非静态方法不能从具有方法和变量[duplicate]的静态上下文中引用

Question

在编写BookStoreApplication时，使用Book、Tape和CD类创建对象。虽然未完成，但application类应该创建新的BookStoreItems，它们是Book、Tape和CD。它们从BookStoreItems类继承。在这个应用程序类中,我不断得到错误：

error: non-static method printMenu() cannot be referenced from a static context
error: non-static method getUserChoice() cannot be referenced from a static context
error: non-static variable input cannot be referenced from a static context

我把它改成静态，然后又改成不是静态，但我继续得到这个错误...

import java.util.Scanner;

public class BookStoreApp2 {

    //constants for options
    static final int ADD_BOOK = 0;
    static final int ADD_TAPE = 1;
    static final int ADD_CD = 2;
    static final int QUIT = -1;

    Scanner input = new Scanner (System.in);

    public static void main(String[] args) {


        BookStoreItem[] item;//declaring array

        item = new BookStoreItem[10];//initializing array

        int itemType = -1;

        printMenu();
        getUserChoice();

        for (int i = 0; i < item.length; i++){
            System.out.print("\n" + i + "\tEnter 0 for Book, 1 for Tape, 2 for CD: ");
            itemType = input.nextInt();

            switch (itemType) {
                case 0:
                    item[i] = new Book();
                    break;
                case 1:
                    item[i] = new Tape();
                    break;
                case 2:
                    item[i] = new CD();
                    break;
                default: 
                    System.out.println("\nInvalid choice.");
            }//end of switch statement

        }//end of for loop

        for (int i = 0; i < item.length; i++) {
            System.out.println("\nAnimal #" + i + ": ");

            System.out.println("\n\tTitle: " + item[i].getTitle()); //polymorphic because they can operate on separate objects
            System.out.println("\n\tAuthor: " + item[i].getAuthor());
        }//end of for


    }//end of main method


//PRINT MENU----------------------------------------------------------  
    public void printMenu(){
        System.out.println("\nPress:");
        System.out.println("\t" + ADD_BOOK + "\tTo add a book to the book store.\n");
        System.out.println("\t" + ADD_TAPE + "\tTo add a tape to the book store.\n");
        System.out.println("\t" + ADD_CD + "\tTo add a CD to the book store.\n");
        System.out.println("\t" + QUIT + "\tTo exit\n");
    }
//---------------------------------------------------------------------
//GET USER CHOICE------------------------------------------------------ 
     public int getUserChoice() {
        int choice;     
        System.out.print("Please enter your choice: ");
        choice = input.nextInt();

        return choice;
    }//end of getUserChoice
//----------------------------------------------------------------------

}//end class

Answer 1

仅仅为了使您的程序正常工作，可以将main（）方法的内容放入构造函数中：

public BookStoreApp2()
{
   // Put contents of main method here
}

然后，在您的main（）方法中。执行以下操作：

public void main( String[] args )
{
  new BookStoreApp2();
}

Answer 2

您需要使您的方法-printmenu()和getUserChoice()static，因为您直接从静态main方法调用它们，而不创建类的实例，这些方法是在中定义的。并且如果没有对定义非静态方法的类的实例的任何引用，则无法调用该方法。

或者，您可以将方法调用部分更改为：

BookStoreApp2 bookStoreApp = new BookStoreApp2();
bookStoreApp.printMenu();
bookStoreApp.getUserChoice();