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Thread: Collections.sort

  1. #1
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    Default Collections.sort

    This compiles and runs well:
    public class IntegerTest implements Comparator<Integer>{
    @Override
    public int compare(Integer o1, Integer o2){
    return (o1<o2 ? -1 : (o1==o2 ? 0 : 1));
    }


    /**
    * @param args the command line arguments
    */
    public static void main(String[] args) {
    List<Integer> arrayList = new ArrayList<>();
    arrayList.add(12);
    arrayList.add(5);
    arrayList.add(7);
    arrayList.add(4);
    arrayList.add(5);
    arrayList.add(16);
    arrayList.add(13);
    arrayList.add(2);
    System.out.println(arrayList);
    Collections.sort (arrayList, new IntegerTest());
    for (Integer integer : arrayList){
    System.out.println (integer);

    }
    }
    }

    This code creates and prints the array but gives me an error at Collections.sort.
    What is wrong with the call?

    public class ListSorter implements Comparator<Integer>{

    @Override
    public int compare(Integer o1, Integer o2){
    return (o1>o2 ? -1 : (o1==o2 ? 0 : 1));
    }

    }
    public static void main(String[] args)throws IOException {
    java.io.File intDat = new java.io.File("intDat");
    try (java.io.PrintWriter create = new java.io.PrintWriter (intDat)) {
    create.print("23 45 67 12 9 44 14 21 78 95 6 89 7 42 28 34");
    }
    Collection<Integer> intList;
    try (Scanner read = new Scanner (intDat)) {
    intList = new ArrayList<>();
    //List<Integer> arrayList = new ArrayList<>();
    while (read.hasNext()){
    intList.add(read .nextInt());


    System.out.println(intList);
    }
    }
    //System.out.println(arrayList);

    Collections.sort (intList, new ListSorter());
    for (Integer integer : intList){
    System.out.println (integer);
    }

    }
    }
    post.jpg

  2. #2
    Super Moderator curmudgeon's Avatar
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    Default Re: Collections.sort

    Please wrap your code in [code] [/code] tags. Please post the actual error message.

  3. #3
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    Default Re: Collections.sort

    I am looking at other posts to see what you mean by "
     
    tags".

  4. #4
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    Default Re: Collections.sort

    Here's another look at code tags:

    [code=java]
    <YOUR CODE HERE>
    [/code]
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
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    Default Re: Collections.sort

    Thanks!

  6. #6
    Super Moderator curmudgeon's Avatar
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    Default Re: Collections.sort

    You can edit your original post and place the tags around your posted code. Again please post the error message?

  7. #7
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    Default Re: Collections.sort

    This compiles and runs well:
    public class IntegerTest implements Comparator<Integer>{
    @Override
    public int compare(Integer o1, Integer o2){
    return (o1<o2 ? -1 : (o1==o2 ? 0 : 1));
    }
     
     
    /**
    * @param args the command line arguments
    */
    public static void main(String[] args) {
    List<Integer> arrayList = new ArrayList<>();
    arrayList.add(12);
    arrayList.add(5);
    arrayList.add(7);
    arrayList.add(4);
    arrayList.add(5);
    arrayList.add(16);
    arrayList.add(13);
    arrayList.add(2);
    System.out.println(arrayList);
    Collections.sort (arrayList, new IntegerTest());
    for (Integer integer : arrayList){
    System.out.println (integer);
     
    }
    }
    }

    The code below gives me an error at 28:
    non-static variable this cannot be referenced from a static context

    no suitable method found for sort(Collection<Integer>,gtleHw4.ListSorter)
    method Collections.<T#1>sort(List<T#1>,Comparator<? super T#1>) is not applicable
    (no instance(s) of type variable(s) T#1 exist so that argument type Collection<Integer> conforms to formal parameter type List<T#1>)
    method Collections.<T#2>sort(List<T#2>) is not applicable
    (cannot instantiate from arguments because actual and formal argument lists differ in length)
    where T#1,T#2 are type-variables:
    T#1 extends Object declared in method <T#1>sort(List<T#1>,Comparator<? super T#1>)
    T#2 extends Comparable<? super T#2> declared in method <T#2>sort(List<T#2>

    The only difference I see is the while lop reading integers from a file.
    1.	    
    2.	   public class ListSorter implements Comparator<Integer>{
    3.	       
    4.	       @Override
    5.	       public int compare(Integer o1, Integer o2){
    6.	           return (o1>o2 ? -1 : (o1==o2 ? 0 : 1));
    7.	       }
    8.	       
    9.	   }
    10.	    public static void main(String[] args)throws IOException {
    11.	        java.io.File intDat = new java.io.File("intDat");
    12.	        try (java.io.PrintWriter create = new java.io.PrintWriter (intDat)) {
    13.	            create.print("23 45 67 12 9 44 14 21 78 95 6 89 7 42 28 34");
    14.	        }
    15.	        Collection<Integer> intList;        
    16.	        try (Scanner read = new Scanner (intDat)) {
    17.	            intList = new ArrayList<>();
    18.	            //List<Integer> arrayList = new ArrayList<>();
    19.	            while (read.hasNext()){
    20.	                intList.add(read .nextInt());
    21.	                
    22.	                   
    23.	             System.out.println(intList);
    24.	            }
    25.	        }
    26.	        //System.out.println(arrayList);
    27.	           
    28.	          Collections.sort (intList, new ListSorter());
    29.	            for (Integer integer : intList){
    30.	                System.out.println (integer);
    31.	            }
    32.	            
    33.	                   }
    34.	    }
    35.	
    36.
    Last edited by gtrankle; March 24th, 2013 at 02:09 PM. Reason: Curmudgeon not happy

  8. #8
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    Default Re: Collections.sort

    Where are the line numbers showing where in the source file the errors happened?
    If you don't understand my answer, don't ignore it, ask a question.

  9. #9
    Super Moderator curmudgeon's Avatar
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    Default Re: Collections.sort

    And again, please edit your original post and add the code tags. Please don't re-ask the same question in the forum.

  10. #10
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    Default Re: Collections.sort

    error at 28

  11. #11
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    Default Re: Collections.sort

    non-static variable this cannot be referenced from a static context

    error at 28
    Where is the "this" variable on line 28?
    If you don't understand my answer, don't ignore it, ask a question.

  12. #12
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    Default Re: Collections.sort

    Idk??? there is no this there.

  13. #13
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    Default Re: Collections.sort

    Use the javac command to get good error messages.
    Here is a sample:
    TestSorts.java:138: cannot find symbol
    symbol  : variable var
    location: class TestSorts
             var = 2;
             ^
    If you don't understand my answer, don't ignore it, ask a question.

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