Thread: 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);
 
 
 
	}
 
}

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.

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?

Originally Posted by GregBrannon

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.

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)