blocks|key|450849|text|为什么我不能在不创建类的新对象的情况下从main调用这些方法和函数？|type|blockquote|depth|inlineStyleRanges|entityRanges|data|450850|因为他们不是static|unstyled|offset|length|style|CODE|450851|您必须创建一个实例才能使用它们。添加修饰符，这样您就可以访问它们。例如,|450852|private+static+getSomething()|code-block|syntax|javascript|450853|然后，从main调用它如下：|450854|getSomething()|450855|也要注意全能者JonSkeet在评论中说的话。|450856|entityMap^0|0|6|6|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Y|8|@$E|Z|F|10|G|H]]|9|@]|A|$]]|$1|I|3|J|5|D|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|M|7|12|8|@]|9|@]|A|$N|O]]|$1|P|3|Q|5|D|7|13|8|@]|9|@]|A|$]]|$1|R|3|S|5|M|7|14|8|@]|9|@]|A|$N|O]]|$1|T|3|U|5|D|7|15|8|@]|9|@]|A|$]]|$1|V|3|-4|5|D|7|16|8|@]|9|@]|A|$]]]|W|$]]

<blockquote>
 Why can I not call those methods and functions from the main without creating a new object of the class?
</blockquote>

Because they are not <code>static</code>

You have to create an instance to be able to use them. Add the modifier, so you can have access to them. For instance,

<pre><code>private static getSomething()
</code></pre>

And then, call it from main as:

<pre><code>getSomething()
</code></pre>

Also pay attention to what the almighty @JonSkeet said in the comments.

blocks|key|1999720|text|这主要是因为您可以拥有同一个类的多个实例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1999721|Dog+bigDog+=+new+Dog();
Dog+littleDog+=+new+Dog();|code-block|syntax|javascript|1999722|如果你想知道你狗的名字呢？Dog.GetName();，对吧？哪个狗的名字会回来？您必须通过调用类实例上的函数将其告诉编译器。|offset|length|style|CODE|1999723|bigDog.GetName();|1999724|entityMap^0|0|0|D|E|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

This is essentially because you could have multiple instances of the same class. 

<pre><code>Dog bigDog = new Dog();
Dog littleDog = new Dog();
</code></pre>

Now what if you wanted to know the name of your dog?
<code>Dog.GetName();</code>, right? Which dog's name would that return? You have to tell that to the compiler by calling the function on an instance of the class.

<pre><code>bigDog.GetName();
</code></pre>

blocks|key|2479576|text|如果你想在你的产品中处理两个以上的人，那么你可以用，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2479577|PersonProgram+p1+=+new+PersonProgram();//+Have+p1.name="First+person"
PersonProgram+p2+=+new+PersonProgram();//+Have+p2.name="Second+person"|code-block|syntax|javascript|2479578|所以..。|2479579|如果您知道只有一个人，那么您可以创建静态类，并且不需要创建新的对象并直接使用而不需要创建实例。|2479580|所以OOP给了你两个选项，选择一个适合你的。|2479581|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

If you want to handle more than 2 persons in your product then you can do with,

<pre><code>PersonProgram p1 = new PersonProgram();// Have p1.name="First person"
PersonProgram p2 = new PersonProgram();// Have p2.name="Second person"
</code></pre>

so on ...

If you know there is only one person then you can create static class and you wont need to create new object and use directly without creating instance.

so OOP gives you both option, select one which suites you.

blocks|key|450895|text|了解创建Object+Oriented+Programming背后的驱动力是将内存/数据与可以在其上运行的操作结合起来。因此，在面向对象(+OOP+)中，对象的大部分时间都是针对它们包含的数据提供服务的，而静态方法基本上是用于无状态进程的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|450896|entityMap^0|4|R|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

Understand that the driving force behind the creation of <code>Object Oriented Programming</code> is to couple memory/data with the operations that can be run on it. So in OOP, Objects are intended most of the time to provide services on the data that they contain, whereas static methods are fundamentally for stateless processes.

