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。

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
    }
}

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

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。

ClassLoader负责加载类文件。让我们看看当我们编写自己的类时会发生什么。

示例1:

class StaticTest {

      static int a;
      int b;
      int c;
}

现在我们可以看到类“StaticTest”有3个字段。但实际上不存在b c成员变量。但是为什么??好的，让我看看。这里b c是实例变量。因为实例变量在对象创建时获得内存。所以这里b c还没有内存。这就是不存在b c的原因。所以只有a的存在。 对于ClassLoader，它只有一个关于a的信息。ClassLoader还不能识别b,c，因为它的对象还没有实例化。

让我们看另一个例子: 示例2:

class StaticTest {

      public void display() {
          System.out.println("Static Test");
      }


      public static void main(String []cmd) {

             display();       
      }

}

现在如果我们尝试编译这段代码，编译器会给出CE错误。 不能从静态上下文中引用非静态方法display()。

对于ClassLoader，它看起来是这样的:

class StaticTest {

      public static void main(String []cmd) {

             display();       
      }

}

在例2中，CE错误是因为我们从静态上下文调用非静态方法。因此ClassLoader不可能在编译时识别方法display()。因此会发生编译时错误。

最基本的事情是静态变量或静态方法是在类级别。类级变量或方法在实例级方法或变量之前加载。显然，没有加载的东西是不能使用的。所以java编译器不让在运行时处理的事情在编译时解决。这就是为什么它会给你错误非静态的东西不能从静态上下文中引用。你只需要阅读类级作用域、实例级作用域和局部作用域。