Java：无法从静态上下文引用非静态变量

Question

  package lab10;

  import java.util.*;
  public class Lab10 {

  class QuadraticEquation{

   double a,b,c;

   QuadraticEquation(){
     Random number1 = new Random();
     a = (int) (number1.nextDouble() * 8 + 1.0);

     Random number2 = new Random();
     b = (int) (number2.nextDouble() * 8 + 1.0);

     Random number3 = new Random();
     c = (int) (number3.nextDouble() * 8 + 1.0);


   }
   double getA(){

       return a;
   }
   double getB(){

       return b;
   }
   double getC(){      

       return c;
   }
   double getDiscriminant(){
       double discriminant;

       discriminant = (Math.pow(b, 2)- 4 *(a *c));
       return discriminant;
     }
   }

public static void main(String[] args) {

    QuadraticEquation equation = new QuadraticEquation(); //<---- error

    System.out.println("Three randomized coefficients are: ");
    System.out.println("a = " + equation.getA());
    System.out.println("b = " + equation.getB());
    System.out.println("c = " + equation.getC());

}
  }

我已经在stackoverflow上阅读了几篇有关此错误的文章，但是即使阅读了这些文章，我似乎也不太明白为什么会得到它。 我正在引用以前的项目，但我不明白为什么我不能声明QuadraticEquation equation = new QuadraticEquation（）

这是我正在引用的项目：

   package InClass05;

   public class TestSimpleSquare {

   public static void main(String[] args) {

    SimpleSquare square1 = new SimpleSquare();
    System.out.println("The area of the square of side lengths " + square1.length + " is " + square1.getArea());
    System.out.println("The perimeter of the square of side lengths " + square1.length + " is " + square1.getPerimeter());
    System.out.println("The diagonal of the square of side lengths " + square1.length + " is " + square1.getDiagonal());

    System.out.println("");
    SimpleSquare square2 = new SimpleSquare(25);
    System.out.println("The area of the square of side lengths " + square2.length + " is " + square2.getArea());
    System.out.println("The perimeter of the square of side lengths " + square2.length + " is " + square2.getPerimeter());
    System.out.println("The diagonal of the square of side lengths " + square2.length + " is " + square2.getDiagonal());

    System.out.println("");
    square2.length = 500;
    System.out.println("The area of the square of side lengths " + square2.length + " is " + square2.getArea());
    System.out.println("The perimeter of the square of side lengths " + square2.length + " is " + square2.getPerimeter());
    System.out.println("The diagonal of the square of side lengths " + square2.length + " is " + square2.getDiagonal());


}
  }
  class SimpleSquare{
  double length;

   SimpleSquare(){
    length = 10;
}

SimpleSquare(double newlength){
    length = newlength;
}

double getArea(){
    return length * length;
}

double getPerimeter(){
    return length + length + length + length;
}

double getDiagonal(){
    return Math.sqrt(Math.pow(length, 2) + Math.pow(length, 2));
}
void setlength(double newlength){
    length = newlength;
}
 }

在这个例子中，我可以声明SimpleSquare square1 = new SimpleSquare（）; 我不明白为什么我现在无法完成的项目无法做到这一点。 任何帮助，将不胜感激。 谢谢。

Answer 1

它应该是：

QuadraticEquation equation = new Lab10().new QuadraticEquation();

因为QuadraticEquation是内部类

Answer 2

您可以添加单词static来使您的类静态：

static class QuadraticEquation

没有该类，您的类将位于Lab10内部，因此应将其附加到封闭的实例。 通常，内部类是在类的成员内部创建的。

它在此之后编译：

Three randomized coefficients are: 
a = 5.0
b = 7.0
c = 3.0

Answer 3

QuadraticEquation被定义为Lab10类中的一个内部类。 这样，除非在外部类的实例中创建它，否则不能创建它，除非您将其更改为静态内部类：

static class QuadraticEquation{
...
}

或者，如果您需要它是非静态的，则可以在创建内部类时创建外部类的实例：

QuadraticEquation equation = new Lab10().new QuadraticEquation();

经过一时的困惑之后，我发现在您的其他情况下，类SimpleSquare并不是内部类。 这是顶层的另一类。

Java允许您在同一文件中的同一级别上定义多个类，只要其中一个是公共的即可。

区别在于（可能是偶然的）末端支架的位置不同！