Java String Methods

The String class provides a large set of methods for inspecting, comparing, transforming, and splitting text. Because Strings are immutable, every method that seems to change a String actually returns a new String object with the result, leaving the original unchanged.

Why it matters

Most everyday string work — trimming input, checking a prefix, splitting a line, comparing ignoring case — is one method call if you know the method exists. The details matter, though: indexes start at 0, the end index of substring is exclusive, and split takes a regular expression. Getting one of those wrong gives an off-by-one result or a runtime exception.

Inspecting and Accessing Characters

These methods let you read information about a String without altering it.

• length() — returns the number of characters (not a field, so it is called with parentheses, unlike arrays).
• charAt(int index) — returns the character at a given zero-based index; throws StringIndexOutOfBoundsException if the index is invalid.
• indexOf(String str) / lastIndexOf(String str) — return the position of the first or last occurrence of a substring, or -1 if not found.
• substring(int begin) and substring(int begin, int end) — extract part of the String; "end" is exclusive.

Comparing Strings

Java gives you several comparison methods, and choosing the wrong one is a very common source of bugs. equals() compares character content exactly (case-sensitive). equalsIgnoreCase() compares content while ignoring case differences. compareTo() compares Strings lexicographically and returns a negative, zero, or positive int, which is useful for sorting. Crucially, == compares object references, not content, so it should never be used to check whether two Strings "say the same thing."

Transforming and Splitting

These methods return new Strings (or arrays of Strings) derived from the original.

• toUpperCase() / toLowerCase() — return a new String with case changed.
• trim() removes leading/trailing ASCII whitespace; strip() (Java 11+) is Unicode-aware and preferred in modern code.
• replace(char/CharSequence old, char/CharSequence new) — replaces all occurrences of one character or sequence with another.
• split(String regex) — splits the String into a String[] using a regular expression delimiter.
• String.format(String pattern, Object... args) — builds a formatted String using placeholders like %s, %d, and %.2f.

Example

public class StringMethodsDemo {
    public static void main(String[] args) {
        String text = "  Java Programming  ";

        System.out.println(text.trim());
        System.out.println(text.trim().length());
        System.out.println(text.trim().toUpperCase());
        System.out.println(text.trim().indexOf("Programming"));
        System.out.println(text.trim().substring(0, 4));
        System.out.println("Java".equals("java"));
        System.out.println("Java".equalsIgnoreCase("java"));

        String[] words = text.trim().split(" ");
        System.out.println(words.length);

        String formatted = String.format("Score: %d/%d (%.1f%%)", 45, 50, 90.0);
        System.out.println(formatted);
    }
}

Java Programming
16
JAVA PROGRAMMING
5
Java
false
true
2
Score: 45/50 (90.0%)

Exercise

Write a method that decides whether a phrase is a palindrome, ignoring spaces and upper or lower case.

Expected output: true for "Never odd or even", then false for "WebNest".

public class Palindrome {

    static boolean isPalindrome(String text) {
        // TODO: remove spaces, ignore case, then compare characters from both ends
        return false;
    }

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

public class Palindrome {

    static boolean isPalindrome(String text) {
        String clean = text.replace(" ", "").toLowerCase();
        for (int i = 0; i < clean.length() / 2; i++) {
            if (clean.charAt(i) != clean.charAt(clean.length() - 1 - i)) {
                return false;
            }
        }
        return true;
    }

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

Quick quiz

Interview questions