具有继承支持的泛型问题的生成器模式

Question

我基于这个答案实现了模式，我有以下asbtract配置：

public abstract class AbstractConfig {

    public static abstract class Builder<B extends Builder<B>> {

        private int calories = 0;

        public Builder() {

        }

        public B setCalories(int calories) {
            this.calories = calories;
            return (B) this;
        }

        public abstract AbstractConfig build();
    }

    private int calories = 0;

    protected AbstractConfig(final Builder builder) {
        calories = builder.calories;
    }
}

我有以下具体配置:

public class DialogConfig extends AbstractConfig {

    public static class DialogConfigBuilder<B extends DialogConfigBuilder<B>> extends Builder<B> {

        private double width;

        private double height;

        public DialogConfigBuilder() {
            //does nothing.
        }

        public B setWidth(final double value) {
            width = value;
            return (B) this;
        }

        public B setHeight(final double value) {
            height = value;
            return (B) this;
        }
        public DialogConfig build() {
            return new DialogConfig(this);
        }
    }

    private final double width;

    private final double height;

    protected DialogConfig(final DialogConfigBuilder builder) {
        super(builder);
        width = builder.width;
        height = builder.height;
    }

    public double getWidth() {
        return width;
    }

    public double getHeight() {
        return height;
    }
}

这就是我使用它的方式

DialogConfig config = new DialogConfig.DialogConfigBuilder()
                .setWidth(0)
                .setCalories(0)
                .setHeight(0) //X LINE
                .build();

在X行我得到-找不到符号方法setHeight。我的错误是什么？

编辑 - 我将有一个扩展的诊断配置，必须扩展对话框配置等。我的意思是会有其他子类。

Answer 1

我发现了我的错误。这是我如何使用DialogConfigBuilder的

DialogConfig config = new DialogConfig.DialogConfigBuilder()
                .setWidth(0)
                .setCalories(0)
                .setHeight(0) //X LINE
                .build();

这就是我应该如何使用DialogConfigBuilder

DialogConfig config = new DialogConfig.DialogConfigBuilder<>()
                .setWidth(0)
                .setCalories(0)
                .setHeight(0) //X LINE
                .build();

请注意＜code＞

Answer 2

您首先将＜code＞setCalories（）

public Builder<B> setCalories(int calories) {
  this.calories = calories;
  return this;
}

去掉强制转换和警告。现在仔细看看这个。您返回一个Builder。这段代码不知道未来的子类。它只返回该基础构建器的一个实例。

因此，当您具有该链接调用时：

 .setHeight(0) .build();

这将返回该基础构建器。然后调用 build（） - 这将构建一个抽象配置。但是您希望将其分配给更具体的对话配置。因此，错误。

一个(丑陋的)解决方法:

DialogConfig.DialogConfigBuilder<?> builder = new DialogConfig.DialogConfigBuilder<>().setHeight(0);
builder.setCalories(0);

...config = builder.build();

还有一个解决方案——再次修改< code > setcales():

@SuppressWarnings("unchecked")
public <T extends B> T setCalories(int calories) {
  this.calories = calories;
  return (T) this;
}

修复编译错误；并且还允许链接＜code＞setCalories（）

记录在案的是“完整”解决方案，包括消除原始类型和其他警告的所有自适应：

abstract class AbstractConfig {
    public static abstract class Builder<B extends Builder<B>> {
        private int calories = 0;

        @SuppressWarnings("unchecked")
        public <T extends B> T setCalories(int calories) {
            this.calories = calories;
            return (T) this;
        }

        public abstract AbstractConfig build();
    }

    private int calories = 0;
    public int getCalories() { return calories; }

    protected <B extends Builder<B>> AbstractConfig(final Builder<B> builder) {
        calories = builder.calories;
    }
}

final class DialogConfig extends AbstractConfig {
    public static class DialogConfigBuilder<B extends DialogConfigBuilder<B>> extends Builder<B> {

        private double width;    
        private double height;

        public DialogConfigBuilder<B> setWidth(final double value) {
            width = value;
            return this;
        }

        public DialogConfigBuilder<B> setHeight(final double value) {
            height = value;
            return this;
        }

        public DialogConfig build() {
            return new DialogConfig(this);
        }
    }

    private final double width;
    private final double height;

    protected <B extends DialogConfigBuilder<B>> DialogConfig(final DialogConfigBuilder<B> builder) {
        super(builder);
        width = builder.width;
        height = builder.height;
    }

    public double getWidth() { return width; }
    public double getHeight() { return height; }
}

public class Builders {
    public static void main(String[] args) {
        DialogConfig config = new DialogConfig.DialogConfigBuilder<>().setHeight(0).setCalories(0).build();
        System.out.println(config);
    }
}