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Thread: Writing my own custom comparator in java

  1. #1
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    Default Writing my own custom comparator in java

    Hello guys,

    I have a java code that should sort an array of names based on the last name. e.g jane a, jane b, jane z, jane d should be
    jane a, jane b, jane d, jane z. I have the following code but for some reasons, the s1 in the comparator method is always null. Any idea why this is so?


     
     
     
    public class ShuffleName {
     
     
    	public static void sortNames(String[] names){
     
    		 Arrays.sort( names, new Comparator<String>() { 
     
    				public int compare( String s1, String s2 ) {
     
    				String s1last = s1.split("\\s+")[1]; 
    				String s2last = s2.split("\\s+")[1]; 
    				return s1last.compareTo(s2last); 
    				} 
    				}); 
     
    				for ( String s : names ) System.out.println(s);
     
     
     
     
    	}
     
     
    	public static void main(String[] args) {
     
    		System.out.println("Enter your firstName and sureName : \n");
    		Scanner st = new Scanner(System.in);
    		String [] namesArray = new String[6];
    		String name = new String();
    		int index = 0;
    		int maxLength = namesArray.length;
     
    		name = st.nextLine();
     
    		while( (!name.trim().equals(""))){
     
    				if (index < maxLength) {
    					namesArray[index] = name;
    					index++;
    				} else {
    					namesArray = Arrays.copyOf(namesArray, maxLength + 4);
    					namesArray[index] = name;
    					index++;
    				}
     
    				System.out.println("Enter your firstName and sureName : \n");
    				name = st.nextLine();
    		}
     
     
    		sortNames(namesArray);
     
     
     
    	}
     
    }


  2. #2
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    Default Re: Writing my own custom comparator in java

    Post a sample run copied from your console that shows the inputs/outputs and the problem you've described. If you're asking for help with an error message, post the complete text of the error message.

  3. #3
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    Default Re: Writing my own custom comparator in java

    INPUT:
     
    Enter your firstName and sureName : 
    john a [ENTER]
     
    Enter your firstName and sureName : 
    john b [ENTER]
     
    Enter your firstName and sureName : 
    john z [ENTER]
     
    Enter your firstName and sureName : 
    john d [ENTER]
     
    Enter your firstName and sureName : 
    [ENTER]
     
     
    OUTPUT:
     
    Exception in thread "main" java.lang.NullPointerException
    	at org.Busy.WeYou.ShuffleName$1.compare(ShuffleName.java:16)
    	at org.Busy.WeYou.ShuffleName$1.compare(ShuffleName.java:1)
    	at java.util.TimSort.binarySort(TimSort.java:265)
    	at java.util.TimSort.sort(TimSort.java:190)
    	at java.util.TimSort.sort(TimSort.java:173)
    	at java.util.Arrays.sort(Arrays.java:659)
    	at org.Busy.WeYou.ShuffleName.sortNames(ShuffleName.java:12)
    	at org.Busy.WeYou.ShuffleName.main(ShuffleName.java:57)
    The reason is because the String s1 is null even though when I checked the names array, all the names entered are complete.
    Last edited by help_desk; July 27th, 2014 at 05:02 PM. Reason: format line

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    Default Re: Writing my own custom comparator in java

    You have an array of names that looks something like:

    [ john a, john b, john z, john d, null, null ]

    How will the null values be handled?

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    Default Re: Writing my own custom comparator in java

    Quote Originally Posted by GregBrannon View Post
    You have an array of names that looks something like:

    [ john a, john b, john z, john d, null, null ]

    How will the null values be handled?
    Yeah that is true but internally the array seems to be something like this :
    [ null, john b, john z, john d, null, null ]

    I don't get why this is so. Please, how could I handle null in my custom comparator? I could do something like this:

    public int compare( String s1, String s2 ) {
                                    if(s1!=null && s2!=null){
    				String s1last = s1.split("\\s+")[1]; 
    				String s2last = s2.split("\\s+")[1]; 
    				return s1last.compareTo(s2last); 
    				} 
                             }

    but that still doesn't explain why the s1 String is null even though, that was not how the names were stored.

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    Default Re: Writing my own custom comparator in java

    You can add some print statements to see what is happening and answer your question. For example, I added a few and got the following as a result. From that you can see it's not as simple as you're imagining.
    Enter your firstName and sureName : 
     
    John a
    Name = John a
    Enter your firstName and sureName : 
     
    John b
    Name = John b
    Enter your firstName and sureName : 
     
    John z
    Name = John z
    Enter your firstName and sureName : 
     
    John d
    Name = John d
    Enter your firstName and sureName : 
     
     
    Array of names = [John a, John b, John z, John d, null, null]
    s1 = John b
    s2 = John a
    s1 = John z
    s2 = John b
    s1 = John d
    s2 = John z
    s1 = John d
    s2 = John b
    s1 = John d
    s2 = John z
    s1 = null
    s2 = John d
    Exception in thread "main" java.lang.NullPointerException
    	at TestClass$1.compare(TestClass.java:16)
    	at TestClass$1.compare(TestClass.java:1)
    	at java.util.TimSort.binarySort(TimSort.java:265)
    	at java.util.TimSort.sort(TimSort.java:190)
    	at java.util.TimSort.sort(TimSort.java:173)
    	at java.util.Arrays.sort(Arrays.java:659)
    	at TestClass.sortNames(TestClass.java:9)
    	at TestClass.main(TestClass.java:60)

  7. The Following User Says Thank You to GregBrannon For This Useful Post:

    help_desk (July 27th, 2014)

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