有没有办法在抽象超级 class 中声明最终属性并在子 class 中初始化值？

Question

我想要一个带有最终属性 locationNumber 的抽象超级 class Location。但是 locationNumber 应该像 Market 或 Mosque 一样在子 class 本身中初始化。有干净的方法吗？我知道这样是行不通的，只是为了说明问题。

public abstract class Location {
    protected final int locationNumber;
    Collection<Figure> visitors;

    public int getLocationNumber() {
        return locationNumber;
    }
}

public class Market extends Location {
    locationNumber = 5;
}

public class Mosque extends Location {
    locationNumber = 10;
}

Answer 1

这就是你的做法：

public abstract class Location {

    private final int locationNumber;
    Collection<Figure> visitors;

    public Location(int locationNumber) {
       this.locationNumber = locationNumber;
    }

    public int getLocationNumber() {
        return locationNumber;
    }
}

public class Market1 extends Location {

    public Market1() {
       super(5);
    }
}

public class Market2 extends Location {

    public Market2() {
       super(10);
    }
}

Location m1 = new Market1();
Location m2 = new Market2();

System.out.println(m1.getLocationNumber()); // prints 5
System.out.println(m2.getLocationNumber()); // prints 10

Answer 2

final class成员变量在声明时不需要初始化。它可以在 class 构造函数或初始化块中赋值。

public abstract class Location {

    protected final int locationNumber;
    Collection<Figure> visitors;

    protected Location(int locNum) {
        locationNumber = locNum;
    }

    public int getLocationNumber() {
        return locationNumber;
    }
}

public class Market extends Location {
    public Market() {
        super(5);
    }
}

或者，您可以使方法 getLocationNumber 抽象化，并让每个子class return 具有相关值。那么你就不需要成员 locationNumber.

public abstract class Location {
    Collection<Figure> visitors;

    public abstract int getLocationNumber();
}

public class Market extends Location {

    public int getLocationNumber() {
        return 5;
    }
}

但如果您仍想保留会员 locationNumber 那么以下方法也有效。

public abstract class Location {
    protected final int locationNumber;

    protected Location() {
        locationNumber = getLocationNumber();
    }

    public abstract int getLocationNumber();
}

public class Market extends Location {

    public int getLocationNumber() {
        return 5;
    }
}

有没有办法在抽象超级 class 中声明最终属性并在子 class 中初始化值？

Is there a way to declare a final attribute in an abstract super class and initialize the value in the sub class?

java

final

subclass

superclass