Java 从 getUsername() 获取 NULL 值； (不同的类文件)

Question

所以我正在练习我的 Java 技能，在没有寻求帮助的情况下尝试和思考 2.5 小时后，我有点迷路了......

问题出在 Welcome 类中，我正在设置用户名和角色。但在终端中我想通过 Getter 使用用户名，但结果为空

检查下面的类

欢迎类:

public class Welcome {

    public static void welcome(){
        Player m = new Player();
        String user;
        String roletype;
        Float startersmoney;
        // welcome msg
        System.out.println("Welcome to 'Roots'.");

        //enter your desired username
        System.out.println("Enter your alias : ");
        Scanner scanner = new Scanner(System.in);
        Ruler.ruler();
        user = scanner.nextLine();
        m.setUsername(user);
        // returns username

        System.out.println("Your alias is now set to : " + m.getUsername());
        // are you a hacker?
        System.out.println("What are you? a 'hacker' or ? ");
        Ruler.ruler();
        roletype = scanner.nextLine();
        m.setRole(roletype);
        m.setCurrency(250.00);
        //checks user input. and gives output
        if (m.getRole().equalsIgnoreCase("hacker")) {
            System.out.println("So you are a : " + m.getRole().toLowerCase());
        } else {
            System.out.println("hahahaha... NOOOOB!!!\n only hackers are allowed to play 'Roots'..");
        }
        System.out.println("Okay lets get started. hint : 'help' ");
        Ruler.ruler();
        Terminal.terminal();
    }

标尺类

  public class Ruler {

    // for input markup '@root:~ > ' <- feels like terminal :D
    public static void ruler(){
        Player r = new Player();
        System.out.print(r.getUsername() + "@root:~ > ");
    }
}

玩家类

public class Player {

    //variables
    private String username;
    private String role;
    private double currency;

    //setters
    public void setUsername(String value)   {   this.username = value;      }
    public void setRole(String role)        {   this.role = role;           }
    public void setCurrency(Double currency){   this.currency = currency;   }

    //getters
    public String getUsername() {   return this.username;        }
    public String getRole()     {   return this.role;            }
    public Double getCurrency() {   return this.currency;        }

}

Answer 1

您的问题很常见——创建对象的新实例并期望它神奇地获取同一类其他对象的状态。这不是 Java 的工作方式。

看看您在代码中看到 new Player() 的次数。每次看到它时，您都在这样做，即创建一个新 Player 对象，一个与其他对象截然不同的对象。因此，设置其中一个的状态不会影响您正在检查其状态的另一个。

解决方案:通过传递引用，只检查先前创建的 Player 对象的状态。不要在您的监听器代码中创建一个新的。

所以:

public class Ruler {
   private Player player;

   public Ruler(Player player) {
      this.player = player;
   }

    // **** this shouldn't be static!
    public void checkState(){
        // Player r = new Player();
        // System.out.print(r.getUsername() + "@root:~ > ");
        System.out.print(player.getUsername() + "@root:~ > ");
    }
}

然后在

public static void main(String[] args) {
    Player player = new Player();
    Ruler ruler = new Ruler(player);

    // now call player and ruler methods as desired
}

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编辑
你声明:

.btw it has to be static otherwise i cant use it.... for some reason..

不!那么你正在修复错误的东西。关键是修复您的代码，使其不必是静态的。

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look here : /home/stefano/intelliJ_Projects/Roots/src/com/stefano/Welcome.java Error:(32, 14) java: non-static method ruler() cannot be referenced from a static context Error:(44, 14) java: non-static method ruler() cannot be referenced from a static context Error:(45, 17) java: non-static method terminal() cannot be referenced from a static context

同样，正如我的代码所示，摆脱静态标尺方法，而只使用非静态方法。 Ruler 类必须有状态，必须有非静态变量，所以你应该在类的对象上调用实例方法，而不是静态方法。恐怕你在倒退。