blocks|key|1999878|text|简单的回答是:因为主要方法是静态的，这意味着它属于类而不是特定的对象。静态上下文不能引用非静态上下文，因为在脑海中没有特定对象的引用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1999879|但是你也可以让你所有的变量和方法static，在那里，问题解决了，对吗？|offset|length|style|CODE|1999880|不是的。别干那事。|1999881|长话短说：|1999882|主要方法是static，因为应用程序的自定义对象在其启动之前不存在。毕竟，在房间存在之前，你不能在房间里有什么东西。但是，JVM需要一个程序的入口点，所以主方法必须不包含所有对象引用，因此是static。|1999883|在应用程序启动时，Java调用PersonProgram.main(args)。现在假设您已经定义了PersonProgram类中的"Person“。也许PersonProgram有一个名称、一个年龄等等，但是您还没有任何PersonProgram对象，那么main方法如何知道它应该使用的名称或年龄呢？|1999884|这里有一种让您更加清楚的方法:创建一个名为Application的新类，并只将您的主要方法放入其中。现在，尝试引用您的PersonProgram实例变量，除非您首先创建一个PersonProgram对象，否则您将无法这样做。|1999885|您刚刚开始学习OOP概念，将main方法放在类定义中的想法可能会变得令人困惑。它使学习代码变得更容易，但在尝试学习OOP原则时，您应该尝试将main方法与类逻辑分开。|1999886|entityMap^0|0|G|6|0|0|0|5|6|2N|6|0|F|O|1E|D|25|D|33|D|3L|4|0|L|B|1N|D|2E|D|0|E|4|1Y|4|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|W|8|@$D|X|E|Y|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|10|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|11|8|@$D|12|E|13|F|G]|$D|14|E|15|F|G]]|9|@]|A|$]]|$1|N|3|O|5|6|7|16|8|@$D|17|E|18|F|G]|$D|19|E|1A|F|G]|$D|1B|E|1C|F|G]|$D|1D|E|1E|F|G]|$D|1F|E|1G|F|G]]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1H|8|@$D|1I|E|1J|F|G]|$D|1K|E|1L|F|G]|$D|1M|E|1N|F|G]]|9|@]|A|$]]|$1|R|3|S|5|6|7|1O|8|@$D|1P|E|1Q|F|G]|$D|1R|E|1S|F|G]]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1T|8|@]|9|@]|A|$]]]|U|$]]

The short answer: Because the main method is static, meaning that it belongs to the class and not a particular object. Static contexts cannot reference non-static context because there is no reference of a particular object in mind.

But you could just make all of your other variables and methods <code>static</code> too, and there, problem solved, right?

No. Don't do that.

The long answer:

The main method is <code>static</code> because none of your application's custom defined objects exist before its started. After all, you can't have something exist in a room before the room exists. The JVM needs an entry point to your program, however, so the main method must be free of all object references, hence <code>static</code>.

Java calls <code>PersonProgram.main(args)</code> just as your application starts. Now let's say you have defined what a "Person" is in your <code>PersonProgram</code> class. Maybe a <code>PersonProgram</code> has a name, an age, etc. But you don't have any <code>PersonProgram</code> objects yet, so how does the <code>main</code> method know what name or age it's supposed to be working with?

Here's a way to make things clearer for you: Make a new class called <code>Application</code> and put only your main method in it. Now, try to refer to your <code>PersonProgram</code>'s instance variables. You won't be able to do it unless you create a <code>PersonProgram</code> object first.

You've just started learning OOP concepts, and the idea of putting the <code>main</code> method in your class definitions can become confusing. It makes it easier to learn to code, but you should try keeping your <code>main</code> method separate from your class logic as you try to learn OOP principles.

blocks|key|452666|text|基本OOP规则：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|452667|类使其所有非私有、非静态成员只能通过实例化对象使用。|452668|即使在从同一个类请求成员时也是如此。尽管您也可以访问类的私有成员。|452669|要使用this.method()，您必须使方法是静态的。|offset|length|style|CODE|452670|entityMap^0|0|0|0|3|D|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|P|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|Q|8|@$H|R|I|S|J|K]]|9|@]|A|$]]|$1|L|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|M|$]]

Basic OOP rule:

A class makes all its non-private, non-static members available only through its instantiated object. 

This applies even when the members are requested from the same class. Although you can access private members of your class as well.

To use <code>this.method()</code>, you will have to make the method static.

OK I am learning OOP at the moment, and there are a few things I don't understand. For example I have a class called PersonProgram that contains my main. I have several methods and functions outside of my main method within the class. Why can I not call those methods and functions from the main without creating a new object of the class? Since they are all in the same class, why doesn't it make sense to just be able to call them like any other method, without 
 PersonProgram p = new PersonProgram();
first?

Why do I need to create an instance of the class I am working in?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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好的，我现在正在学习OOP，有些事情我不明白。例如，我有一个名为PersonProgram的类，它包含我的main。在类的主方法之外，我有几个方法和函数。为什么我不能在不创建类的新对象的情况下从main调用这些方法和函数？既然它们都在同一个类中，那么为什么不能像任何其他方法那样调用它们呢？ PersonProgram ...

问为什么我需要创建我正在工作的类的实例？
EN

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问为什么我需要创建我正在工作的类的实例？EN