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Generics bounded parameter not applied for interface
The Next CEO of Stack OverflowWhy can this generic method with a bound return any type?How to generate a random alpha-numeric string?Create Generic method constraining T to an EnumCollections.emptyList() returns a List<Object>?How do I generate random integers within a specific range in Java?How do I make the method return type generic?How to create a generic array in Java?Does Java support default parameter values?Java bounded parameters in generic methodsBounded Generic Types in Method Parameters C#Multiple generic bounds including type parameter
Below is the code where I declared two methods whose return type are bounded parameters but one is based on interface whereas other is based on a class. In the test() method, assigning return type to String fails for method where bounded parameter extends from Class which is expected, but compiler doesn't throw error when the bounded parameter is based on interface. Can you explain why does this happen?
import java.io.FileInputStream;
import java.util.function.Predicate;
public class Dummy
<T extends Predicate<String>> T getBoundedByInterface()
return null;
<T extends FileInputStream> T getBoundedByClass()
return null;
public void test()
String compilationError = getBoundedByClass(); //Expected
String works = getBoundedByInterface(); //No compilation error. Why?
java generics bounded-types
add a comment |
Below is the code where I declared two methods whose return type are bounded parameters but one is based on interface whereas other is based on a class. In the test() method, assigning return type to String fails for method where bounded parameter extends from Class which is expected, but compiler doesn't throw error when the bounded parameter is based on interface. Can you explain why does this happen?
import java.io.FileInputStream;
import java.util.function.Predicate;
public class Dummy
<T extends Predicate<String>> T getBoundedByInterface()
return null;
<T extends FileInputStream> T getBoundedByClass()
return null;
public void test()
String compilationError = getBoundedByClass(); //Expected
String works = getBoundedByInterface(); //No compilation error. Why?
java generics bounded-types
Possible duplicate stackoverflow.com/questions/29670018/…
– Radiodef
Mar 8 at 19:43
add a comment |
Below is the code where I declared two methods whose return type are bounded parameters but one is based on interface whereas other is based on a class. In the test() method, assigning return type to String fails for method where bounded parameter extends from Class which is expected, but compiler doesn't throw error when the bounded parameter is based on interface. Can you explain why does this happen?
import java.io.FileInputStream;
import java.util.function.Predicate;
public class Dummy
<T extends Predicate<String>> T getBoundedByInterface()
return null;
<T extends FileInputStream> T getBoundedByClass()
return null;
public void test()
String compilationError = getBoundedByClass(); //Expected
String works = getBoundedByInterface(); //No compilation error. Why?
java generics bounded-types
Below is the code where I declared two methods whose return type are bounded parameters but one is based on interface whereas other is based on a class. In the test() method, assigning return type to String fails for method where bounded parameter extends from Class which is expected, but compiler doesn't throw error when the bounded parameter is based on interface. Can you explain why does this happen?
import java.io.FileInputStream;
import java.util.function.Predicate;
public class Dummy
<T extends Predicate<String>> T getBoundedByInterface()
return null;
<T extends FileInputStream> T getBoundedByClass()
return null;
public void test()
String compilationError = getBoundedByClass(); //Expected
String works = getBoundedByInterface(); //No compilation error. Why?
java generics bounded-types
java generics bounded-types
asked Mar 8 at 19:21
PrafullPrafull
284
284
Possible duplicate stackoverflow.com/questions/29670018/…
– Radiodef
Mar 8 at 19:43
add a comment |
Possible duplicate stackoverflow.com/questions/29670018/…
– Radiodef
Mar 8 at 19:43
Possible duplicate stackoverflow.com/questions/29670018/…
– Radiodef
Mar 8 at 19:43
Possible duplicate stackoverflow.com/questions/29670018/…
– Radiodef
Mar 8 at 19:43
add a comment |
1 Answer
1
active
oldest
votes
The Java compiler will determine if the variable type can satisfy the generic return type of the method called. With a class such as FileInputStream
, it knows that no class can be both a FileInputStream
and a String
because a class can't inherit from more than one class, so this is disallowed. Additionally we know that String
is also final
, but that doesn't figure into the compiler's logic.
However, with an interface, it's possible for a subclass to extend the bound and be a subclass of another class.
class Thing extends FileInputStream implements Predicate<String>
// Implementation
This compiles for that reason.
Thing works = getBoundedByInterface();
Notice that it also works with String
because the compiler doesn't take into account that String
is final
.
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
The Java compiler will determine if the variable type can satisfy the generic return type of the method called. With a class such as FileInputStream
, it knows that no class can be both a FileInputStream
and a String
because a class can't inherit from more than one class, so this is disallowed. Additionally we know that String
is also final
, but that doesn't figure into the compiler's logic.
However, with an interface, it's possible for a subclass to extend the bound and be a subclass of another class.
class Thing extends FileInputStream implements Predicate<String>
// Implementation
This compiles for that reason.
Thing works = getBoundedByInterface();
Notice that it also works with String
because the compiler doesn't take into account that String
is final
.
add a comment |
The Java compiler will determine if the variable type can satisfy the generic return type of the method called. With a class such as FileInputStream
, it knows that no class can be both a FileInputStream
and a String
because a class can't inherit from more than one class, so this is disallowed. Additionally we know that String
is also final
, but that doesn't figure into the compiler's logic.
However, with an interface, it's possible for a subclass to extend the bound and be a subclass of another class.
class Thing extends FileInputStream implements Predicate<String>
// Implementation
This compiles for that reason.
Thing works = getBoundedByInterface();
Notice that it also works with String
because the compiler doesn't take into account that String
is final
.
add a comment |
The Java compiler will determine if the variable type can satisfy the generic return type of the method called. With a class such as FileInputStream
, it knows that no class can be both a FileInputStream
and a String
because a class can't inherit from more than one class, so this is disallowed. Additionally we know that String
is also final
, but that doesn't figure into the compiler's logic.
However, with an interface, it's possible for a subclass to extend the bound and be a subclass of another class.
class Thing extends FileInputStream implements Predicate<String>
// Implementation
This compiles for that reason.
Thing works = getBoundedByInterface();
Notice that it also works with String
because the compiler doesn't take into account that String
is final
.
The Java compiler will determine if the variable type can satisfy the generic return type of the method called. With a class such as FileInputStream
, it knows that no class can be both a FileInputStream
and a String
because a class can't inherit from more than one class, so this is disallowed. Additionally we know that String
is also final
, but that doesn't figure into the compiler's logic.
However, with an interface, it's possible for a subclass to extend the bound and be a subclass of another class.
class Thing extends FileInputStream implements Predicate<String>
// Implementation
This compiles for that reason.
Thing works = getBoundedByInterface();
Notice that it also works with String
because the compiler doesn't take into account that String
is final
.
answered Mar 8 at 19:29
rgettmanrgettman
152k21209293
152k21209293
add a comment |
add a comment |
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Possible duplicate stackoverflow.com/questions/29670018/…
– Radiodef
Mar 8 at 19:43