Functions

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来自 Duke University 的课程

Java 程序设计：使用软件解题

2645 个评分

Duke University

Java 程序设计：使用软件解题

2645 个评分

课程 1（共 4 门，Specialization Object Oriented Programming in Java）

Building upon Duke’s “Programming and the Web for Beginners” course, you will grow in your ability to be a problem-solver and programmer by writing Java programs. We are excited that you've chosen to learn Java with us! You will soon be able to solve problems that you find interesting and useful by practicing a design process you will learn in this course. Java concepts presented in this course include basic control flow; object-oriented concepts including classes and methods; custom, open-source classes; and a design-implement-debug process that you can continue to leverage as you build your experience and knowledge of programming principles. The custom, open-source classes will provide a consistent application programming interface (API) so that you can write programs that access and transform images, websites, comma separated value (CSV) files and other data files using simple and accessible coding idioms built on the Java Iterable interface. You will learn to design, test, and debug programs and to find and read Java documentation that can serve as the basis for continued study of Java and programming. After completing this course you will be able to: 1. Edit, compile, and run a Java program; 2. Use conditionals and for loops appropriately in a Java program; 3. Use Java API documentation in writing programs. 4. Debug a Java program using the scientific method; 5. Develop 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 more than one method.

从本节课中

Fundamental Java Syntax and Semantics

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.

Conditionals3:25

与讲师见面

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

The next important idea to understand is function calls.

Functions extract a computation out, giving it a name and parameters.

You then can use the function to perform that computation without rewriting it.

You can also think about what the function does and not how it does it.

Technically speaking, Java doesn't have functions.

It has methods since all code in Java is inside of objects.

However, before we learn the more complex behavior of objects and methods,

we'll learn the basic principles of function calls.

These concepts will then lay the foundation for understanding method calls.

We have three functions here.

My function, f and g.

Why are we starting at g?

We'll assume for now that you chose to call g from the BlueJ interface.

Later, you'll learn about the main method which is where programs start when you run

them outside of BlueJ.

We start with a frame for g and

with the execution right at the start of that function.

The first line declares a, so we create it's box inside of the frame for g.

Next, we're going to set a equal to the value of the function call

myFunction(3, 7).

To evaluate this expression,

we need to create a frame of the function that we're calling.

In this case, myFunction.

This will hold the parameters and variables of myFunction.

Next, we pass the parameters to myFunction.

We create a box for

each parameter with the names coming from the function declaration, x and y.

We initialize these by copying in the values of the expressions that were

passed.

Here, 3 and 7.

Next, we need to note where to return when we finish executing my function.

This location in the code is named the call site.

The place where the function was called.

We'll note it with a marker one in the code and

put the same marker in the corner of the frame.

Finally, we move the execution arrow into myFunction and start executing code there.

Here, we declared an initial of z.

Evaluating the expression 2 times x minus y.

The values for x and y come from the frame from myFunctions, 3 and 7 respectively.

So, z will be -1.

Now, we have reached a return statement.

Return statements tell us to leave the correct function,

returning to the call site noted in the frame.

They also tell us the value to return to the caller.

The first thing we need to do is evaluate this expression

to obtain the return value.

Here, the expression is z times x.

So we evaluate -1x3 and we get -3.

Next, find where we should return.

This is the call site we noted in the frame,

then we copy the return value back to the call site.

The function call evaluates to this return value.

We move the execution arrow back to the call site and destroy the frame for

the function we just returned from.

Now, we're back in g.

The call to my function evaluated to -3.

So, this line behaves like a gets -3.

We'll finish that assignment, putting -3 in its box.

Our next line again has a variable declaration and a function call.

We make a box for b and go through the same process to call f.

You make a frame for f and past parameters.

This time, there's one parameter n, whose value is a times a which is 9.

We know where to return and begin executing code inside of f.

Our next statement is a return statement, but

the expression involves a function call.

So, we have to evaluate that call before anything else.

We start with a frame and pass parameters.

X gets a value of n, which is 9.

And y gets a value of n+1, which is 10.

We know the call site will use two this time since we are already using one

somewhere else and move the executioner to the start of myFunction, and

start executing code there.

We declare z and initialize it to a.

Now, we are ready to return from my function.

We evaluate z times x which is 72,

then we find the call site noted in the frame and copy the return value 3.

Finally, we move our execution arrow back to the call site.

Destroying the frame from myFunction.

Now, we pick up where we left off in f using 72 for

the value of the call to myFunction.

We evaluate 3 plus 72 to get 75.

Since we're evaluating the return statement, this is the return value of f.

So we find the call site and copy the return value there,

then we return to that call site.

Destroying the frame for f.

Now, we can finish the initialization of b.

B gets 75.

Lastly, we reach the return statement for g which is where we started.

When we return from the function where we started, we are done.

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