The first thing is to know the difference between an instance of a class, and the class itself. A class models certain properties, and the behaviour of the whole in the context of those properties. An instance will define specific values for those properties. Anything bound to the static keyword is available in the context of the class rather than in the context of an instance of the class As a corollary to the above variables within a method can not be static static fields, and methods must be invoked using the class-name e.g. MyProgram7.main(...) The lifetime of a static field/method is equivalent to the lifetime of your application

如。 例如，汽车具有属性颜色，并表现出行为“运动”。 以25公里每小时的速度行驶的红色大众甲壳虫为例。

现在，汽车的静态属性将是道路上的车轮数量(4)，这将适用于所有汽车。

HTH

The first thing is to know the difference between an instance of a class, and the class itself. A class models certain properties, and the behaviour of the whole in the context of those properties. An instance will define specific values for those properties. Anything bound to the static keyword is available in the context of the class rather than in the context of an instance of the class As a corollary to the above variables within a method can not be static static fields, and methods must be invoked using the class-name e.g. MyProgram7.main(...) The lifetime of a static field/method is equivalent to the lifetime of your application

如。 例如，汽车具有属性颜色，并表现出行为“运动”。 以25公里每小时的速度行驶的红色大众甲壳虫为例。

现在，汽车的静态属性将是道路上的车轮数量(4)，这将适用于所有汽车。

HTH

在这个程序中，你想使用count，所以将count方法声明为静态方法

class MyProgram<br>
{
    int count = 0;
    public static void main(String[] args)
    {
        System.out.println(count);
    }
}

您可以将此方法声明为公共私有并受保护。如果使用此方法，则可以创建安全应用程序。

class MyProgram
{
    static int count = 0;
    public static void main(String[] args)
    {
        System.out.println(count);
    }
}

您必须理解类和类的实例之间的区别。如果你在街上看到一辆车，即使你看不清车型，你也会立刻知道这是一辆车。这是因为您将所看到的内容与类“car”进行比较。该类包含了类似于所有汽车的which。把它看作一个模板或一个想法。

同时，你所看到的汽车是类“car”的一个实例，因为它具有你所期望的所有属性:有人驾驶它，它有引擎，轮子。

因此，类说“所有汽车都有颜色”，实例说“这辆特定的汽车是红色的”。

在OO世界中，定义类，在类内部定义一个类型为Color的字段。当类被实例化时(当您创建一个特定的实例时)，内存为颜色保留，您可以给这个特定的实例一个颜色。因为这些属性是特定的，所以它们是非静态的。

静态字段和方法与所有实例共享。它们用于特定于类的值，而不是特定的实例。对于方法，这通常是全局辅助方法(如Integer.parseInt())。对于字段，它通常是常量(如汽车类型，即你有一个有限的集合，不经常改变)。

为了解决这个问题，您需要实例化类的一个实例(创建一个对象)，这样运行时就可以为该实例保留内存(否则，不同的实例就会相互覆盖，这是您不希望看到的)。

在你的情况下，试试下面的代码作为起点:

public static void main (String[] args)
{
    try
    {
        MyProgram7 obj = new MyProgram7 ();
        obj.run (args);
    }
    catch (Exception e)
    {
        e.printStackTrace ();
    }
}

// instance variables here

public void run (String[] args) throws Exception
{
    // put your code here
}

新的main()方法创建它所包含的类的实例(听起来很奇怪，但因为main()是与类一起创建的，而不是与实例一起创建的，所以它可以这样做)，然后调用实例方法(run())。

static关键字修改类中方法或变量的生命周期。静态方法或变量是在装入类时创建的。未声明为静态的方法或变量仅在类实例化为对象(例如使用new操作符)时创建。

广义来说，类的生命周期是:

类的源代码是创建模板或 图案或邮票，然后可以使用 使用类创建一个带有new操作符的对象，使类的实例作为实际对象，然后在处理对象时 在垃圾收集期间销毁对象，回收它所持有的资源，例如内存。

为了为应用程序提供一个初始入口点，Java采用了这样的约定:Java程序必须有一个类，该类包含一个具有一致同意的或特殊名称的方法。这个特殊的方法称为main()。因为无论包含main方法的类是否已被实例化，方法都必须存在，所以main()方法必须用静态修饰符声明，以便一旦装入类，main()方法就可用。

The result is that when you start your Java application by a command line such as java helloworld a series of actions happen. First of all a Java Virtual Machine is started up and initialized. Next the helloworld.class file containing the compiled Java code is loaded into the Java Virtual Machine. Then the Java Virtual Machine looks for a method in the helloworld class that is called main(String [] args). this method must be static so that it will exist even though the class has not actually been instantiated as an object. The Java Virtual Machine does not create an instance of the class by creating an object from the class. It just loads the class and starts execution at the main() method.

因此，您需要创建类的实例作为对象，然后可以访问未使用静态修饰符声明的类的方法和变量。一旦您的Java程序开始使用main()函数，您就可以使用任何具有static修饰符的变量或方法，因为它们作为正在加载的类的一部分存在。

However, those variables and methods of the class which are outside of the main() method which do not have the static modifier can not be used until an instance of the class has been created as an object within the main() method. After creating the object you can then use the variables and methods of the object. An attempt to use the variables and methods of the class which do not have the static modifier without going through an object of the class is caught by the Java compiler at compile time and flagged as an error.

import java.io.*;

class HelloWorld {
    int myInt;      // this is a class variable that is unique to each object
    static int myInt2;  // this is a class variable shared by all objects of this class

    static void main (String [] args) {
        // this is the main entry point for this Java application
        System.out.println ("Hello, World\n");
        myInt2 = 14;    // able to access the static int
        HelloWorld myWorld = new HelloWorld();
        myWorld.myInt = 32;   // able to access non-static through an object
    }
}

Before you call an instance method or instance variable It needs a object(Instance). When instance variable is called from static method compiler doesn't know which is the object this variable belongs to. Because static methods doesn't have an object (Only one copy always). When you call an instance variable or instance methods from instance method it refer the this object. It means the variable belongs to whatever object created and each object have it's own copy of instance methods and variables.

静态变量被标记为静态，实例变量没有特定的关键字。