Arrays asList of an array

Question

What is wrong with this conversion   public int getTheNumber int   factors        ArrayList lt Integer gt  f   new ArrayList Arrays asList factors          Collections sort f       return f get 0  f get f size  -1       I made this after reading the solution found in Create ArrayList from array  The second line  sorting  in getTheNumber      causes the following exception      Exception in thread  main  java lang ClassCastException   I cannot be cast to java lang Comparable    What is wrong here  I do realize that sorting could be done with Arrays sort    I m just curious about this one

User · Answer

Use java utils Arrays   public int getTheNumber int   factors        int   f    int   factors clone        Arrays sort f       return f 0  f  f length-1       Or if you want to be efficient avoid all the object allocation just actually do the work   public static int getTheNumber int   array        if  array length    0          throw new IllegalArgumentException        int min   array 0       int max   array 0       for  int i   1  i lt  array length   i            int v   array i           if  v  lt  min                min   v            else if  v  gt  max                max   v                      return min   max

User · Answer

I think you have found an example where auto-boxing doesn t really work  Because Arrays asList T    a  has a varargs parameter the compiler apparently considers the int   and returns a List lt int   gt  with a single element in it   You should change the method into this   public int getTheNumber Integer   factors        ArrayList lt Integer gt  f   new ArrayList lt Integer gt  Arrays asList factors          Collections sort f       return f get 0    f get f size   - 1       and possibly add this for compatibility  public int getTheNumber int   factors        Integer   factorsInteger   new Integer factors length       for int ii 0  ii lt factors length    ii            factorsInteger ii    factors ii              return getTheNumber factorsInteger

User · Answer

As far as I understand it  the sort function in the collection class can only be used to sort collections implementing the comparable interface   You are supplying it a array of integers  You should probably wrap this around one of the know Wrapper classes such as Integer  Integer implements comparable   Its been a long time since I have worked on some serious Java  however reading some matter on the sort function will help

User · Answer

Let s consider the following simplified example   public class Example       public static void main String   args            int   factors    1  2  3           ArrayList lt Integer gt  f   new ArrayList Arrays asList factors            System out println f             At the println line this prints something like    I 190d11   which means that you have actually constructed an ArrayList that contains int arrays   Your IDE and compiler should warn about unchecked assignments in that code  You should always use new ArrayList lt Integer gt    or new ArrayList lt  gt    instead of new ArrayList    If you had used it  there would have been a compile error because of trying to pass List lt int   gt  to the constructor   There is no autoboxing from int   to Integer    and anyways autoboxing is only syntactic sugar in the compiler  so in this case you need to do the array copy manually   public static int getTheNumber int   factors        List lt Integer gt  f   new ArrayList lt Integer gt         for  int factor   factors            f add factor      after autoboxing the same as  f add Integer valueOf factor              Collections sort f       return f get 0    f get f size   - 1

User · Answer

You are trying to cast int   to Integer    this is not possible   You can use commons-lang s ArrayUtils to convert the ints to Integers before getting the List from the array   public int getTheNumber int   factors        Integer   integers   ArrayUtils toObject factors       ArrayList lt Integer gt  f   new ArrayList lt Integer gt  Arrays asList integers        Collections sort f       return f get 0  f get f size  -1

User · Answer

this is from Java API   sort  public static void sort List list  Sorts the specified list into ascending order  according to the natural ordering of its elements  All elements in the list must implement the Comparable interface  Furthermore  all elements in the list must be mutually comparable  that is  e1 compareTo e2  must not throw a ClassCastException for any elements e1 and e2 in the list     it has to do with implementing the Comparable interface

User · Answer

This works from Java 5 to 7   public int getTheNumber Integer    factors        ArrayList lt Integer gt  f   new ArrayList lt Integer gt  Arrays asList factors        Collections sort f       return f get 0  f get f size  -1       In Java 4 there is no vararg     -

User · Answer

Arrays asList factors  returns a List lt int   gt   not a List lt Integer gt   Since you re doing new ArrayList instead of new ArrayList lt Integer gt  you don t get a compile error for that  but create an ArrayList lt Object gt  which contains an int   and you then implicitly cast that arraylist to ArrayList lt Integer gt   Of course the first time you try to use one of those  Integers  you get an exception

User · Answer

there are two cause of this exception   1  Arrays asList factors  returns a List lt int   gt  where factors is an int array  2  you forgot to add the type parameter to    ArrayList lt Integer gt  f   new ArrayList Arrays asList factors      with   ArrayList lt Integer gt  f   new ArrayList lt Integer gt  Arrays asList factors        resulting in a compile-time error    found     java util List lt int    required  java util List lt java lang Integer

[java] Arrays.asList() of an array

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