Generic method to copy a list into a list of lists in Java

I'm trying to create a method as much generic as possible to copy a List into a List of Lists. I've tried with this one:

public static <E> void addToMatrix(List<E> list, List<List<? extends E>> matrix) {

    matrix.add(list);

}

Here is the main I've used to test the method:

public static void main(String args) {

    List< List<? extends Number> > matrix = new ArrayList<>();

    List<Integer> list = new ArrayList<>(); 



    addToMatrix(list, matrix);

}

Why it is not working ?
How can I do this in a proper way without changing the logic of the method ?

edited Nov 25 '18 at 15:36

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

Is there an issue you're having with the current method?

– Joe C
Nov 25 '18 at 15:28

I'm getting the following error on compile time: The method addToMatrix(List<E>, List<List<? extends E>>) in the type Main is not applicable for the arguments (List<Integer>, List<List<? extends Number>>)

– Aleksandar Stojkovski
Nov 25 '18 at 15:39

Why do you need a method? Calling matrix.add(list) directly seems to do what you need.

– Andy Turner
Nov 25 '18 at 16:27

add a comment |

I'm trying to create a method as much generic as possible to copy a List into a List of Lists. I've tried with this one:

public static <E> void addToMatrix(List<E> list, List<List<? extends E>> matrix) {

    matrix.add(list);

}

Here is the main I've used to test the method:

public static void main(String args) {

    List< List<? extends Number> > matrix = new ArrayList<>();

    List<Integer> list = new ArrayList<>(); 



    addToMatrix(list, matrix);

}

Why it is not working ?
How can I do this in a proper way without changing the logic of the method ?

edited Nov 25 '18 at 15:36

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

Is there an issue you're having with the current method?

– Joe C
Nov 25 '18 at 15:28

I'm getting the following error on compile time: The method addToMatrix(List<E>, List<List<? extends E>>) in the type Main is not applicable for the arguments (List<Integer>, List<List<? extends Number>>)

– Aleksandar Stojkovski
Nov 25 '18 at 15:39

Why do you need a method? Calling matrix.add(list) directly seems to do what you need.

– Andy Turner
Nov 25 '18 at 16:27

add a comment |

I'm trying to create a method as much generic as possible to copy a List into a List of Lists. I've tried with this one:

public static <E> void addToMatrix(List<E> list, List<List<? extends E>> matrix) {

    matrix.add(list);

}

Here is the main I've used to test the method:

public static void main(String args) {

    List< List<? extends Number> > matrix = new ArrayList<>();

    List<Integer> list = new ArrayList<>(); 



    addToMatrix(list, matrix);

}

Why it is not working ?
How can I do this in a proper way without changing the logic of the method ?

edited Nov 25 '18 at 15:36

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

I'm trying to create a method as much generic as possible to copy a List into a List of Lists. I've tried with this one:

public static <E> void addToMatrix(List<E> list, List<List<? extends E>> matrix) {

    matrix.add(list);

}

Here is the main I've used to test the method:

public static void main(String args) {

    List< List<? extends Number> > matrix = new ArrayList<>();

    List<Integer> list = new ArrayList<>(); 



    addToMatrix(list, matrix);

}

Why it is not working ?
How can I do this in a proper way without changing the logic of the method ?

java generics wildcard

edited Nov 25 '18 at 15:36

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

edited Nov 25 '18 at 15:36

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

edited Nov 25 '18 at 15:36

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

asked Nov 25 '18 at 15:26

Aleksandar Stojkovski

133

Is there an issue you're having with the current method?

– Joe C
Nov 25 '18 at 15:28

I'm getting the following error on compile time: The method addToMatrix(List<E>, List<List<? extends E>>) in the type Main is not applicable for the arguments (List<Integer>, List<List<? extends Number>>)

– Aleksandar Stojkovski
Nov 25 '18 at 15:39

Why do you need a method? Calling matrix.add(list) directly seems to do what you need.

– Andy Turner
Nov 25 '18 at 16:27

add a comment |

Is there an issue you're having with the current method?

– Joe C
Nov 25 '18 at 15:28

I'm getting the following error on compile time: The method addToMatrix(List<E>, List<List<? extends E>>) in the type Main is not applicable for the arguments (List<Integer>, List<List<? extends Number>>)

– Aleksandar Stojkovski
Nov 25 '18 at 15:39

Why do you need a method? Calling matrix.add(list) directly seems to do what you need.

– Andy Turner
Nov 25 '18 at 16:27

Is there an issue you're having with the current method?

– Joe C
Nov 25 '18 at 15:28

I'm getting the following error on compile time: The method addToMatrix(List<E>, List<List<? extends E>>) in the type Main is not applicable for the arguments (List<Integer>, List<List<? extends Number>>)

– Aleksandar Stojkovski
Nov 25 '18 at 15:39

Why do you need a method? Calling matrix.add(list) directly seems to do what you need.

– Andy Turner
Nov 25 '18 at 16:27

add a comment |

2 Answers
2

active

oldest

votes

I think the most general you can make it is like this (replace "E" with "? extends E" in the first List parameter):

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix)

{

    matrix.add(list);

}

This compiles and allows your method to accept any subtypes of E (including Integer and another subtype of Number).

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:58

@AleksandarStojkovski when you put in "? extends E" you are telling your program to expect a subtype of E. In your case, you are expecting a subtype of Number. But then when you are also requesting to initiate a list of integer (List<Integer>) the compiler gets confused, since it assigns Integer to E and then expects one of E's subtypes in List<List<? extends E>>. Problem is that a subtype of Number (as in "? extends Number") will not necessarily be a subtype of Integer too, so the compiler complains.

