协变器返回类型在界面不通过Javac进行编译 [英] Covariant Return Type in Interface not compiling via Javac

问题描述

我有以下结构：

  public interface BarReturn {} 
 public interface FooReturn {} 
 public interface FooBarReturn扩展FooReturn，BarReturn {} 
 
 public interface Foo {
 FooReturn fooBar（）; 
} 
 
 public interface Bar {
 BarReturn fooBar（）; 
} 
 
 public interface FooBar扩展Foo，Bar {
 FooBarReturn fooBar（）; 
} 
  

Javac失败，并显示以下消息：

  FooBar.java:2：类型Bar和Foo不兼容;都定义fooBar（），但是使用不相关的返回类型
 public interface FooBar extends Foo，Bar {
 ^ 
 1 error 
   
 
 
这是Eclipse编译还是javac中的错误？还是有办法说服javac来编译它？作为参考，我的javac选项如下所示：
  javac -d / tmp / covariant / target / classes -sourcepath / tmp / covariant / src / main / java：/tmp/covariant/src/main/java/Foo.java /tmp/covariant/src/main/java/BarReturn.java /tmp/covariant/src/main/java/FooBarReturn.java / tmp / covariant / src / main / java / Bar.java /tmp/covariant/src/main/java/FooReturn.java /tmp/covariant/src/main/java/FooBar.java -g -nowarn -target 1.6 -source 1.6 
  
 
 
解决方案
您正在FooBar中扩展Foo和Bar接口。因此，您继承了具有不兼容返回类型的两种方法。只有当跟随Liskov替换时，才允许Java协方差。 Aka，首选的候选类型必须几乎是覆盖的返回类型的子类。 
 
 
 
在上面的例子中，这样的应该编译：
公共接口BarReturn {} 
 public interface FooReturn {} 
 public interface FooBarReturn扩展FooReturn，BarReturn {} 
 
 public interface Foo {
 FooReturn fooBar（）; 
} 
 
 public interface FooBar extends Foo {
 FooBarReturn fooBar（）; 
} 
  
 
I have the following structure:
public interface BarReturn {}
public interface FooReturn {}
public interface FooBarReturn extends FooReturn, BarReturn {}

public interface Foo {  
  FooReturn fooBar( );
}

public interface Bar {
  BarReturn fooBar();
}

public interface FooBar extends Foo, Bar {
  FooBarReturn fooBar();
}
Javac fails with the following message:
FooBar.java:2: types Bar and Foo are incompatible; both define fooBar(), but with unrelated return types
public interface FooBar extends Foo, Bar {
       ^
1 error
However, Eclipse can compile it fine, and as far as I can see it should compile - FooBar's fooBar() method satisfies the contract of both Foo and Bar's fooBar() method by using covariant returns.

Is this a bug in the Eclipse compile or in javac?  Or is there a way to persuade javac to compile it?  For reference my javac options look like this:
javac -d /tmp/covariant/target/classes -sourcepath /tmp/covariant/src/main/java: /tmp/covariant/src/main/java/Foo.java /tmp/covariant/src/main/java/BarReturn.java /tmp/covariant/src/main/java/FooBarReturn.java /tmp/covariant/src/main/java/Bar.java /tmp/covariant/src/main/java/FooReturn.java /tmp/covariant/src/main/java/FooBar.java -g -nowarn -target 1.6 -source 1.6

解决方案
You are extending both Foo and Bar in your FooBar interface. As such you are inheriting two methods with incompatible return types. Java co-variance is only allowed when it follows Liskov substitution. Aka, the overriding candidate types must pretty much be a subclass of the overridden return type. 

In your example above something like this should compile:
public interface BarReturn {}
public interface FooReturn {}
public interface FooBarReturn extends FooReturn, BarReturn {}

public interface Foo {  
  FooReturn fooBar( );
}

public interface FooBar extends Foo{
  FooBarReturn fooBar();
}


                        
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