尝试创建对象的多个实例并打印其信息

Question

尝试创建一个以NFL球队统计数据为特色的程序。"Team"类创建对象，main 方法将打印统计信息。当我尝试打印统计数据时，所有团队只会打印上一支团队的属性。所以当我尝试打印亚利桑那州时，它打印的是布法罗。这是代码：

//Team class that creates the Team Objects
public class Team {
    public static int offenseRating;
    public static int defenseRating;
    public static int kickerRating;
    public static String teamname;
    public Team(String teamname, int offenseRating, int defenseRating, 
    int kickerRating) {
        Team.teamname = teamname;
        Team.offenseRating = offenseRating;
        Team.defenseRating = defenseRating;
        Team.kickerRating = kickerRating;
    }
    static Team Arizona = new Team("Arizona Cardinals", 50, 50, 50);
    static Team Atlanta = new Team("Atlanta Falcons", 50, 50, 50);
    static Team Baltimore = new Team("Baltimore Ravens", 50, 50, 50);
    static Team Buffalo = new Team("Buffalo Bills", 50, 50, 50);

public class footballMain {
//Main method
    public static void main(String[] args) {
    System.out.print(Team.Arizona.teamname);
    }
}

Answer 1

static表示"该类的所有实例将共享同一个变量"。这意味着当调用构造函数时，每次都会更改值。因此，只有最后一个团队仍然存在。

这是学习static与non-static的好资源

要解决此问题，您应该删除

public static int offenseRating;
public static int defenseRating;
public static int kickerRating;
public static String teamname;

并在构造函数中使用this.而不是Team.

Answer 2

您必须从字段和构造函数中删除 static 关键字。在这里试试这个：

//Team class that creates the Team Objects
public class Team {
   public int offenseRating;
   public int defenseRating;
   public int kickerRating;
   public String teamname;
   public Team(String teamname, int offenseRating, int defenseRating, 
   int kickerRating) {
       this.teamname = teamname;
       this.offenseRating = offenseRating;
       this.defenseRating = defenseRating;
       this.kickerRating = kickerRating;
   }
   static Team Arizona = new Team("Arizona Cardinals", 50, 50, 50);
   static Team Atlanta = new Team("Atlanta Falcons", 50, 50, 50);
   static Team Baltimore = new Team("Baltimore Ravens", 50, 50, 50);
   static Team Buffalo = new Team("Buffalo Bills", 50, 50, 50);
//Main method
public static void main(String[] args) {
   System.out.print(Team.Arizona.teamname);
}

this 关键字表示将构造函数参数的值写入当前实例的字段。

Answer 3

正如所指出的，静态变量是"类变量"，在声明它们时始终可以访问。它们通常用于声明有关程序的常量(您应该检查可能适合您需求的枚举结构(。类的字段是"实例变量"，它们的目的是保留在内存中创建的特定数据结构的特定状态。出于安全原因，保护这些内存空间非常重要。

不重现此错误的一个很好的方法是专注于数据的用例 --> 永远一样然后是静态的;特定对象(类的实例(的特定变量，然后是实例变量。

这是一个代码来说明这一点，并且可以编译

package Question_5669067;
import java.util.Objects;
public class Team
{
    // We ensure that the static variable Buffalo will never change.
    public final static Team Buffalo = new Team("Buffalo Bills");
    // We protect the instance class from calling code.
    private String teamName;
    private int offenseRating;
    public Team(String teamName){
        this.teamName = Objects.requireNonNull(teamName, "Null teamName");
    }
    // We ensure that the String name is returned.
    public String getTeamName() {
        return this.teamName;
    }
    public int getOffenseRating(){
        return offenseRating;
    }
    // We ensure that the rate could not get under 0.
    public void setOffenseRating(int offenseRating){
        if (offenseRating < 0) {
            throw new IllegalArgumentException("rate < 0");
        }
        this.offenseRating = offenseRating;
    }
    public static void main(String[] args) {
        System.out.println(Team.Buffalo.getTeamName());
        // Whe are still within the class,
        // so we can reach the team name directly.
        System.out.println(Buffalo.teamName);
        Buffalo.setOffenseRating(13);
        System.out.println(Buffalo.getOffenseRating());
    }
}