Java - What is the sense of: List unmodifiableList(List list) [duplicate]
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Understanding upper and lower bounds on ? in Java Generics
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Generic type parameter naming convention for Java (with multiple chars)?
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This is a static method from the Collections class. The type argument "? extends T" is bounded, in particular it has an upper bound represented by the type variable T which identifies the generic's method one and only type parameter which is implicitly bounded by the type Object.
However, I do not know what the point of writing "? extends T" compared to "?" is, since the type parameter is never replaced by a type argument?
java generics type-parameter generic-type-argument
asked Nov 20 at 17:56
chipsmonster91
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Nov 20 at 18:04
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
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This question already has an answer here:
Understanding upper and lower bounds on ? in Java Generics
5 answers
Generic type parameter naming convention for Java (with multiple chars)?
5 answers
This is a static method from the Collections class. The type argument "? extends T" is bounded, in particular it has an upper bound represented by the type variable T which identifies the generic's method one and only type parameter which is implicitly bounded by the type Object.
However, I do not know what the point of writing "? extends T" compared to "?" is, since the type parameter is never replaced by a type argument?
java generics type-parameter generic-type-argument
asked Nov 20 at 17:56
chipsmonster91
6
marked as duplicate by Makoto java
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Nov 20 at 18:04
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
The List<T> probably wants to deal with a specific kind of object. Having a <? extends T> would allow the class to narrow down what it will work with.<? extends T>is the difference between me creating aList<String>, and being able toAdd()an int as an element. Because of <? extends String> would fail on an int, it won't allow me to add it, a good thing of course.
– Frontear
Nov 20 at 17:57
Thanks, but that was not the question. The question is what's the difference between "? extends T" and "?" whereby T just refers to the type parameter of the generic method.
– chipsmonster91
Nov 20 at 18:02
?will allow for anything to pass through.? extends Twill allow only Objects that are either of typeT, or extend typeTto pass through.
– Frontear
Nov 20 at 18:03
1
List<?>would allow this to compile:List<String> list = Collections.unmodifiableList(Arrays.asList(1, 2, 3));
– shmosel
Nov 20 at 18:08
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It's defined right there at the beginning. What are you saying?
– shmosel
Nov 20 at 18:40
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up vote
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down vote
favorite
This question already has an answer here:
Understanding upper and lower bounds on ? in Java Generics
5 answers
Generic type parameter naming convention for Java (with multiple chars)?
5 answers
This is a static method from the Collections class. The type argument "? extends T" is bounded, in particular it has an upper bound represented by the type variable T which identifies the generic's method one and only type parameter which is implicitly bounded by the type Object.
However, I do not know what the point of writing "? extends T" compared to "?" is, since the type parameter is never replaced by a type argument?
java generics type-parameter generic-type-argument
asked Nov 20 at 17:56
chipsmonster91
6
This question already has an answer here:
Understanding upper and lower bounds on ? in Java Generics
5 answers
Generic type parameter naming convention for Java (with multiple chars)?
5 answers
This is a static method from the Collections class. The type argument "? extends T" is bounded, in particular it has an upper bound represented by the type variable T which identifies the generic's method one and only type parameter which is implicitly bounded by the type Object.
However, I do not know what the point of writing "? extends T" compared to "?" is, since the type parameter is never replaced by a type argument?
This question already has an answer here:
Understanding upper and lower bounds on ? in Java Generics
5 answers
Generic type parameter naming convention for Java (with multiple chars)?
5 answers
java generics type-parameter generic-type-argument
java generics type-parameter generic-type-argument
asked Nov 20 at 17:56
chipsmonster91
6
asked Nov 20 at 17:56
chipsmonster91
6
asked Nov 20 at 17:56
chipsmonster91
6
asked Nov 20 at 17:56
chipsmonster91
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asked Nov 20 at 17:56
chipsmonster91
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6
marked as duplicate by Makoto java
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Nov 20 at 18:04
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
marked as duplicate by Makoto java
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Nov 20 at 18:04
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
The List<T> probably wants to deal with a specific kind of object. Having a <? extends T> would allow the class to narrow down what it will work with.<? extends T>is the difference between me creating aList<String>, and being able toAdd()an int as an element. Because of <? extends String> would fail on an int, it won't allow me to add it, a good thing of course.
