Java中一些重要的核心特性：

Android relies heavily on these Java fundamentals. The Android SDK includes many standard Java libraries (data structure libraries, math libraries, graphics libraries, networking libraries and everything else you could want) as well as special Android libraries that will help you develop awesome Android applications.

Android对这些Java基本原理依赖性很强。Android SDK包含很多标准的Java库文件（数据结构库，数学库，图形库，网络库和任何你想要使用的库）和一些特殊的Android库，以便于帮助你开发出漂亮的应用程序。

为什么Java易学（Why is Java Easy to Learn?）

Java is easy to learn for a variety of reasons. There’s certainly no shortage of Java resources out there to help you learn the language, including websites, tutorials, books, and classes. Java is one of the most widely discussed, taught, and used programming languages on the planet. It’s used for many different types of programming projects, no matter their scale, from web applications to desktop applications to mobile applications.

Java易学是由很多原因的。这些Java资源在帮助你学习这门语言中是不可或缺的，包括网站、教程、书籍和课程。Java是世界上讨论、教授和使用最多的编程语言。Java被用于很多不同类型的编程项目，无论这些项目的范围，从web应用程序到桌面应用程序，再到移动应用程序。

If you’re coming from a traditional programming background like C or C++, you’ll find Java syntax quite similar. If you’re not, then take comfort in knowing that you’ve chosen one of the easiest languages to learn. You’ll be up and running in no time at all.

如果你有传统的编程语言的经验，例如C和C++，你会发现Java的语法与其很相似。如果你没有相关经验，那么放松下来，要知道你选择的Java是最简单的编程语言之一。你会学会并且很快进入到状态中。

Finally, Java is one of the most human-readable languages out there, by which we mean that a person who knows nothing about programming can often look at some Java code and have at least an inkling what it’s doing. Consider the following example:

最后，Java是最易于人阅读的编程语言之一，言下之意就是没有任何编程经验的人也可以看懂Java代码并且也能大概理解这些代码的作用。考虑下面的例子：

char character = 'a';
if(character=='a')
{
    doSomething();
} else {
    doSomethingElse();
}

public class Fish {
    private String mSpecies;
    private String mColor;
    private int mSize;
    Fish() {
        // generic fish
        mSpecies = "unknown";
        mColor = "unknown";
        mSize = 0;
    }
    Fish(String species, String color, int size) {
        mSpecies = species;
        mColor = color;
        mSize = size;
    }
    public void eat() {
        // eat some algae
    };
    public void eat(Fish fishToEat) {
        // eat another fish!
    };
    public void sleep() {
        // sleep
    };
    public void makeBabyFish(Fish fishSpouse, int numBabies) {
        // Make numBabies worth of baby fish with Fish spouse
    };
}

构造方法是一种特殊类型的方法，用于创建和初始化一个对象；构造方法的名字与所在的类一致并且还包含一些参数。下面的Fish类中包含两个构造方法：一个用来创建一个普通的Fish对象，另一个用来构造一个具有一些初始化数据特定的Fish对象。你可以看到Fish类中有两个eat（）方法：一个作用是随机吃东西，另一个是吃其他的鱼，其可以被其他的Fish类实例所代表。

public class Fish {
    private String mSpecies;
    private String mColor;
    private int mSize;
    Fish() {
        // generic fish
        mSpecies = "unknown";
        mColor = "unknown";
        mSize = 0;
    }
    Fish(String species, String color, int size) {
        mSpecies = species;
        mColor = color;
        mSize = size;
    }
    public void eat() {
        // eat some algae
    };
    public void eat(Fish fishToEat) {
        // eat another fish!
    };
    public void sleep() {
        // sleep
    };
    public void makeBabyFish(Fish fishSpouse, int numBabies) {
        // Make numBabies worth of baby fish with Fish spouse
    };
}

Classes can be organized into hierarchies, where a derived class (or subclass) includes all the features of its parent class (or superclass), but refines and adds to them to define a more specific object using the extends keyword. This is called inheritance.

类可以被组织进不同的层级，衍生类（或者子类）包含父类（或者超类）的所有特性，但是定义一个更加高效的对象通过使用改善和添加扩展字来实现。这就是继承。

For example, the Fish class might have two subclasses: FreshwaterFish and SaltwaterFish. These subclasses would have all the features of the Fish class, but could further customize the objects through new attributes and behaviors or modified behaviors from the parent class Fish. For example, the FreshwaterFish class might include information about the type of freshwater environment lived in (e.g. river, lake, pond, or puddle). Similarly, the SaltwaterFish class might customize the makeBabyFish() method such that the fish eats its mate after breeding (as defined in the super class) by using the override mechanism, like this:

public class SaltwaterFish extends Fish
{
    @Override
    public void makeBabyFish(Fish fishSpouse, int numBabies) {
        // call parent method
        super.makeBabyFish(fishSpouse, numBabies);
        // eat mate
        eat(fishSpouse);
    }
}

