Java 静态方法 + 类

Question

首先，我想说的是，我是 Java 的新手，具有 C++ 背景。我永远无法联系上我的老师，所以我想在这里尝试 post 这个我一直想知道的问题（希望我能正确表达）：

如何在不使用 static 的情况下创建方法？我知道我可能需要为它制作一个 class 但我该怎么做呢？只是 class 没有变量，只有函数？我是否制作第二个 class 来包含 main 而不是以 .java 文件命名的 class？例如：

package musiclibrary;
import java.util.Scanner;

/**
 * This class implements a user to create a playlist from a selection of artists and songs
 * @author ahb5190
 */
public class MusicLibrary {
static String divider = "*****************************************************";
//Scanner class
static Scanner input = new Scanner(System.in);

/**
 * Welcome menu
 */
public static void welcomeMenu()
{
    System.out.println(divider);
    System.out.println();
    System.out.println("Welcome to Art's personal music library!");
    System.out.println();
    System.out.println("Choose an option:");
    System.out.println("1) Create Playlist");
    System.out.println("2) Delete Playlist");
    System.out.println("3) Add Selection to Playlist");
    System.out.println("4) Remove Selection from Playlist");
    System.out.println("5) Quit");
    System.out.println();
    System.out.print("Your choice?: ");
}

/**
 * 
 * @param min error check low bound
 * @param max error check high bound
 * @return 
 */
public static int getData(int min, int max)
{
   boolean goodInput = false;
    int choice = 1; //Will be overwritten
    while(!goodInput)
    {    
        choice = input.nextInt();
        if(choice < min || choice > max)
        {
            System.out.print(choice + " is not a valid choice. Please enter a number between " + min + " and " + max + ": ");
            goodInput = false;
        }
        else
        {
            goodInput = true;
        }
    }

    return choice;
}

/**
 * @param args the command line arguments
 */
public static void main(String[] args)
{
    //Variables
    int getDataMin = 1;
    int getDataMax = 5;
    boolean quit = false;
    int userInput;

    do {
        welcomeMenu();
        userInput = getData(getDataMin, getDataMax);
        if (userInput == 1)
        {
            quit = false;
        }
        else  if (userInput == 2)
        {
            quit = false;
        }
        else  if (userInput == 3)
        {
            quit = false;
        }
        else if (userInput == 4)
        {
            quit = false;
        }
        else  if (userInput == 5)
        {
            quit = true;
        }
    } while(!quit);

}

}

是分配的 java 程序的开头。如果我从 public static void welcomeMenu() 中删除 static，当我尝试在 main 中调用 welcomeMenu(); 时，它会给我 non-static method welcomeMenu() cannot be referenced from a static context。

另一段代码（不是很整洁，是定时考试的一部分）：

package lalala;

/**
 *
 * @author ahb5190
 */
public class Lalala {


    public class Movie
    {
        private String title;
        private String genre;
        private String director;
        private String star;

        public Movie (String t, String g, String d, String s)
        {
            title = t;
            genre = g;
            director = d;
            star = s;
        }
        public String gettitle()
        {
            return title;
        }
        public String getGenre()
        {
            return genre;
        }
        public String getDirector()
        {
            return director;
        }
        public String getStar()
        {
            return star;
        }
        public void setTitle(String x)
        {
            title = x;
        }
        public void setGenre(String x)
        {
            genre = x;
        }
        public void setDirector(String x)
        {
            director = x;
        }
        public void setsStar(String x)
        {
            star = x;
        }
        public boolean equals(Movie otherMovie)
        {
            if(otherMovie == null)
            {
                return false;
            }
            else
            {
                return title.equals(otherMovie.title) && genre.equals(otherMovie.genre) && director.equals(otherMovie.director) && star.equals(otherMovie.star);
            }
        }
        @Override
        public String toString()
        {
            return(title + " " + genre + " " + director + " " + star);
        }
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) 
    {
        Movie a;
        a = new Movie("Star Trek into Darkness", "Sci-fi", "J.J. Abrams", "Chris Pine");  //error: non-static variables this cannot be referenced from a static context
        Movie b = new Movie("Star Trek", "Sci-Fi", "J.J. Abrams", "Chris Pine");  //error: non-static variables this cannot be referenced from a static context
        Movie c = new Movie("Independence Day", "Action", "Roland Emmerich", "Will Smith"); //error: non-static variables this cannot be referenced from a static context

        System.out.println("Movies");

        System.out.println("Title: " + a.title);
        System.out.println("Genre: " + a.genre);
        System.out.println("Director: " + a.director);
        System.out.println("Star: " + a.star);
        System.out.println();
        System.out.println("Title: " + b.title);
        System.out.println("Genre: " + b.genre);
        System.out.println("Director: " + b.director);
        System.out.println("Star: " + b.star);
        System.out.println();
        System.out.println("Title: " + c.title);
        System.out.println("Genre: " + c.genre);
        System.out.println("Director: " + c.director);
        System.out.println("Star: " + c.star);
        System.out.println();

        a.equals(b);

    }

}

给我和以前一样的静态变量错误，如上面代码中所评论的那样。在那种情况下，我如何将它变成 'work' 是从 public static void main(String args[]) 中删除 static。

真的在努力学习 Java 的正确方法，我们将不胜感激。我也希望这符合MCV。

Answer 1

要访问 class 的非静态成员，您需要 class 的实例。

所以new Movie("a", "b", "c", "d").getGenre()是合法的。

从 main 中删除 static 关键字是不合法的，因为它是程序的入口点，因此必须存在。

编辑：在第一个源代码中，main() 方法不会创建 MusicLibrary 的任何实例，这就是为什么您需要对所有成员使用静态。

添加 MusicLibrary lib = new MusicLibrary(); 然后调用 lib.welcomeMenu(); 就可以去掉 static 关键字了。

Answer 2

首先，非静态方法只能通过 class、

的实例调用

这个link might help, to understand it a bit.And this type of question is already been answered here: this

Java 静态方法 + 类

Java static methods + classes

java

methods

static

static-methods

main