– Elia Anagnostou
Nov 25 '18 at 16:04

add a comment |

You can also change your method definition to:

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix) {

    matrix.add(list); 

}

since the add API for List<E> expects the argument E e which in your case is list of type List<? extends E>.

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

This seems to work. Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:54

@AleksandarStojkovski have added the details of what the List.add API contracts are.

– Naman
Nov 25 '18 at 15:55

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

I think the most general you can make it is like this (replace "E" with "? extends E" in the first List parameter):

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix)

{

    matrix.add(list);

}

This compiles and allows your method to accept any subtypes of E (including Integer and another subtype of Number).

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:58

@AleksandarStojkovski when you put in "? extends E" you are telling your program to expect a subtype of E. In your case, you are expecting a subtype of Number. But then when you are also requesting to initiate a list of integer (List<Integer>) the compiler gets confused, since it assigns Integer to E and then expects one of E's subtypes in List<List<? extends E>>. Problem is that a subtype of Number (as in "? extends Number") will not necessarily be a subtype of Integer too, so the compiler complains.

– Elia Anagnostou
Nov 25 '18 at 16:04

add a comment |

I think the most general you can make it is like this (replace "E" with "? extends E" in the first List parameter):

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix)

{

    matrix.add(list);

}

This compiles and allows your method to accept any subtypes of E (including Integer and another subtype of Number).

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:58

@AleksandarStojkovski when you put in "? extends E" you are telling your program to expect a subtype of E. In your case, you are expecting a subtype of Number. But then when you are also requesting to initiate a list of integer (List<Integer>) the compiler gets confused, since it assigns Integer to E and then expects one of E's subtypes in List<List<? extends E>>. Problem is that a subtype of Number (as in "? extends Number") will not necessarily be a subtype of Integer too, so the compiler complains.

– Elia Anagnostou
Nov 25 '18 at 16:04

add a comment |

I think the most general you can make it is like this (replace "E" with "? extends E" in the first List parameter):

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix)

{

    matrix.add(list);

}

This compiles and allows your method to accept any subtypes of E (including Integer and another subtype of Number).

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

I think the most general you can make it is like this (replace "E" with "? extends E" in the first List parameter):

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix)

{

    matrix.add(list);

}

This compiles and allows your method to accept any subtypes of E (including Integer and another subtype of Number).

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

answered Nov 25 '18 at 15:51

Elia Anagnostou

261

Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:58

@AleksandarStojkovski when you put in "? extends E" you are telling your program to expect a subtype of E. In your case, you are expecting a subtype of Number. But then when you are also requesting to initiate a list of integer (List<Integer>) the compiler gets confused, since it assigns Integer to E and then expects one of E's subtypes in List<List<? extends E>>. Problem is that a subtype of Number (as in "? extends Number") will not necessarily be a subtype of Integer too, so the compiler complains.

– Elia Anagnostou
Nov 25 '18 at 16:04

add a comment |

Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:58

@AleksandarStojkovski when you put in "? extends E" you are telling your program to expect a subtype of E. In your case, you are expecting a subtype of Number. But then when you are also requesting to initiate a list of integer (List<Integer>) the compiler gets confused, since it assigns Integer to E and then expects one of E's subtypes in List<List<? extends E>>. Problem is that a subtype of Number (as in "? extends Number") will not necessarily be a subtype of Integer too, so the compiler complains.

– Elia Anagnostou
Nov 25 '18 at 16:04

Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:58

@AleksandarStojkovski when you put in "? extends E" you are telling your program to expect a subtype of E. In your case, you are expecting a subtype of Number. But then when you are also requesting to initiate a list of integer (List<Integer>) the compiler gets confused, since it assigns Integer to E and then expects one of E's subtypes in List<List<? extends E>>. Problem is that a subtype of Number (as in "? extends Number") will not necessarily be a subtype of Integer too, so the compiler complains.

– Elia Anagnostou
Nov 25 '18 at 16:04

add a comment |

You can also change your method definition to:

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix) {

    matrix.add(list); 

}

since the add API for List<E> expects the argument E e which in your case is list of type List<? extends E>.

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

This seems to work. Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:54

@AleksandarStojkovski have added the details of what the List.add API contracts are.

– Naman
Nov 25 '18 at 15:55

add a comment |

You can also change your method definition to:

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix) {

    matrix.add(list); 

}

since the add API for List<E> expects the argument E e which in your case is list of type List<? extends E>.

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

This seems to work. Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:54

@AleksandarStojkovski have added the details of what the List.add API contracts are.

– Naman
Nov 25 '18 at 15:55

add a comment |

You can also change your method definition to:

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix) {

    matrix.add(list); 

}

since the add API for List<E> expects the argument E e which in your case is list of type List<? extends E>.

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

You can also change your method definition to:

public static <E> void addToMatrix(List<? extends E> list, List<List<? extends E>> matrix) {

    matrix.add(list); 

}

since the add API for List<E> expects the argument E e which in your case is list of type List<? extends E>.

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

answered Nov 25 '18 at 15:48

Naman

44.4k11102204

This seems to work. Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:54

@AleksandarStojkovski have added the details of what the List.add API contracts are.

– Naman
Nov 25 '18 at 15:55

add a comment |

This seems to work. Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:54

@AleksandarStojkovski have added the details of what the List.add API contracts are.

– Naman
Nov 25 '18 at 15:55

This seems to work. Could you please explain why it is working, and what is wrong with my implementation ?

– Aleksandar Stojkovski
Nov 25 '18 at 15:54

@AleksandarStojkovski have added the details of what the List.add API contracts are.

– Naman
Nov 25 '18 at 15:55

add a comment |

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