比如说，Fish类有两个子类：FreshwaterFish和SaltwaterFish。这两个子类有Fish类的所有特性，但是可以通过添加新的属性和行为或者重新定义Fish父类的行为来自定义对象。例如，FreshwaterFish类中包含淡水环境的信息（例如，河流，湖泊，池塘或者水潭）。与此相似的是，SaltwaterFish类自定义makeBabyFish（）方法，即鱼儿在繁殖后吃了同伴，通过使用override机制，如下述代码：

public class SaltwaterFish extends Fish
{
    @Override
    public void makeBabyFish(Fish fishSpouse, int numBabies) {
        // call parent method
        super.makeBabyFish(fishSpouse, numBabies);
        // eat mate
        eat(fishSpouse);
    }
}

通过接口组织对象的行为（Organizing Object Behavior with Interfaces）

In Java, you can organize object behaviors in what are called interfaces. While a class defines an object, an interface defines some behavior that can be applied to an object. For example, we could define a Swimmer interface that provides a set of methods that are common across all objects that can swim, whether they are fish, otters, or submergible androids. The Swimmer interface might specify four methods: startSwimming(), stopSwimming(), dive(), and surface().

public interface Swimmer
{
    void startSwimming();
    void stopSwimming();
    void dive(); 
    void surface(); 
}

public class Fish implements Swimmer {
    // Provide implementations of the four methods within the Swimmer interface 
}

在Java中，你可以通过使用接口来组织对象行为。虽然类定义了一个对象，但是接口定义的一些行为也可以应用于对象。例如，我们可以定义一个名为Swimmer的接口，这个接口提供了一系列的方法来实现所有对象都具有游泳的行为，无论是鱼，水獭还是安卓潜艇。Swimmer接口指定了四个方法：startSwimming(), stopSwimming(), dive(), and surface()。

public interface Swimmer
{
    void startSwimming();
    void stopSwimming();
    void dive(); 
    void surface(); 
}

像Fish这样的类可以实现Swimmer接口并且提供游泳行为的实现：

public class Fish implements Swimmer {
    // Provide implementations of the four methods within the Swimmer interface 
}

通过包组织类和接口（Organizing Classes and Interfaces with Packages）

Class hierarchies, such as our fish classes, can then be organized into packages. A package is simply a set of classes and interfaces, bundled together. Developers use namespaces to uniquely name packages. For example, we could use com.mamlambo.aquarium or com.ourclient.project.subproject as our package name to keep track of our fish-related classes.

类的层级，比如我们的fish类们，可以使用packages来组织起来。包就是一系列类和接口简单的集合在一起。开发者使用命名空间（namespaces）来命名不同的包。例如，我们可以使用com.mamlambo.aquarium或者com.ourclient.project.subproject来作为包名来追踪我们的与鱼相关的类。

Wrapping Up

Wow! You’ve just embarked on a crash-course in Java for Android development. We’ve covered a pretty intense amount of material here, so let things settle for a bit before moving on to the next lesson of this tutorial series. In Lesson 2, we switch our focus to the nitty-gritty details of Java syntax.

哇!你刚刚完成了Java for Android development的课程。我们讨论了很多重要的知识，因此在进入下一节课程之前先把本课的内容吸收一下。在第二节中，我们将注意力转移到Java语法上来。

You’ve only scratched the surface of Java development for Android development. Check out all the other great tutorials on Mobiletuts+ to dive deeper into Java and Android development. Good luck!

你仅仅是接触了Android开发中需要的Java知识的皮毛。查看Mobiletuts+上其他好的教程来深入学习Java和Android开发。祝你好运！

About the Authors Mobile developers Lauren Darcey and Shane Conder have coauthored several books on Android development: an in-depth programming book entitled Android Wireless Application Development and Sams TeachYourself Android Application Development in 24 Hours. When not writing, they spend their time developing mobile software at their company and providing consulting services. They can be reached at via email to androidwirelessdev+mt@gmail.com, via their blog at androidbook.blogspot.com, and on Twitter @androidwireless.

关于作者 Mobile developers Lauren Darcey 和 Shane Conder合著了一些有关Android开发的书籍：《深入学习Android无线程序开发》和《Sams24小时自学Android开发》。在写书之前，他们在他们公司做移动软件开发。你可以联系他们通过邮件：androidwirelessdev+mt@gmail.com，通过他们的博客androidbook.blogspot.com,或者通过Twitter @androidwireless.

原文地址：http://code.tutsplus.com/tutorials/java-tutorial—mobile-2604

译者：chiahaolu 校对：校对者ID

本文由AWCNTT 原创翻译，AndroidWeekly中国 荣誉推出

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