[java] Java get String CompareTo as a comparator object

I would like to sort and binary search a static array of strings via the String.CompareTo comparator.

The problem is that both sorting, and binary searching requires that a Comparator object be passed in -- So how do I pass in the built in string comparator?

We can use the String.CASE_INSENSITIVE_ORDER comparator to compare the strings in case insensitive order.

Arrays.binarySearch(someStringArray, "The String to find.",String.CASE_INSENSITIVE_ORDER);

This is a generic Comparator for any kind of Comparable object, not just String:

package util;

import java.util.Comparator;

/**
 * The Default Comparator for classes implementing Comparable.
 *
 * @param <E> the type of the comparable objects.
 *
 * @author Michael Belivanakis (michael.gr)
 */
public final class DefaultComparator<E extends Comparable<E>> implements Comparator<E>
{
    @SuppressWarnings( "rawtypes" )
    private static final DefaultComparator<?> INSTANCE = new DefaultComparator();

    /**
     * Get an instance of DefaultComparator for any type of Comparable.
     *
     * @param <T> the type of Comparable of interest.
     *
     * @return an instance of DefaultComparator for comparing instances of the requested type.
     */
    public static <T extends Comparable<T>> Comparator<T> getInstance()
    {
        @SuppressWarnings("unchecked")
        Comparator<T> result = (Comparator<T>)INSTANCE;
        return result;
    }

    private DefaultComparator()
    {
    }

    @Override
    public int compare( E o1, E o2 )
    {
        if( o1 == o2 )
            return 0;
        if( o1 == null )
            return 1;
        if( o2 == null )
            return -1;
        return o1.compareTo( o2 );
    }
}

How to use with String:

Comparator<String> stringComparator = DefaultComparator.getInstance();

Also, if you want case-insensitive comparison, in recent versions of Java the String class contains a public static final field called CASE_INSENSITIVE_ORDER which is of type Comparator<String>, as I just recently found out. So, you can get your job done using String.CASE_INSENSITIVE_ORDER.

Regarding Nambari's answer there was a mistake. If you compare values using double equal sign == program will never reach compare method, unless someone will use new keyword to create String object which is not the best practice. This might be a bit better solution:

public int compare(String o1, String o2) {
        if (o1 == null && o2 == null){return 0;}
        if (o1 == null) { return -1;}
        if (o2 == null) { return 1;}
        return o1.compareTo(o2);
    }

P.S. Thanks for comments ;)

Solution for Java 8 based on java.util.Comparator.comparing(...):

Comparator<String> c = Comparator.comparing(String::toString);

or

Comparator<String> c = Comparator.comparing((String x) -> x);

To generalize the good answer of Mike Nakis with String.CASE_INSENSITIVE_ORDER, you can also use :

Collator.getInstance();

See Collator

Ok this is a few years later but with java 8 you can use Comparator.naturalOrder():

http://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#naturalOrder--

From javadoc:

static <T extends Comparable<? super T>> Comparator<T> naturalOrder()

Returns a comparator that compares Comparable objects in natural order. The returned comparator is serializable and throws NullPointerException when comparing null.

If you do find yourslef needing a Comparator, and you already use Guava, you can use Ordering.natural().

You can use the StringUtils.compare("a", "b")

You may write your own comparator

public class ExampleComparator  implements Comparator<String> {
  public int compare(String obj1, String obj2) {
    if (obj1 == obj2) {
        return 0;
    }
    if (obj1 == null) {
        return -1;
    }
    if (obj2 == null) {
        return 1;
    }
    return obj1.compareTo(obj2);
  }
}

Again, don't need the comparator for Arrays.binarySearch(Object[] a, Object key) so long as the types of objects are comparable, but with lambda expressions this is now way easier.

Simply replace the comparator with the method reference: String::compareTo

E.g.:

Arrays.binarySearch(someStringArray, "The String to find.", String::compareTo);

You could also use

Arrays.binarySearch(someStringArray, "The String to find.", (a,b) -> a.compareTo(b));

but even before lambdas, there were always anonymous classes:

Arrays.binarySearch(
                someStringArray,
                "The String to find.",
                new Comparator<String>() {
                    @Override
                    public int compare(String o1, String o2) {
                        return o1.compareTo(o2);
                    }
                });