– Frontear
Nov 20 at 17:57
Thanks, but that was not the question. The question is what's the difference between "? extends T" and "?" whereby T just refers to the type parameter of the generic method.
– chipsmonster91
Nov 20 at 18:02
?will allow for anything to pass through.? extends Twill allow only Objects that are either of typeT, or extend typeTto pass through.
– Frontear
Nov 20 at 18:03
1
List<?>would allow this to compile:List<String> list = Collections.unmodifiableList(Arrays.asList(1, 2, 3));
– shmosel
Nov 20 at 18:08
1
It's defined right there at the beginning. What are you saying?
– shmosel
Nov 20 at 18:40
|
show 9 more comments
The List<T> probably wants to deal with a specific kind of object. Having a <? extends T> would allow the class to narrow down what it will work with.<? extends T>is the difference between me creating aList<String>, and being able toAdd()an int as an element. Because of <? extends String> would fail on an int, it won't allow me to add it, a good thing of course.
– Frontear
Nov 20 at 17:57
Thanks, but that was not the question. The question is what's the difference between "? extends T" and "?" whereby T just refers to the type parameter of the generic method.
– chipsmonster91
Nov 20 at 18:02
?will allow for anything to pass through.? extends Twill allow only Objects that are either of typeT, or extend typeTto pass through.
– Frontear
Nov 20 at 18:03
1
List<?>would allow this to compile:List<String> list = Collections.unmodifiableList(Arrays.asList(1, 2, 3));
– shmosel
Nov 20 at 18:08
1
It's defined right there at the beginning. What are you saying?
– shmosel
Nov 20 at 18:40
The List<T> probably wants to deal with a specific kind of object. Having a <? extends T> would allow the class to narrow down what it will work with.
<? extends T> is the difference between me creating a List<String>, and being able to Add() an int as an element. Because of <? extends String> would fail on an int, it won't allow me to add it, a good thing of course.– Frontear
Nov 20 at 17:57
The List<T> probably wants to deal with a specific kind of object. Having a <? extends T> would allow the class to narrow down what it will work with.
<? extends T> is the difference between me creating a List<String>, and being able to Add() an int as an element. Because of <? extends String> would fail on an int, it won't allow me to add it, a good thing of course.– Frontear
Nov 20 at 17:57
Thanks, but that was not the question. The question is what's the difference between "? extends T" and "?" whereby T just refers to the type parameter of the generic method.
– chipsmonster91
Nov 20 at 18:02
Thanks, but that was not the question. The question is what's the difference between "? extends T" and "?" whereby T just refers to the type parameter of the generic method.
– chipsmonster91
Nov 20 at 18:02
? will allow for anything to pass through. ? extends T will allow only Objects that are either of type T, or extend type T to pass through.– Frontear
Nov 20 at 18:03
? will allow for anything to pass through. ? extends T will allow only Objects that are either of type T, or extend type T to pass through.– Frontear
Nov 20 at 18:03
1
1
List<?> would allow this to compile: List<String> list = Collections.unmodifiableList(Arrays.asList(1, 2, 3));– shmosel
Nov 20 at 18:08
List<?> would allow this to compile: List<String> list = Collections.unmodifiableList(Arrays.asList(1, 2, 3));– shmosel
Nov 20 at 18:08
1
1
It's defined right there at the beginning. What are you saying?
– shmosel
Nov 20 at 18:40
It's defined right there at the beginning. What are you saying?
– shmosel
Nov 20 at 18:40
|
show 9 more comments
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The List<T> probably wants to deal with a specific kind of object. Having a <? extends T> would allow the class to narrow down what it will work with.
<? extends T>is the difference between me creating aList<String>, and being able toAdd()an int as an element. Because of <? extends String> would fail on an int, it won't allow me to add it, a good thing of course.– Frontear
Nov 20 at 17:57
Thanks, but that was not the question. The question is what's the difference between "? extends T" and "?" whereby T just refers to the type parameter of the generic method.
– chipsmonster91
Nov 20 at 18:02
?will allow for anything to pass through.? extends Twill allow only Objects that are either of typeT, or extend typeTto pass through.– Frontear
Nov 20 at 18:03
1
List<?>would allow this to compile:List<String> list = Collections.unmodifiableList(Arrays.asList(1, 2, 3));– shmosel
Nov 20 at 18:08
1
It's defined right there at the beginning. What are you saying?
– shmosel
Nov 20 at 18:40