两个非静态变量这不能从静态上下文错误中引用

Question

我现在正在编写这段代码，它总是在“教师x1=new Teacher（）；学生y1=new Student（）”部分返回错误“非静态变量this cannot reference from a static context”。我不知道该怎么做才能让这个项目运转起来。谢谢你的帮助！

编辑：添加了其他类编辑2：此代码的目标是有一个类Person（带有姓名，年龄和性别），另一个类学生（带有等级）和教师（带有工资）。学生和教师类必须从Person类继承。

我的代码：

import java.util.*;
import java.util.Scanner;

public class Java2Technical {

public static void main(String[] args) {
System.out.println("DATABASE");
System.out.println(" ");
System.out.println("1. Teacher");
System.out.println("2. Student");
    
System.out.print("Choice: ");
Scanner scan = new Scanner(System.in);
int choice = scan.nextInt();   
System.out.println(" ");
    
if(choice == 1) {
    Teacher x1 = new Teacher();
    System.out.print("Name: ");
    Scanner scan1 = new Scanner(System.in);
    x1.name = scan1.nextLine();
    System.out.println(" ");
    
    System.out.print("Age: ");
    Scanner scan2 = new Scanner(System.in);
    x1.age = scan2.nextInt();
    System.out.println(" ");
    
    System.out.print("Gender: ");
    Scanner scan3 = new Scanner(System.in);
    x1.gen = scan3.next().charAt(0);
    System.out.println(" ");
    
    System.out.print("Salary: " + x1.salary);
    System.out.println(" ");
    }
    
    else if(choice == 2){
    Student y1 = new Student();
    
    System.out.print("Name: ");
    Scanner scan1 = new Scanner(System.in);
    y1.name = scan1.nextLine();
    System.out.println(" ");
    
    System.out.print("Age: ");
    Scanner scan2 = new Scanner(System.in);
    y1.age = scan2.nextInt();
    System.out.println(" ");
    
    System.out.print("Gender: ");
    Scanner scan3 = new Scanner(System.in);
    y1.gen = scan3.next().charAt(0);
    System.out.println(" ");
    
    System.out.print("Grade: " + y1.grade);
    System.out.println(" ");
    }
    else{
        System.out.println("Invalid Input");
    }
}
     public class Person {

     String name;
     int age;
     char gen;
     double grade;
    double salary;
 }

public class Student extends Person {
    public void setName(String Brand) {
        this.name = name;
   }
public void setAge(int age) {
    this.age = age;
}
public void setGen(char gend) {
    this.gen = gen;
}
public void setGrade(double grade){
    this.grade = grade;
}
public String getName(String name) {
    return this.name;
}
public int getAge(int age) {
    return this.age;    
}
public char getGen(char gend) {
    return this.gen;
}
public double getGrade(double grade) {
    return this.grade;
    }   
}
public class Teacher extends Person {
public void setName(String Brand) {
    this.name = name;
}
public void setAge(int age) {
    this.age = age;
}
public void setGen(char gen) {
    this.gen = gen;
}
public void setSalary(double salary) {
    this.salary = salary;
}
public String getName(String name) {
    return this.name;
}
 public int getAge(int age) {
    return this.age;
}
  public char getGen(char gen) {
    return this.gen;
}
  public double getSalary(double salary) {
    return this.salary;
    }  
}
}

Answer 1

您的主要方法是静态的，因此您处于一个静态上下文中，这个不存在。

静态成员或静态方法与类相关联，而不是类的实例。静态成员每个类存在一次，非静态成员对每个实例都是唯一的。

如果您有一个内部类，就像您在示例中所做的那样，那么只有当您已经有一个外部类的实例时，才能创建内部类的实例。

public class Outer {
    public class Inner {  }

    public static void main (String[] args) {
        var outer = new Outer();
        var inner = outer.new Inner(); // notice the use of the outer instance here
    }
}

如果您不希望将内部类的每个实例绑定到外部类的特定实例，并且希望能够独立实例化它，则需要使内部类为静态：

public class Outer {
    public static class Inner {  }

    public static void main (String[] args) {
        var inner = new Inner(); // works now
    }
}

注意内部类上的static关键字，它使得实例化外部类变得不必要。

另外，当您已经处于实例上下文中时，您不需要显式地使用外部类：

public class Outer {
    public class Inner {  }

    Inner inner;

    Outer() {
        var inner = new Inner(); // `this` exists, we are in an instance of the outer class and can thus instantiate the inner class because we are in the proper object context
        this.inner = inner;
    }

    public static void main (String[] args) {
        var outer = new Outer();
        var inner = outer.inner; // access the outer classes field and get the instance of the inenr class that was created in the constructor of the outer class
    }
}

_{请注意，适当的缩进使代码更容易阅读。}