Java Programs›Strings›Sort Characters in a String

Sort Characters in a String in Java

beginner· Strings · String Manipulation

Problem

Sorting a string's characters means rearranging every character it contains into ascending order, based on each character's code.

Given a string, arrange its characters in alphabetical order.

Input

banana

Output

aaabnn

Java Program

Java

import java.util.Arrays;

public class SortCharacters {
    public static void main(String[] args) {
        String str = "banana";
        char[] chars = str.toCharArray(); // copy the characters out into a mutable array
        Arrays.sort(chars); // sorts the array in place, ascending by character code
        System.out.println(new String(chars)); // rebuilds a String from the sorted array
    }
}

▶ Run Code

Output

aaabnn

Core Logic

Converting the string into a character array, sorting that array in place, and rebuilding a string from it reorders every character.

How It Works

1str.toCharArray() converts the string into a char[] holding every character.
2Arrays.sort(chars) sorts that array in place, in ascending order by character code.
3new String(chars) builds a brand-new String from the now-sorted array.

For "banana", the array ['b','a','n','a','n','a'] sorts to ['a','a','a','b','n','n'], producing "aaabnn".

💡

Key Point: Arrays.sort() works directly on the char[], not the original String — strings are immutable in Java, so the characters have to be copied out into a mutable array first.

Complexity

Time Complexity: O(n log n)Space Complexity: O(n)

Why: toCharArray() copies every character into a new array, and Arrays.sort() then runs an O(n log n) comparison sort over it.

Key Concepts

toCharArray()Arrays.sort()String constructor

Approach 2: Java 8

Java

import java.util.stream.Collectors;

public class SortCharactersStream {
    public static void main(String[] args) {
        String str = "banana";
        // sorted() sorts the character codes; the rest reassembles them into a String
        String result = str.chars()
            .sorted()
            .mapToObj(c -> String.valueOf((char) c))
            .collect(Collectors.joining());
        System.out.println(result);
    }
}

▶ Run Code

Output

aaabnn

Core Logic

The same sort can be expressed as a stream pipeline — sort the character codes directly, then join them back into a string.

How It Works

1str.chars() returns an IntStream of the string's character codes.
2.sorted() puts those codes into ascending order, the stream equivalent of Arrays.sort() on a char[].
3.mapToObj(c -> String.valueOf((char) c)) converts each sorted code back into a one-character String.
4.collect(Collectors.joining()) concatenates them all back into the final sorted string.

Sorting "banana"'s character codes and joining them produces the same result as the array version: "aaabnn".

💡

Key Point: This does the same O(n log n) sort as the array version, just without ever touching a char[] directly.

Complexity

Time Complexity: O(n log n)Space Complexity: O(n)

Why: sorted() still performs the same comparison sort over every character, and Collectors.joining() builds a result string holding all n characters.

Key Concepts

Streamchars()sorted()Collectors.joining()

← Previous

Reverse Words in a String

Sort Words in a String

Related Programs

Check Palindrome String

Stringsintermediate

Check Pangram

Stringsbeginner

Check String Contains Only Alphabets