Python开发者转Java指南：从HelloWorld到类与数据结构

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"《Java for Python程序员》是一本专门为Python开发者设计的Java入门教程，旨在帮助那些已经熟悉Python语言的开发者快速过渡到Java领域。此书在2015年发布第三版，由Brad Miller和James Riely共同编著。本书内容详实，结构清晰，从基础概念到高级特性都有涉及。首先，作者通过介绍Java与Python这两种语言的对比，阐述了Java的动态与静态类型的特点，帮助读者理解Java的优势和适用场景，如处理大数据和面向对象编程等。在'HelloWorld'示例中，读者可以学习如何在Java中创建和运行简单的程序，了解程序的基本构成。接着，书中详细讲解了Java的基础知识，包括基本数据类型（如int、float等）、字符串操作、类型转换，以及如何进行温度单位（华氏转摄氏）的计算。这部分内容强调了实践中常见的错误和命名规范，同时推荐在线查找Java文档资源，以便深入学习。随着内容的深入，章节转向列表（List）和映射（Map）的数据结构，讲解了数值和字符串类型的列表操作，数组和字典的使用，这些都是数据处理和编程中必不可少的部分。这些章节展示了Java如何组织和管理数据，提升代码效率。进入核心部分，书中探讨了类和对象的概念，如何定义类、编写构造函数，以及方法（成员函数）的实现。接着，作者解释了继承、接口和多态性在Java中的应用，这些都是面向对象编程的核心概念。静态成员变量和静态方法的介绍让读者了解到Java中不同类型的成员的使用场景。此外，书中的一个完整示例——Fraction类的实现，展示了如何在实际项目中构造和使用类，提供了实用的代码参考。全书还包括索引和PDF版本以及源代码供读者查阅和实践。《Java for Python程序员》是一本帮助Python开发者系统学习Java语言的实用指南，旨在帮助他们在扩展技能栈的同时，能够无缝地将Python的经验迁移到Java的世界中。无论你是想进一步掌握大数据处理还是想在算法工程领域尝试新挑战，这本书都是一个理想的起点。"

CHAPTER

TWO

GETTING STARTED

2.1 Why Java? Dynamic versus Static Languages

Python is a nice language for beginning programming for several reasons.

• The syntax is sparse and clear.

• The underlying model is very simple. Everything is an object.

• You can write powerful and interesting programs without a lot of work.

Python is representative of one kind of language, called a dynamic language. Dynamic languages can be interpreted

directly, which means that the actual text of the program — the source code — is used while the program is running.

In contrast, a static language is executed in two phases: ﬁrst the program is translated from source code to binary

code, and then the binary code is interpreted. Although the terms dynamic and static language are widely used, the

distinction is a fuzzy one. Most execution engines do both translation and interpretation.

• Static refers to what the translater does. The translater is called a compiler.

• Dynamic refers to what the interpreter does.

Dynamic languages do more in the interpreter. By doing more in the interpreter, dynamic languages have more

ﬂexibility. By doing more in the compiler, static languages gain their own advantages.

• Compiled languages are fast. They gain speed in at least two ways:

First, by distinguishing objects from builtin types — aka base types. In order to do basic arithmetic, a dynamic

language may need to convert from an object to base type and then back; a static language avoids the conversions.

Second, by performing name lookups in advance. Dynamic languages use the name of a method (or a de-

rived selector) to look up the method during interpretation. Compilation makes this much more efﬁcient, often

resolving the actual method in advance.

• Compiled languages are excellent for writing large programs that are maintained over time. Python and other

scripting languages require that the programmer keep track of a lot of information. For example if you set vari-

able x to reference a turtle, and forget later that x is a turtle but try to invoke a string method on it, you will

get an error. Compiled languages track this information using types for variables. This means that program-

mers need to keep track of less information informally, reducing the chance of miscommunication between to

programmers.

In addition compiled languages support a notion of hiding. Java allows you to declare some data to be private,

meaning that it is only accessible by certain method and not others. Python uses naming conventions for the

same thing; variables starting with __ are meant to be ignored by most methods, but there is nothing that

enforces this.

Python is representative of a whole class of languages, sometimes referred to as scripting languages. Other languages

in the same category as Python are Ruby and Perl. Java is representative of what I will call industrial strength

java4python, Release 3.0

languages, which include C++, C# and Scala. Industrial strength languages are good for projects with several people

working on the project where being formal and careful about what you do may impact lots of other people.

Why Java?

• Java is the most widely taught programming language.

• Java is one of the most widely used programming languages.

• Java is industrial strength used for large systems by large groups of people

• Java is simpler than C++ or Objective-C.

Although Java is an enormous language, the core language is very small. The features that we will use are common to

every static language. If you want to program in C++ or Objective-C, you should learn Java ﬁrst.

2.2 Hello World

A time honored tradition in Computer Science is to write a program called “hello world.” The “hello world” program

is simple and easy. There are no logic errors to make, so getting it to run relies only on understanding the syntax. To

be clear lets look a a “complicated” version of hello world for Python:

def main():

print "Hello World!"

Remember that we can deﬁne this program right at the Python command line and then run it:

>>> main()

"Hello World!"

>>>

Now lets look at the same program written in Java:

1 public class Hello {

2 public static void main(String[] args) {

3 System.out.println("Hello World!");

4 }

5 }

What we see is that at the core there are a few similarities, such as a main and the string “Hello World” However there

is a lot more stuff around the edges that make it harder to see the core of the program. Do not worry! An important

skill for a computer scientist is to learn what to ignore and what to look at carefully. You will soon ﬁnd that there are

some elements of Java that will fade into the background as you become used to seeing them. One thing that will help

you is to learn a little bit about Java Naming Conventions.

2.3 Running from the command line

The ﬁrst question you probably have about this little program is “How do I run it?” Running a Java program is not as

simple as running a Python program. The ﬁrst thing you need to do with a Java program is compile it. First we must

type the hello world program into a ﬁle and save that ﬁle using the name Hello.java The ﬁle name must be the

same as the public class you deﬁne in the ﬁle. Once we have saved the ﬁle we compile it from the command line as

follows:

1 $ ls -l Hello.

2 -rw-r--r-- 1 bmiller bmiller 117 Jul 19 17:46 Hello.java

3 $ javac Hello.java

4 $ ls -l Hello.

6 Chapter 2. Getting Started

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