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Why is generic of a return type erased when there is an unchecked conversion of a method parameter in Java 8?

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野趣味
野趣味 2020-12-05 13:22

Consider the following code sample:

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
4条回答
  • 自闭症患者
    自闭症患者 (楼主)
    2020-12-05 13:49

    This looks like a known compatibility issue reported here and here.

    From the second link:

    The following code which compiled, with warnings, in JDK 7 will not compile in JDK 8:

    import java.util.List;
class SampleClass {

     static class Baz {
         public static List> sampleMethod(Baz param) {
             return null;
         }
     }

     private static void bar(Baz arg) {
         Baz element = Baz.sampleMethod(arg).get(0);
     }
}



    
  
    Compiling this code in JDK 8 produces the following error:
    
  
  
    SampleClass.java:12: error:incompatible types: Object cannot be
  converted to Baz
    


    Baz element = Baz.sampleMethod(arg).get(0);

    
  
  
    Note: SampleClass.java uses unchecked or unsafe operations. Note:
  Recompile with -Xlint:unchecked for details. 1 error
    

    


    Deriving from this, the OP's code can be fixed by replacing this line (the type declartion on the right hand side threw me off - I read it as a typed array list which it is not):
    


    List list = new ArrayList();

    


    with
    


    List list = new ArrayList();

    


    which will not result in type being being erased from return type of method getProducer(List list)
    


    Quote from second link again:
    


    
  
    In this example, a raw type is being passed to the
  sampleMethod(Baz) method which is applicable by subtyping (see
  the JLS, Java SE 7 Edition, section 15.12.2.2). An unchecked
  conversion is necessary for the method to be applicable, so its return
  type is erased (see the JLS, Java SE 7 Edition, section 15.12.2.6). In
  this case the return type of sampleMethod(Baz) is
  java.util.List instead of java.util.List> and thus the
  return type of get(int) is Object, which is not assignment-compatible
  with Baz.
    

    
    
             
    
                                                        
    
            
            
    
              
                
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