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Java Strings: Methods, Comparison with equals, Immutability and Formatting

Learn Java strings: creating strings, length and charAt, substring, indexOf and contains, replace, split and join, trim and strip, case conversion, comparing with equals vs ==, immutability and the string pool, String.format, and text blocks.

Beginner12 min readUpdated
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Text is everywhere in programs: names, messages, file contents, commands. In Java, text is represented by the String class, which comes with dozens of useful methods. Strings are immutable: no method ever changes a string; methods that "modify" text return a new string.

Creating and joining strings#

Java
public class Main {
    public static void main(String[] args) {
        String first = "Asha";
        String last = "Rao";

        String full = first + " " + last;
        full += "!";

        System.out.println(full);
        System.out.println("Length: " + full.length());
        System.out.println("-".repeat(10));
    }
}
Output
Asha Rao!
Length: 9
----------

Characters and positions#

Each character has an index starting at 0. charAt(i) returns one char:

Java
public class Main {
    public static void main(String[] args) {
        String word = "Java";
        System.out.println(word.charAt(0) + " " + word.charAt(word.length() - 1));

        for (char c : word.toCharArray()) {
            System.out.print(c + "-");
        }
        System.out.println();
    }
}
Output
J a
J-a-v-a-

Searching and substrings#

Java
public class Main {
    public static void main(String[] args) {
        String email = "asha.rao@example.com";

        int at = email.indexOf('@');
        String user = email.substring(0, at);       // from 0 up to (not including) at
        String domain = email.substring(at + 1);    // from at + 1 to the end

        System.out.println(user + " | " + domain);
        System.out.println(email.contains("example") + " " + email.endsWith(".com"));
        System.out.println(email.indexOf("xyz"));
        System.out.println(email.lastIndexOf('a'));
    }
}
Output
asha.rao | example.com
true true
-1
11
  • substring(start, end) includes start and excludes end.
  • indexOf returns -1 when the text is not found.
  • startsWith, endsWith and contains answer yes/no questions.

Changing text (returns a new string)#

Java
public class Main {
    public static void main(String[] args) {
        String s = "  I like tea  ";

        System.out.println("[" + s.trim() + "]");
        System.out.println(s.strip().replace("tea", "coffee"));
        System.out.println(s.toUpperCase().strip());
        System.out.println("[" + s + "]");   // unchanged: strings are immutable
    }
}
Output
[I like tea]
I like coffee
I LIKE TEA
[  I like tea  ]

strip() (Java 11) removes all kinds of whitespace, including Unicode spaces; trim() removes only basic ASCII whitespace.

Splitting and joining#

Java
import java.util.Arrays;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        String csv = "Asha,Pune,21";
        String[] parts = csv.split(",");
        System.out.println(parts[1] + " (" + parts.length + " fields)");

        String sentence = "practice   makes  progress";
        String[] words = sentence.split("\\s+");   // one or more spaces
        System.out.println(Arrays.toString(words));

        System.out.println(String.join(" | ", List.of("a", "b", "c")));
    }
}
Output
Pune (3 fields)
[practice, makes, progress]
a | b | c

split takes a regular expression: "\\s+" means "one or more whitespace characters". Characters with special meaning in regex, such as . and |, must be escaped: split("\\.").

Comparing strings: equals, never ==#

== checks whether two variables refer to the same object. Use equals to compare the text:

Java
public class Main {
    public static void main(String[] args) {
        String a = "hello";
        String b = "hello";
        String c = new String("hello");

        System.out.println(a == b);        // true: same pooled literal
        System.out.println(a == c);        // false: different objects
        System.out.println(a.equals(c));   // true: same text
        System.out.println("Hello".equalsIgnoreCase(a));
        System.out.println("apple".compareTo("banana") < 0);
    }
}
Output
true
false
true
true
true

Identical string literals share one object in the string pool, which is why a == b happens to be true. Strings created at run time (from input, files or new String) are different objects, so == gives unreliable results. Always use equals. compareTo orders strings alphabetically (negative, zero or positive).

Immutability#

Because strings never change, they can be shared safely and used as keys in maps. The cost: building a string piece by piece with += in a loop creates a new string every time. For that, use StringBuilder.

Formatting#

String.format (and its shortcut "...".formatted(...)) builds strings from placeholders, like printf:

Java
public class Main {
    public static void main(String[] args) {
        String name = "Asha";
        double balance = 2500.756;

        System.out.println(String.format("%s has Rs %.2f", name, balance));
        System.out.println("|%-8s|%8s|".formatted("left", "right"));
        System.out.println("%05d".formatted(42));
    }
}
Output
Asha has Rs 2500.76
|left    |   right|
00042
PlaceholderMeaning
%sAny value as text
%dWhole number
%.2fDecimal with 2 places
%-8s / %8sLeft / right aligned in 8 characters
%nLine break

Text blocks#

Java 15 added text blocks for multi-line strings, written between triple quotes. Indentation common to all lines is removed automatically:

Java
public class Main {
    public static void main(String[] args) {
        String json = """
            {
              "name": "Asha",
              "city": "Pune"
            }
            """;
        System.out.print(json);
    }
}
Output
{
  "name": "Asha",
  "city": "Pune"
}

Common mistakes#

Practice#

Practice exercise

Write boolean isPalindrome(String s) that ignores spaces and case, and test it with "Never odd or even" and "Java".

Show solution
Java
public class Palindrome {
    static boolean isPalindrome(String s) {
        String cleaned = s.replace(" ", "").toLowerCase();
        String reversed = new StringBuilder(cleaned).reverse().toString();
        return cleaned.equals(reversed);
    }

    public static void main(String[] args) {
        System.out.println(isPalindrome("Never odd or even"));
        System.out.println(isPalindrome("Java"));
    }
}
Output
true
false

Practice exercise

Capitalize the first letter of every word in "the quick brown fox".

Show solution
Java
public class TitleCase {
    public static void main(String[] args) {
        String sentence = "the quick brown fox";
        String[] words = sentence.split(" ");
        for (int i = 0; i < words.length; i++) {
            String w = words[i];
            words[i] = Character.toUpperCase(w.charAt(0)) + w.substring(1);
        }
        System.out.println(String.join(" ", words));
    }
}
Output
The Quick Brown Fox

Key takeaway#

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