Sorting collection of String and StringBuffer in Java

Sorting a collection of objects can be done in two ways:

By implementing Comparable (Natural Order) e.g. Strings are sorted ASCIIbetically. meaning B comes before A and 10 is before 2.
By implementing Comparator (Custom order) and it can be in any order.
Using Collections.sort() method of Collections utility class.

For sorted Collection we can use below collections:

TreeSet
TreeMap

In case of String, sorting will be done automatically in natural order.

// Java program to sort String objects using TreeSet 
import java.util.Set; 
import java.util.TreeSet; 
  
public class Test { 
    public static void main(String[] args) 
    { 
        Set<String> str = new TreeSet<String>(); 
        str.add("Sohan"); 
        str.add("Raja"); 
        str.add("Harish"); 
        str.add("Ram"); 
        System.out.println(str); 
    } 
} 

Output:

[Harish, Raja, Ram, Sohan]

In case of StringBuffer or StringBuilder an exception will be thrown.

// Java program to demonstrate that simple sorting 
// StringBuffer objects doesn't work. 
import java.util.Set; 
import java.util.TreeSet; 
  
public class Test { 
  
    public static void main(String[] args) 
    { 
        Set<StringBuffer> str = new TreeSet<>(); 
        str.add(new StringBuffer("Sohan")); 
        str.add(new StringBuffer("Raja")); 
        str.add(new StringBuffer("Harish")); 
        str.add(new StringBuffer("Ram")); 
        System.out.println(str); 
    } 
} 

Output:

Exception in thread "main" java.lang.ClassCastException: java.lang.StringBuffer cannot be cast to java.lang.Comparable
    at java.util.TreeMap.compare(Unknown Source)
    at java.util.TreeMap.put(Unknown Source)
    at java.util.TreeSet.add(Unknown Source)
    at Test.main(Test.java:11)

String class implements Comparable interface whereas StringBuffer and StringBuilder classes do not implement Comparable interface. See below signatures of String, StringBuffer and StringBuilder classes:

public final class String 
    extends Object 
        implements Serializable, 
    Comparable, CharSequence 

public final class StringBuffer 
    extends Object 
        implements Serializable, 
    CharSequence 

public final class StringBuilder 
    extends Object 
        implements Serializable, 
    CharSequence 

There are many ways of sorting StringBuffer, StringBuilder classes. Some of the ways are given below:

By implementing Comparator interface
By converting StringBuffer to String using StringBuffer.toString() method

// Java program to demonstrate sorting 
// of StringBuffer objects using Comparator 
// interface. 
import java.util.Comparator; 
import java.util.Set; 
import java.util.TreeSet; 
  
public class Test implements Comparator<StringBuffer> { 
    @Override public int compare(StringBuffer s1, StringBuffer s2) 
    { 
        return s1.toString().compareTo(s2.toString()); 
    } 
  
    public static void main(String[] args) 
    { 
        Set<StringBuffer> str = new TreeSet<>(new Test()); 
        str.add(new StringBuffer("Sohan")); 
        str.add(new StringBuffer("Raja")); 
        str.add(new StringBuffer("Harish")); 
        str.add(new StringBuffer("Ram")); 
        System.out.println(str); 
    } 
} 

Output:

[Harish, Raja, Ram, Sohan]

This article is contributed by Sajid Ali Khan. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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