继承泛型构建器的子类的子类

huangapple
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英文:

Subclassing a subclass of a (generic) builder

问题

  1. public class StackOverflow {
  2.     public static class Builder<T extends Builder<T>> {
  3.         public Builder() {}
  5.         public T doA() {
  6.             System.out.println("A");
  7.             return (T) this;
  8.         }
  9.     }
  10.     
  11.     public static class BuilderSubclass<T extends BuilderSubclass<T>> 
  12.             extends Builder<BuilderSubclass<T>> {
  13.         
  14.         public BuilderSubclass() {}
  16.         public T doB() {
  17.             System.out.println("B");
  18.             return (T) this;
  19.         }
  20.     }
  22.     public static class BuilderSubclassSubclass
  23.             extends BuilderSubclass<BuilderSubclassSubclass> {
  25.         public BuilderSubclassSubclass() {}
  27.         public BuilderSubclassSubclass doC() {
  28.             System.out.println("C");
  29.             return this;
  30.         }
  31.     }
  32. }

I would like to do something like this:
new StackOverflow.BuilderSubclassSubclass().doA().doC().doB()

However, when I call new StackOverflow.BuilderSubclassSubclass().doA(), the returned object is not a BuilderSubclassSubclass but a BuilderSubclass<BuilderSubclassSubclass>. I know why this is the case: Builder's generic type is BuilderSubclass<BuilderSubclassSubclass> which is assigned in the class signature of BuilderSubclass.

Any ideas on how I can avoid this problem?

Update: I guess I could avoid using the builder pattern, and instead call the functions like this:

  1. StackOverflow.BuilderSubclassSubclass builder = new StackOverflow.BuilderSubclassSubclass();
  2. builder.doA();
  3. builder.doB();
  4. builder.doC();

In which case the code becomes:

  1. public class StackOverflowBatchHelperFunctions {
  2.     public static class Builder {
  4.         public Builder() {
  5.         }
  7.         public void doA() {
  8.             System.out.println("A");
  9.         }
  10.     }
  12.     public static class BuilderSubclass extends Builder {
  13.         public BuilderSubclass() {
  14.             super();
  15.         }
  17.         public void doB() {
  18.             System.out.println("B");
  19.         }
  20.     }
  22.     public static class BuilderSubclassSubclass extends BuilderSubclass {
  23.         public BuilderSubclassSubclass() {
  24.             super();
  25.         }
  27.         public void doC() {
  28.             System.out.println("C");
  29.         }
  30.     }
  31. }

However, I think this is less "pretty", so I would still love to see an answer to the initial problem.

  2. <details>
  3. <summary>英文:</summary>
  5. I&#39;m trying to implement a builder object with multi-level inheritance. I took some inspiration from https://stackoverflow.com/questions/17164375/subclassing-a-java-builder-class and this is my (simplified) code:
  7. ```java
  8. public class StackOverflow {
  9.     public static class Builder&lt;T extends Builder&lt;T&gt;&gt; {
  10.         public Builder() {}
  12.         public T doA() {
  13.             System.out.println(&quot;A&quot;);
  14.             return (T) this;
  15.         }
  16.     }
  17.     
  18.     public static class BuilderSubclass &lt;T extends BuilderSubclass&lt;T&gt;&gt; 
  19.             extends Builder&lt;BuilderSubclass&lt;T&gt;&gt; {
  20.         
  21.         public BuilderSubclass() {}
  23.         public T doB() {
  24.             System.out.println(&quot;B&quot;);
  25.             return (T) this;
  26.         }
  27.     }
  29.     public static class BuilderSubclassSubclass
  30.             extends BuilderSubclass&lt;BuilderSubclassSubclass&gt; {
  32.         public BuilderSubclassSubclass() {}
  34.         public BuilderSubclassSubclass doC() {
  35.             System.out.println(&quot;C&quot;);
  36.             return this;
  37.         }
  38.     }
  39. }

I would like do something like this:
new StackOverflow.BuilderSubclassSubclass().doA().doC().doB()

However, when I call new StackOverflow.BuilderSubclassSubclass().doA(), the returned object is not a BuilderSubclassSubclass but a BuilderSubclass&lt;BuilderSubclassSubclass&gt;. I know why this is the case: Builder's generic type is BuilderSubclass&lt;BuilderSubclassSubclass&gt; which is assigned in the class signature of BuilderSubclass.

Any ideas on how I can avoid this problem?

Update: I guess I could avoid using the builder pattern, and instead call the functions like this:

  1. StackOverflow.BuilderSubclassSubclass builder = new StackOverflow.BuilderSubclassSubclass();
  2. builder.doA();
  3. builder.doB();
  4. builder.doC();

In which case the code becomes:

  1. public class StackOverflowBatchHelperFunctions {
  2.     public static class Builder {
  4.         public Builder() {
  5.         }
  7.         public void doA() {
  8.             System.out.println(&quot;A&quot;);
  9.         }
  10.     }
  12.     public static class BuilderSubclass extends Builder {
  13.         public BuilderSubclass() {
  14.             super();
  15.         }
  17.         public void doB() {
  18.             System.out.println(&quot;B&quot;);
  19.         }
  20.     }
  22.     public static class BuilderSubclassSubclass extends BuilderSubclass {
  23.         public BuilderSubclassSubclass() {
  24.             super();
  25.         }
  27.         public void doC() {
  28.             System.out.println(&quot;C&quot;);
  29.         }
  30.     }
  31. }

However I think this is less "pretty", so I would still love to see an answer to the initial problem.

答案1

得分: 0

你应该将 public static class BuilderSubclass&lt;T extends BuilderSubclass&lt;T&gt;&gt; extends Builder&lt;BuilderSubclass&lt;T&gt;&gt; 替换为 public static class BuilderSubclass&lt;T extends BuilderSubclass&lt;T&gt;&gt; extends Builder&lt;T&gt;。

英文:

You should replace public static class BuilderSubclass&lt;T extends BuilderSubclass&lt;T&gt;&gt; extends Builder&lt;BuilderSubclass&lt;T&gt;&gt; with public static class BuilderSubclass&lt;T extends BuilderSubclass&lt;T&gt;&gt; extends Builder&lt;T&gt;.

huangapple
  • 本文由 发表于 2020年4月8日 14:36:19
  • 转载请务必保留本文链接：https://go.coder-hub.com/61094648.html
  • builder
  • generics
  • java
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