Java avoid class cast warning in hierarchical Builder pattern

generics, java

Solution

As said before, this can't be done, because it is not safe. `B` extends `BaseBuilder<T,B>`, but `BaseBuilder<T,B>` (type of `this`) does not extend `B`. Recursive bounds are almost NEVER useful in Java, and do not give you the self-type. You should get rid of it.

You can add an abstract method such that implementing classes must give an instance of `B`:

public abstract class BaseBuilder <T, B> {

  abstract public B getB();

  public B setB1(String b1) {
    this.b1 = b1;
    return getB();
  }

  abstract public T build();

  String b1;
}

Problem

Is there a way to avoid the unchecked class cast in this hierarchical Builder pattern? ``` public abstract class BaseBuilder <T, B extends BaseBuilder<T,B>> { public B setB1(String b1) { this.b1 = b1; return (B) this; // can I make this unchecked cast go away? } abstract public T build(); String b1; } ``` and no, the answer is not: ``` return B.class.cast(this); ``` and yes, I know I could use @SuppressWarnings

Original source

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