2. Defining and Using Classes 2. 定义和使用类

If you do not have prior Java experience, we recommend that you work through the exercises in HW0 before reading this chapter. It will cover various syntax issues that we will not discuss in the book.
如果您没有 Java 经验，我们建议您在阅读本章之前完成HW0中的练习。它将涵盖我们在书中不会讨论的各种语法问题。

Static vs. Non-Static Methods
静态方法与非静态方法

Static Methods 静态方法

All code in Java must be part of a class (or something similar to a class, which we'll learn about later). Most code is written inside of methods. Let's consider an example:
所有 Java 代码必须属于一个类（或类似类的结构，我们将在后面学习）。大多数代码都写在方法内部。让我们来看一个例子：

public class Dog {
    public static void makeNoise() {
        System.out.println("Bark!");
    }
}

If we try running the Dog class, we'll simply get an error message:
如果我们尝试运行Dog类，我们只会收到一个错误信息：

$ java Dog
Error: Main method not found in class Dog, please define the main method as:
       public static void main(String[] args)

The Dog class we've defined doesn't do anything. We've simply defined something that Dog can do, namely make noise. To actually run the class, we'd either need to add a main method to the Dog class, as we saw in chapter 1.1. Or we could create a separate DogLauncher class that runs methods from the Dog class. For example, consider the program below:
我们定义的Dog类什么也不做。我们只是定义了Dog可以做的事情，即发出声音。要实际运行这个类，我们需要在Dog类中添加一个主方法，正如我们在第 1.1 章中所见。或者，我们可以创建一个单独的DogLauncher类来运行Dog类中的方法。例如，考虑下面的程序：

public class DogLauncher {
    public static void main(String[] args) {
        Dog.makeNoise();
    }
}

$ java DogLauncher
Bark!

A class that uses another class is sometimes called a "client" of that class, i.e. DogLauncher is a client of Dog. Neither of the two techniques is better: Adding a main method to Dog may be better in some situations, and creating a client class like DogLauncher may be better in others. The relative advantages of each approach will become clear as we gain additional practice throughout the course.
使用另一个类的类有时被称为该类的“客户端”，例如 DogLauncher 是 Dog 的客户端。这两种技术没有优劣之分：在某些情况下，向 Dog 添加 main 方法可能更好，而在其他情况下，创建一个像 DogLauncher 这样的客户端类可能更合适。随着我们在课程中不断练习，每种方法的相对优势将会变得更加清晰。

Instance Variables and Object Instantiation
实例变量和对象实例化

Not all dogs are alike. Some dogs like to yap incessantly, while others bellow sonorously, bringing joy to all who hear their glorious call. Often, we write programs to mimic features of the universe we inhabit, and Java's syntax was crafted to easily allow such mimicry.
并非所有的狗都一样。有些狗喜欢不停地吠叫，而另一些狗则深沉地吼叫，给所有听到它们美妙叫声的人带来欢乐。通常，我们编写程序来模仿我们所居住的宇宙的特征，而 Java 的语法正是为了方便这种模仿而设计的。

One approach to allowing us to represent the spectrum of Dogdom would be to create separate classes for each type of Dog.
一种允许我们表现狗类多样性的方法是为每种类型的狗创建单独的类。

public class TinyDog {
    public static void makeNoise() {
        System.out.println("yip yip yip yip");
    }
}

public class MalamuteDog {
    public static void makeNoise() {
        System.out.println("arooooooooooooooo!");
    }
}

As you should have seen in the past, classes can be instantiated, and instances can hold data. This leads to a more natural approach, where we create instances of the Dog class and make the behavior of the Dog methods contingent upon the properties of the specific Dog. To make this more concrete, consider the class below:
正如您在过去所看到的，类可以被实例化，实例可以持有数据。这导致了一种更自然的方法，我们创建狗类的实例，并使狗方法的 behavior 取决于特定狗的属性。为了更具体地说明这一点，请考虑下面的类：

public class Dog {
    public int weightInPounds;

    public void makeNoise() {
        if (weightInPounds < 10) {
            System.out.println("yipyipyip!");
        } else if (weightInPounds < 30) {
            System.out.println("bark. bark.");
        } else {
            System.out.println("woof!");
        }
    }    
}

As an example of using such a Dog, consider:
作为使用此类“狗”的示例，请考虑：

public class DogLauncher {
    public static void main(String[] args) {
        Dog d;
        d = new Dog();
        d.weightInPounds = 20;
        d.makeNoise();
    }
}

When run, this program will create a Dog with weight 20, and that Dog will soon let out a nice "bark. bark.".
当运行此程序时，将创建一个重量为 20 的狗，并且该狗很快会发出一声悦耳的“汪汪。汪汪。”。

Some key observations and terminology:
一些关键观察和术语：

• An Object in Java is an instance of any class.
  Java 中的Object是任何类的实例。

• The Dog class has its own variables, also known as instance variables or non-static variables. These must be declared inside the class, unlike languages like Python or Matlab, where new variables can be added at runtime.
  Dog类拥有自己的变量，也称为实例变量或非静态变量。这些变量必须在类内部声明，与 Python 或 Matlab 等语言不同，后者可以在运行时添加新变量。

• The method that we created in the Dog class did not have the static keyword. We call such methods instance methods or non-static methods.
  我们在Dog类中创建的方法没有使用static关键字。我们称这样的方法为实例方法或非静态方法。

• To call the makeNoise method, we had to first instantiate a Dog using the new keyword, and then make a specific Dog bark. In other words, we called d.makeNoise() instead of Dog.makeNoise().

• Once an object has been instantiated, it can be assigned to a declared variable of the appropriate type, e.g. d = new Dog();

• Variables and methods of a class are also called members of a class.

• Members of a class are accessed using dot notation.

