The String API you use daily — searching, slicing, splitting, formatting — plus Java 15 text blocks for multi-line literals.

Method cheat sheet

Method	Returns	Example → result
`length()`	int	`"hello".length()` → `5`
`charAt(i)`	char	`"hello".charAt(1)` → `'e'`
`substring(b, e)`	String	`"hello".substring(1, 3)` → `"el"` (end exclusive)
`indexOf(x)` / `lastIndexOf(x)`	int	`-1` when not found
`contains`, `startsWith`, `endsWith`	boolean	`"hello".startsWith("he")` → `true`
`toUpperCase()` / `toLowerCase()`	String	new string
`trim()` / `strip()`	String	`strip()` (Java 11) is Unicode-aware
`isEmpty()` / `isBlank()`	boolean	`" ".isBlank()` → `true` (Java 11)
`replace(a, b)`	String	literal replace of all occurrences
`replaceAll(regex, b)`	String	regex-based
`split(regex)`	String[]	trailing empty strings removed
`toCharArray()` / `chars()`	char[] / IntStream	iterate characters
`compareTo(s)`	int	lexicographic: negative / 0 / positive
`repeat(n)`	String	`"ab".repeat(3)` → `"ababab"` (Java 11)
`String.valueOf(x)`	String	any type → string (`"null"` for null objects)
`String.format` / `formatted`	String	printf-style formatting

Searching & slicing

Java

public class Main {
    public static void main(String[] args) {
        String s = "Hello, World";
        System.out.println(s.length() + " " + s.charAt(7));
        System.out.println(s.substring(7) + "|" + s.substring(0, 5));
        System.out.println(s.indexOf('o') + " " + s.lastIndexOf('o') + " " + s.indexOf("xyz"));
        System.out.println(s.contains("World") + " " + s.startsWith("Hell"));
        System.out.println(s.toLowerCase().replace('l', 'L'));
        System.out.println("  padded  ".strip() + "|" + "   ".isBlank() + "|" + "".isEmpty());
        System.out.println("apple".compareTo("banana") + " " + "b".compareTo("a"));
        System.out.println("Java".equalsIgnoreCase("JAVA"));
    }
}

Output

12 W
World|Hello
4 8 -1
true true
heLLo, worLd
padded|true|true
-1 1
true

Splitting & joining

Java

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        System.out.println(Arrays.toString("a,b,,c,,".split(",")));      // trailing empties dropped
        System.out.println(Arrays.toString("a,b,,c,,".split(",", -1)));  // keep them
        System.out.println(Arrays.toString("1.2.3".split("\\.")));       // "." is a regex!
        System.out.println(Arrays.toString("  one   two three ".trim().split("\\s+")));
        System.out.println(String.join(" | ", Arrays.asList("x", "y", "z")));
    }
}

Output

[a, b, , c]
[a, b, , c, , ]
[1, 2, 3]
[one, two, three]
x | y | z

Characters

Java

public class Main {
    public static void main(String[] args) {
        String word = "Level42";
        int letters = 0, digits = 0;
        for (char c : word.toCharArray()) {
            if (Character.isLetter(c)) letters++;
            else if (Character.isDigit(c)) digits++;
        }
        System.out.println(letters + " letters, " + digits + " digits");
        System.out.println((int) 'a' + " " + (char) ('a' + 2) + " " + ('7' - '0'));
        System.out.println(new StringBuilder(word).reverse());
        System.out.println(word.chars().filter(Character::isUpperCase).count());
    }
}

Output

5 letters, 2 digits
97 c 7
24leveL
1

Formatting

Java

public class Main {
    public static void main(String[] args) {
        System.out.println(String.format("%-6s|%5d|%.2f|%05d", "id", 42, 3.14159, 7));
        System.out.println("%s is %d years".formatted("Asha", 30));    // Java 15
        System.out.printf("%x %o %e %b%n", 255, 8, 12345.678, true);
        System.out.println(String.format("%,d", 1234567));
    }
}

Output

id    |   42|3.14|00007
Asha is 30 years
ff 10 1.234568e+04 true
1,234,567

Text blocks (Java 15)

A text block starts with """ followed by a newline. Incidental indentation (the common leading whitespace) is stripped, and line endings are normalised to \n — ideal for JSON, SQL and HTML.

Java

public class Main {
    public static void main(String[] args) {
        String json = """
            {
              "name": "Asha",
              "age": 30
            }
            """;
        System.out.print(json);
        String oneLine = """
            SELECT * \
            FROM users""";                  // \ joins lines
        System.out.println(oneLine);
        System.out.println("""
            Hi %s!""".formatted("Ravi"));
    }
}

Output

{
  "name": "Asha",
  "age": 30
}
SELECT * FROM users
Hi Ravi!

What is the difference between trim() and strip()?

What is the difference between replace and replaceAll?

How do you check if a string is a palindrome?