问题描述
我想通过继承实现Builder模式。因此，我有以下4类：一个抽象类（ClassA），ClassB，ClassC。 TestTest类用于查看所有工作原理：
I want to implement Builder Pattern with inheritance. So I have 4 following classes: one abstract class (ClassA), ClassB, ClassC. TestTest class is used to see how all this works:
public abstract class ClassA {
private String aString;
public String getaString() {
return aString;
}
public abstract class ClassABuilder<T extends ClassABuilder>{
public T setaString(String str) {
ClassA.this.aString = str;
return (T)this;
}
public abstract ClassA build();
}
}
public class ClassB extends ClassA{
private String bString;
public String getbString() {
return bString;
}
public class ClassBBuilder<T extends ClassBBuilder> extends ClassA.ClassABuilder<T>{
public T setbString(String str) {
ClassB.this.bString = str;
return (T)this;
}
@Override
public ClassB build(){
return ClassB.this;
}
}
}
public class ClassC extends ClassB{
private String cString;
public String getcString() {
return cString;
}
public static ClassCBuilder<ClassCBuilder> newBuilder(){
return new ClassC().new ClassCBuilder();
}
public class ClassCBuilder<T extends ClassCBuilder> extends ClassB.ClassBBuilder<T>{
public T setcString(String str) {
ClassC.this.cString = str;
return (T)this;
}
@Override
public ClassC build(){
return ClassC.this;
}
}
}
public class TestTest {
public static void main(String[] args) {
// TODO code application logic here
ClassC C=ClassC.newBuilder()
.setaString(null)
.setbString(null)
.setcString(null) //LINE XXX
.build();
}
}
问题是在XXX线的TestTest，我得到了找不到符号 setcString。我该怎么办？
The problem is that at TestTest at LINE XXX I get can't find symbol "setcString". What do I do wrong?
推荐答案
让我们沿着层次结构进行跟踪：
Let's track it down along the hierarchy:
首先考虑此签名：
class ClassABuilder<T extends ClassABuilder>
调用 setaString（null）时返回的 T 将是扩展 ClassABuilder 的对象。编译器知道这是 ClassBBuilder ，因此将允许您调用 setbString（null）。
When you call setaString(null) the returned T will be an object that extends ClassABuilder. The compiler knows that this is a ClassBBuilder and thus will allow you to call setbString(null).
但是，由于定义指出 T 需要扩展 raw ClassBBuilder 只会丢失有关 ClassBBuilder 泛型类型的任何信息。因此，编译器仅知道 T 是 ClassBBuilder ，而不是实际上是 ClassCBuilder 扩展了 ClassBBuilder< ClassCBuilder> ，因此不知道 setcString（）
However, since the definition states T is required to extend a raw ClassBBuilder only any information on ClassBBuilder's generic types will be lost. Thus the compiler only knows that T is a ClassBBuilder but not that it's actually a ClassCBuilder which extends ClassBBuilder<ClassCBuilder> and hence doesn't know about setcString() on the returned type.
如前所述，使用 T扩展ClassABuilder< T> 可以解决此问题。编译器知道还有另一种通用类型要沿层次结构传递。
As has already been mentioned, using T extends ClassABuilder<T> will fix that since now the compiler knows there's another generic type to be passed down the hierarchy.
newBuilder（）然后看起来像
public static ClassCBuilder<?> newBuilder(){
//you have too create a raw type here so you'll have to ignore/suppress/live with the warning
return (new ClassC().new ClassCBuilder());
}
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