Constructors in Java

As you've hopefully seen before, we usually construct objects in object oriented languages using a constructor:

public class DogLauncher {
    public static void main(String[] args) {
        Dog d = new Dog(20);
        d.makeNoise();
    }
}

Here, the instantiation is parameterized, saving us the time and messiness of manually typing out potentially many instance variable assignments. To enable such syntax, we need only add a "constructor" to our Dog class, as shown below:

public class Dog {
    public int weightInPounds;

    public Dog(int w) {
        weightInPounds = w;
    }

    public void makeNoise() {
        if (weightInPounds < 10) {
            System.out.println("yipyipyip!");
        } else if (weightInPounds < 30) {
            System.out.println("bark. bark.");
        } else {
            System.out.println("woof!");
        }    
    }
}

The constructor with signature public Dog(int w) will be invoked anytime that we try to create a Dog using the new keyword and a single integer parameter. For those of you coming from Python, the constructor is very similar to the __init__ method.

Terminology Summary

Array Instantiation, Arrays of Objects

As we saw in HW0, arrays are also instantiated in Java using the new keyword. For example:

public class ArrayDemo {
    public static void main(String[] args) {
        /* Create an array of five integers. */
        int[] someArray = new int[5];
        someArray[0] = 3;
        someArray[1] = 4;
    }
}

Similarly, we can create arrays of instantiated objects in Java, e.g.

public class DogArrayDemo {
    public static void main(String[] args) {
        /* Create an array of two dogs. */
        Dog[] dogs = new Dog[2];
        dogs[0] = new Dog(8);
        dogs[1] = new Dog(20);

        /* Yipping will result, since dogs[0] has weight 8. */
        dogs[0].makeNoise();
    }
}

Observe that new is used in two different ways: Once to create an array that can hold two Dog objects, and twice to create each actual Dog.

Class Methods vs. Instance Methods

Java allows us to define two types of methods:

• Class methods, a.k.a. static methods.

• Instance methods, a.k.a. non-static methods.

Instance methods are actions that can be taken only by a specific instance of a class. Static methods are actions that are taken by the class itself. Both are useful in different circumstances. As an example of a static method, the Math class provides a sqrt method. Because it is static, we can call it as follows:

x = Math.sqrt(100);

If sqrt had been an instance method, we would have instead the awkward syntax below. Luckily sqrt is a static method so we don't have to do this in real programs.

Math m = new Math();
x = m.sqrt(100);

Sometimes, it makes sense to have a class with both instance and static methods. For example, suppose want the ability to compare two dogs. One way to do this is to add a static method for comparing Dogs.

public static Dog maxDog(Dog d1, Dog d2) {
    if (d1.weightInPounds > d2.weightInPounds) {
        return d1;
    }
    return d2;
}

This method could be invoked by, for example:

Dog d = new Dog(15);
Dog d2 = new Dog(100);
Dog.maxDog(d, d2);

Observe that we've invoked using the class name, since this method is a static method.

We could also have implemented maxDog as a non-static method, e.g.

public Dog maxDog(Dog d2) {
    if (this.weightInPounds > d2.weightInPounds) {
        return this;
    }
    return d2;
}

Above, we use the keyword this to refer to the current object. This method could be invoked, for example, with:

Dog d = new Dog(15);
Dog d2 = new Dog(100);
d.maxDog(d2);

Here, we invoke the method using a specific instance variable.

Exercise 1.2.1: What would the following method do? If you're not sure, try it out.

public static Dog maxDog(Dog d1, Dog d2) {
    if (weightInPounds > d2.weightInPounds) {
        return this;
    }
    return d2;
}

Static Variables

It is occasionally useful for classes to have static variables. These are properties inherent to the class itself, rather than the instance. For example, we might record that the scientific name (or binomen) for Dogs is "Canis familiaris":

public class Dog {
    public int weightInPounds;
    public static String binomen = "Canis familiaris";
    ...
}

Static variables should be accessed using the name of the class rather than a specific instance, e.g. you should use Dog.binomen, not d.binomen.

While Java technically allows you to access a static variable using an instance name, it is bad style, confusing, and in my opinion an error by the Java designers.

Exercise 1.2.2: Complete this exercise:

• Video: link

• Slide: link

• Solution Video: link

public static void main(String[] args)

With what we've learned so far, it's time to demystify the declaration we've been using for the main method. Breaking it into pieces, we have:

• public: So far, all of our methods start with this keyword.

• static: It is a static method, not associated with any particular instance.

• void: It has no return type.

• main: This is the name of the method.

• String[] args: This is a parameter that is passed to the main method.

Command Line Arguments

Since main is called by the Java interpreter itself rather than another Java class, it is the interpreter's job to supply these arguments. They refer usually to the command line arguments. For example, consider the program ArgsDemo below:

public class ArgsDemo {
    public static void main(String[] args) {
        System.out.println(args[0]);
    }
}

This program prints out the 0th command line argument, e.g.

$ java ArgsDemo these are command line arguments
these

In the example above, args will be an array of Strings, where the entries are {"these", "are", "command", "line", "arguments"}.

Summing Command Line Arguments

Exercise 1.2.3: Try to write a program that sums up the command line arguments, assuming they are numbers. For a solution, see the webcast or the code provided on GitHub.

Using Libraries

One of the most important skills as a programmer is knowing how to find and use existing libraries. In the glorious modern era, it is often possible to save yourself tons of work and debugging by turning to the web for help.

In this course, you're welcome to do this, with the following caveats:

• Do not use libraries that we do not provide.

• Cite your sources.

• Do not search for solutions for specific homework or project problems.

For example, it's fine to search for "convert String integer Java". However, it is not OK to search for "Project 2048 Berkeley".

For more on collaboration and academic honesty policy, see the course syllabus.