines in a document or webpage or multiple files in a directory.

Java Programming: Solving Problems with Software

デューク大学（Duke University）

Object Oriented Programming in Java 専門講座における 4 の 1 コース

Learn to code in Java and improve your programming and problem-solving skills. You will learn to design algorithms as well as develop and debug programs. Using custom open-source classes, you will write programs that access and transform images, websites, and other types of data. At the end of the course you will build a program that determines the popularity of different baby names in the US over time by analyzing comma separated value (CSV) files. After completing this course you will be able to: 1. Edit, compile, and run a Java program; 2. Use conditionals and loops in a Java program; 3. Use Java API documentation in writing programs. 4. Debug a Java program using the scientific method; 5. Write a Java method to solve a specific problem; 6. Develop a set of test cases as part of developing a program; 7. Create a class with multiple methods that work together to solve a problem; and 8. Use divide-and-conquer design techniques for a program that uses multiple methods.

シラバスを表示

学習するスキル

Algorithms, Problem Solving, String (Computer Science), Java Programming

講師

Owen Astrachan
Professor of the Practice
Robert Duvall
Lecturer
Andrew D. Hilton
Associate Professor of the Practice
Susan H. Rodger
Professor of the Practice

字幕

As you continue learning how Java works, it is important to talk for

a bit about objects and classes.

Before we delve into the specific semantics, let's talk for

a minute about the high level concepts.

When you are writing a program, you often have data in the form of variables,

which store the values you are computing on.

And code,

when it manipulates that data according to the algorithm you have designed.

Object-oriented programming is a paradigm of programming languages that groups data,

and the code that manipulates it, together into logical units called objects.

This language design aims to help programmers think about their program

by grouping the code and data together into one logical unit.

As you write larger and larger programs, this principle becomes more and

more helpful.

As you progress and learn more about Java,

you will learn about a lot of important principles that go along with these ideas.

For now, however, we're just going to start with the basics.

Here, you can see an example of a class declaration.

A class is a template that specifies how to make objects.

Let's look at each piece of this declaration.

The first line tells Java that we are declaring a class, called Point.

As with variables,

we have a lot of freedom in what we name classes that we create.

But we should name them descriptevly.

Here, we are making a class which represents a two-dimensional point, so

Point is a good name for it.

Next we declare two fields, int x and int y.

Field is the name for a variable that is inside of an object.

They're also called instance variables since they are variables that are in each

instance of the objects created from the class.

These look like variable declarations except

that the word private comes before them.

Private means that only code inside of this class can directly manipulate

these fields.

You'll learn more about why that is important as you become more skilled in

Java, but for now we'll just make all fields private.

Next is the declaration of a constructor for the class.

A constructor specifies how to create objects of this class.

It is code that gets run when an object is created to initialize that object.

Note that the constructor looks like a function but has no return type.

And is named the same as the class.

These are the hallmarks of a constructor declaration.

In front of the constructor, we have the word public,

which means that any code can use this constructor to create a point.

After the constructor, we have three methods, getX, getY and distance.

Methods are functions that are inside of classes.

In Java everything is inside of a class, so

technically all functions in Java are methods.

These methods are invoked or called on a particular object and

implicitly act on that object.

You can see a method call here where the code says otherPt.getX.

This calls the get X method on the other point object.

It will get the X of that particular point object.

Last, we have the Declaration of a static method.

These behave a little differently from regular methods.

They don't act on any particular instance of a class,

they just belong to the class in general.

That concept is a little tricky and we'll explain it more later.

This method is called main, which is a special method.

If you run your programs outside of Blue Jay, main is the starting point.

Execution begins in main before any objects are even created.

The ideas of objects are supposed to help programmers think about their data

in terms of objects that make logical sense.

For example, if we make a new point, we are creating an object which represents

something we can concretely think about, in this case a point on a plane.

We can then make another point which has its own X and Y and represents

a different instance of the same type of thing, another point on the plane.

You can, of course, create as many of an object as you need for your algorithm.

Once you have some objects, you can invoke methods on them, such as p1.distance(p3),

which you can think of as asking p1 to compute the distance to p3.

That is, you can think of this line of code as saying p1

go figure out how far you are from p3.

You can think of the code that executes for

this method call as logically belonging to the p1 object.

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