java - 为什么不是所有最终变量默认都是静态的？

Question

为什么您需要在非静态上下文 中使用final 变量？编译器是否自动将 static 分配给所有 final 变量？

编辑:我知道 static 和 final 之间的区别，我想问的是是否有任何情况下您需要 final int x 而不是 static final int x。当您无法修改 x 时，为什么在每个实例中都需要它的副本？

Answer 1

关键字 final 用于在构造时允许分配值仅一次。

public class Person {
    public final String name;

    public Person(String name) {
        this.name = name;
    }
}

Person p = new Person("a");
p.name = "newName"; // COMPILE ERROR name is marked as "final" thus can not be changed.

when you would need final int x instead of static final int x?

考虑这些类:

public class OriginConstants {
      // static final fields allows to access constants via class accessor i. e. OriginConstants.x (no need to create instance)
      public static final int x = 0;
      public static final int y = 0;
}

public class ImmutablePoint {
    public final int x;
    public final int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

public class MutablePoint {
    public int x;
    public int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

使用示例

// create immutable point shifted from coordinate system origin by 5
ImmutablePoint ip = new ImmutablePoint(OriginConstants.x + 5, OriginConstants.y + 5);

// updating point coordinate by 10
ip.x += 10; // COMPILE ERROR 
ip.y += 10; // COMPILE ERROR

// we cannot modify final fields, but we can create new instance with shifted coordinates
ImmutablePoint shiftedIp = new ImmutablePoint(ip.x + 10, ip.y + 10);

// create mutable point shifted from coordinate system origin by 5
MutablePoint mp = new MutablePoint(OriginConstants.x + 5, OriginConstants.y + 5);

// updating point coordinate by 10
ip.x += 10; // OK
ip.y += 10; // OK

我会为一些不能及时改变的坐标使用不可变点。假设，我们有 height = 100，width = 100 的 Canvas 。我们可以按如下方式创建辅助常量点:

public class Canvas {
    public static final int HEIGHT = 100;
    public static final int WIDTH = 100;

    // anchor points
    public static final ImmutablePoint topLeft = new ImmutablePoint(0,0);
    public static final ImmutablePoint topRight = new ImmutablePoint(Canvas.WIDTH, 0);
    public static final ImmutablePoint bottomLeft = new ImmutablePoint(0, Canvas.HEIGHT);
    public static final ImmutablePoint bottomRight = new ImmutablePoint(Canvas.WIDTH, Canavas.HEIGHT);
}

这样我们就可以确定 topLeft 始终为 0,0 并且不会因错误而更改。