变量（可能）超出范围

Question

好的，所以有两个类Player 和子类Classes。我使用子类通过 switch 语句在主类中设置玩家统计信息，让用户选择他的类。但是由于某种原因，设置变量在主类中执行时超出了范围。为了我的爱，我不知道为什么会这样

这是Player 类：

public class Player {
private int HP;
private int ATK;
private int DEF;
private int DAMAGE;

    void setHP(int a){
    HP=a;
    }

    void setATK(int a){
    ATK=a;
    }

    void setDEF(int a){
    DEF=a;
    }

    void setDAMAGE(){
    DAMAGE = damageCalculation();
    }

    int damageCalculation(){
    int damage = (2*ATK);    
    return damage;
    }

Player(String Class){
    Classes classes = new Classes();
switch(Class){
    case "Barbarian":
        classes.Barbar();
        System.out.println("Done");
        break;
        case "Rogue":
        break;
    default: System.out.println("Error");
}
}

void getStats(){
System.out.format("Player stats:"
        + "\nHP: %d"
        + "\nATK: %d"
        + "\nDEF: %d"
        + "\nDAMAGE: %d\n",HP,ATK,DEF,DAMAGE);
}
}

这是Classes 类（注意在子类中获取统计信息返回有效值）：

public class Classes extends Player {

public void Barbar(){
Player player = new Player();   
player.setHP(60);
player.setATK(15);
player.setDEF(25);
player.setDAMAGE();
player.getStats();
 }
} 

这里是main 类：

import java.util.Scanner;
public class SurvivalGame {

   public static void main(String[] args) {
    Scanner scan = new Scanner(System.in,"Windows-1250");
    System.out.println("Enter your class");
    Player player = new Player(scan.nextLine());
    player.getStats(); //here getStats return zeros,
    //possibly out of scope variables if player enters Barbarian
 }
}

Answer 1

public void Barbar(){
Player player = new Player();   

如果您仔细观察，您正在创建一个新的 Player 并向其添加值，而不是 main 方法中的 Player。

Answer 2

当你扩展一个类时，你得到的东西是超类的一个实例，但是你选择在子类中添加额外的方法和字段。所以当你写

public class Classes extends Player {
    //...
}

您的Classes 实例中已经包含HP（等）字段。您的Barbar 方法可以修改它们而无需创建Player 的新实例：

public void Barbar(){
    setHP(60);
    setATK(15);
    setDEF(25);
    setDAMAGE();
    getStats();
}

你混淆了组合和继承。

Answer 3

您正在创建一个永远不会访问的 Player 的额外实例。只需浏览您的代码：

/* Main Method */
// You get a new Player object using direct user input
// (Which is somewhat dangerous, you should do some data validation first)
Player player = new Player(scan.nextLine())

/* Player constructor */
// Let's say user entered correctly "Barbarian". You are in the process of constructing
// a new Player object when you call this:
classes.Barbar();

/* Barbar method */
// The very first line creates a brand new player - different from one you were
// in the process of constructing. 
Player player = new Player(); 
// Then you assign values to the new Player, which goes out of scope as soon as the
// method ends. 

而不是让Classes 扩展Player（老实说，这对我来说有点奇怪），我只是让 Barbar 方法属于 Player 类本身（然后可以在切换期间调用它陈述）。然后你可以直接修改当前 Player 的值，而不是依赖不同的类来为你做这件事。

例如：

public class Player 
{
    public Player(String class)
    {
        switch (class)
        {
            case "Barbarian": 
                Barbar();
                break;
        }
    }

    public void Barbar()
    {
        HP = 60;
        ATK = 15;
        DEF = 25;
        setDAMAGE();
    }
}