Collections emptyList returns a List Object

Question

I m having some trouble navigating Java s rule for inferring generic type parameters  Consider the following class  which has an optional list parameter  import java util Collections  import java util List   public class Person     private String name    private List lt String gt  nicknames       public Person String name        this name  Collections emptyList              public Person String name  List lt String gt  nicknames        this name   name      this nicknames   nicknames         My Java compiler gives the following error  Person java 9  The constructor Person String  List lt Object gt   is undefined  But Collections emptyList   returns type  lt T gt  List lt T gt   not List lt Object gt   Adding a cast doesn t help public Person String name      this name  List lt String gt  Collections emptyList        yields Person java 9  inconvertible types  Using EMPTY LIST instead of emptyList   public Person String name      this name  Collections EMPTY LIST      yields Person java 9  warning   unchecked  unchecked conversion  Whereas the following change makes the error go away  public Person String name      this name   name    this nicknames   Collections emptyList       Can anyone explain what type-checking rule I m running up against here  and the best way to work around it  In this example  the final code example is satisfactory  but with larger classes  I d like to be able to write methods following this  quot optional parameter quot  pattern without duplicating code  For extra credit  when is it appropriate to use EMPTY LIST as opposed to emptyList

User · Answer

You want to use   Collections  lt String gt emptyList      If you look at the source for what emptyList does you see that it actually just does a   return  List lt T gt  EMPTY LIST

User · Answer

You want to use   Collections  lt String gt emptyList      If you look at the source for what emptyList does you see that it actually just does a   return  List lt T gt  EMPTY LIST

User · Answer

You want to use   Collections  lt String gt emptyList      If you look at the source for what emptyList does you see that it actually just does a   return  List lt T gt  EMPTY LIST

User · Answer

the emptyList method has this signature   public static final  lt T gt  List lt T gt  emptyList     That  lt T gt  before the word List means that it infers the value of the generic parameter T from the type of variable the result is assigned to  So in this case   List lt String gt  stringList   Collections emptyList      The return value is then referenced explicitly by a variable of type List lt String gt   so the compiler can figure it out  In this case   setList Collections emptyList       There s no explicit return variable for the compiler to use to figure out the generic type  so it defaults to Object

User · Answer

You want to use   Collections  lt String gt emptyList      If you look at the source for what emptyList does you see that it actually just does a   return  List lt T gt  EMPTY LIST

User · Answer

the emptyList method has this signature   public static final  lt T gt  List lt T gt  emptyList     That  lt T gt  before the word List means that it infers the value of the generic parameter T from the type of variable the result is assigned to  So in this case   List lt String gt  stringList   Collections emptyList      The return value is then referenced explicitly by a variable of type List lt String gt   so the compiler can figure it out  In this case   setList Collections emptyList       There s no explicit return variable for the compiler to use to figure out the generic type  so it defaults to Object

User · Answer

The issue you re encountering is that even though the method emptyList   returns List lt T gt   you haven t provided it with the type  so it defaults to returning List lt Object gt    You can supply the type parameter  and have your code behave as expected  like this   public Person String name      this name Collections  lt String gt emptyList         Now when you re doing straight assignment  the compiler can figure out the generic type parameters for you   It s called type inference   For example  if you did this   public Person String name      List lt String gt  emptyList   Collections emptyList      this name  emptyList       then the emptyList   call would correctly return a List lt String gt

User · Answer

the emptyList method has this signature   public static final  lt T gt  List lt T gt  emptyList     That  lt T gt  before the word List means that it infers the value of the generic parameter T from the type of variable the result is assigned to  So in this case   List lt String gt  stringList   Collections emptyList      The return value is then referenced explicitly by a variable of type List lt String gt   so the compiler can figure it out  In this case   setList Collections emptyList       There s no explicit return variable for the compiler to use to figure out the generic type  so it defaults to Object

User · Answer

The issue you re encountering is that even though the method emptyList   returns List lt T gt   you haven t provided it with the type  so it defaults to returning List lt Object gt    You can supply the type parameter  and have your code behave as expected  like this   public Person String name      this name Collections  lt String gt emptyList         Now when you re doing straight assignment  the compiler can figure out the generic type parameters for you   It s called type inference   For example  if you did this   public Person String name      List lt String gt  emptyList   Collections emptyList      this name  emptyList       then the emptyList   call would correctly return a List lt String gt

User · Answer

Since Java 8 this kind of code compiles as expected and the type parameter gets inferred by the compiler  public Person String name        this name  Collections emptyList        Inferred to List lt String gt  in Java 8    public Person String name  List lt String gt  nicknames        this name   name      this nicknames   nicknames     The new thing in Java 8 is that the target type of an expression will be used to infer type parameters of its sub-expressions  Before Java 8 only direct assignments and arguments to methods where used for type parameter inference  In this case the parameter type of the constructor will be the target type for Collections emptyList    and the return value type will get chosen to match the parameter type  This mechanism was added in Java 8 mainly to be able to compile lambda expressions  but it improves type inferences generally  Java is getting closer to proper Hindley   Milner type inference with every release

User · Answer

Since Java 8 this kind of code compiles as expected and the type parameter gets inferred by the compiler  public Person String name        this name  Collections emptyList        Inferred to List lt String gt  in Java 8    public Person String name  List lt String gt  nicknames        this name   name      this nicknames   nicknames     The new thing in Java 8 is that the target type of an expression will be used to infer type parameters of its sub-expressions  Before Java 8 only direct assignments and arguments to methods where used for type parameter inference  In this case the parameter type of the constructor will be the target type for Collections emptyList    and the return value type will get chosen to match the parameter type  This mechanism was added in Java 8 mainly to be able to compile lambda expressions  but it improves type inferences generally  Java is getting closer to proper Hindley   Milner type inference with every release

User · Answer

the emptyList method has this signature   public static final  lt T gt  List lt T gt  emptyList     That  lt T gt  before the word List means that it infers the value of the generic parameter T from the type of variable the result is assigned to  So in this case   List lt String gt  stringList   Collections emptyList      The return value is then referenced explicitly by a variable of type List lt String gt   so the compiler can figure it out  In this case   setList Collections emptyList       There s no explicit return variable for the compiler to use to figure out the generic type  so it defaults to Object

User · Answer

The issue you re encountering is that even though the method emptyList   returns List lt T gt   you haven t provided it with the type  so it defaults to returning List lt Object gt    You can supply the type parameter  and have your code behave as expected  like this   public Person String name      this name Collections  lt String gt emptyList         Now when you re doing straight assignment  the compiler can figure out the generic type parameters for you   It s called type inference   For example  if you did this   public Person String name      List lt String gt  emptyList   Collections emptyList      this name  emptyList       then the emptyList   call would correctly return a List lt String gt

User · Answer

The issue you re encountering is that even though the method emptyList   returns List lt T gt   you haven t provided it with the type  so it defaults to returning List lt Object gt    You can supply the type parameter  and have your code behave as expected  like this   public Person String name      this name Collections  lt String gt emptyList         Now when you re doing straight assignment  the compiler can figure out the generic type parameters for you   It s called type inference   For example  if you did this   public Person String name      List lt String gt  emptyList   Collections emptyList      this name  emptyList       then the emptyList   call would correctly return a List lt String gt

[java] Collections.emptyList() returns a List<Object>?

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