String Common Methods

What does length() method return?

The length() method returns the total number of characters in the string as an integer, including spaces, special characters, and digits. It counts from 1 (not 0-indexed like arrays).

Interview Example:

String str = "Hello World";
System.out.println(str.length()); // Output: 11
String empty = "";
System.out.println(empty.length()); // Output: 0
String spaces = "   ";
System.out.println(spaces.length()); // Output: 3

How do charAt(int index) and substring(int begin, int end) work?

charAt(index) returns a single character at the specified index (0-based). substring(begin, end) returns a new String from begin index (inclusive) to end index (exclusive).

Interview Example:

String str = "Interview";
// charAt() - returns char
char ch = str.charAt(0); // 'I'
char last = str.charAt(str.length() - 1); // 'w'
// substring(begin, end) - returns String
String sub1 = str.substring(0, 5); // "Inter" (0 to 4)
String sub2 = str.substring(5); // "view" (5 to end)
String sub3 = str.substring(2, 7); // "tervi"
// charAt throws StringIndexOutOfBoundsException
// char invalid = str.charAt(20); // Exception

What is the difference between substring() and subSequence()?

Both extract a portion of the string, but substring() returns a String object, while subSequence() returns a CharSequence interface. substring() is more commonly used.

Interview Example:

String str = "Java Programming";
// substring() returns String
String sub = str.substring(5, 16);
System.out.println(sub); // "Programming"
// subSequence() returns CharSequence
CharSequence seq = str.subSequence(5, 16);
System.out.println(seq); // "Programming"
// substring() can use String methods directly
System.out.println(sub.toUpperCase()); // Works
// subSequence() needs casting for String methods
// System.out.println(seq.toUpperCase()); // Compile error
System.out.println(((String)seq).toUpperCase()); // Needs cast

How to check if a string contains another string?

Use contains() method which returns boolean (true/false). It checks for exact substring match and is case-sensitive.

Interview Example:

String str = "Java is awesome";
System.out.println(str.contains("Java")); // true
System.out.println(str.contains("awesome")); // true
System.out.println(str.contains("python")); // false
System.out.println(str.contains("java")); // false (case-sensitive)
// Case-insensitive check
System.out.println(str.toLowerCase().contains("java")); // true
// Alternative using indexOf
if (str.indexOf("Java") != -1) {
  System.out.println("Found"); // Found
}

How to find the index of a substring using indexOf() and lastIndexOf()?

indexOf() returns the first occurrence index of substring/character. lastIndexOf() returns the last occurrence index. Both return -1 if not found.

Interview Example:

String str = "Java Programming in Java";
// indexOf() - first occurrence
System.out.println(str.indexOf("Java")); // 0
System.out.println(str.indexOf("a")); // 1
System.out.println(str.indexOf("Python")); // -1
// lastIndexOf() - last occurrence
System.out.println(str.lastIndexOf("Java")); // 20
System.out.println(str.lastIndexOf("a")); // 23
// indexOf with start position
System.out.println(str.indexOf("Java", 5)); // 20 (search from index 5)
// Check if substring exists
if (str.indexOf("Java") != -1) {
  System.out.println("Java found");
}

How to convert a string to uppercase/lowercase?

Use toUpperCase() to convert all characters to uppercase and toLowerCase() to convert to lowercase. Both return a new String (original unchanged).

Interview Example:

String str = "Java Interview";
String upper = str.toUpperCase();
System.out.println(upper); // "JAVA INTERVIEW"
String lower = str.toLowerCase();
System.out.println(lower); // "java interview"
System.out.println(str); // "Java Interview" (original unchanged)
// Case-insensitive comparison
String s1 = "JAVA";
String s2 = "java";
System.out.println(s1.toLowerCase().equals(s2.toLowerCase())); // true
// Or use equalsIgnoreCase()
System.out.println(s1.equalsIgnoreCase(s2)); // true

How to trim spaces from start and end of a string using trim() and strip()?

Both remove whitespace from start and end, but strip() (Java 11+) handles Unicode whitespace better. trim() only removes ASCII spaces (≤ U+0020).

Interview Example:

String str = "   Hello World   ";
// trim() - removes ASCII whitespace
String trimmed = str.trim();
System.out.println(trimmed); // "Hello World"
// strip() - removes Unicode whitespace (Java 11+)
String stripped = str.strip();
System.out.println(stripped); // "Hello World"
// stripLeading() - removes from start only
System.out.println(str.stripLeading()); // "Hello World   "
// stripTrailing() - removes from end only
System.out.println(str.stripTrailing()); // "   Hello World"

What’s the difference between trim() and strip()?

trim() removes ASCII whitespace (char ≤ 32 or U+0020), while strip() removes all Unicode whitespace characters. strip() is more comprehensive and introduced in Java 11.

Interview Example:

String str1 = "  Hello  "; // Regular spaces
String str2 = "\u2003Hello\u2003"; // Unicode em-space
// trim() - handles only ASCII
System.out.println(str1.trim()); // "Hello" ✓
System.out.println(str2.trim()); // "\u2003Hello\u2003" ✗ (unchanged)
// strip() - handles Unicode (Java 11+)
System.out.println(str1.strip()); // "Hello" ✓
System.out.println(str2.strip()); // "Hello" ✓
// Use strip() for better Unicode support
// Use trim() for backward compatibility with Java < 11

How to replace a character or substring (replace(), replaceAll(), replaceFirst())?

replace() replaces all exact matches (literal). replaceAll() uses regex to replace all matches. replaceFirst() uses regex to replace only the first match.

Interview Example:

String str = "Java is Java";
// replace() - literal replacement, all occurrences
String r1 = str.replace("Java", "Python");
System.out.println(r1); // "Python is Python"
String r2 = str.replace('a', 'A');
System.out.println(r2); // "JAvA is JAvA"
// replaceAll() - regex replacement, all occurrences
String s = "Java123Programming456";
String r3 = s.replaceAll("\\d+", "-");
System.out.println(r3); // "Java-Programming-"
// replaceFirst() - regex replacement, first occurrence only
String r4 = s.replaceFirst("\\d+", "-");
System.out.println(r4); // "Java-Programming456"

How to split a string into an array (split() method)?

split(regex) divides a string into an array based on a delimiter (regex pattern). Returns String[] array. Can specify limit for maximum splits.

Interview Example:

String str = "Java,Python,C++,JavaScript";
// Basic split
String[] langs = str.split(",");
for (String lang : langs) {
  System.out.println(lang); // Java, Python, C++, JavaScript
}
// Split with limit
String[] limited = str.split(",", 2);
System.out.println(limited[0]); // "Java"
System.out.println(limited[1]); // "Python,C++,JavaScript"
// Split by whitespace
String text = "Hello World from Java";
String[] words = text.split("\\s+");
System.out.println(words.length); // 4
// Split by multiple delimiters
String data = "a,b;c:d";
String[] parts = data.split("[,;:]"); // Output: ["a", "b", "c", "d"]

How to join multiple strings (String.join(), StringJoiner())?

String.join() joins strings with a delimiter. StringJoiner provides more control with prefix/suffix options for building joined strings.

Interview Example:

// String.join() - simple and direct
String[] words = {"Java", "Python", "C++"};
String joined = String.join(", ", words);
System.out.println(joined); // "Java, Python, C++"
// Join with varargs
String result = String.join("-", "2025", "10", "21");
System.out.println(result); // "2025-10-21"
// StringJoiner - with prefix and suffix
StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.add("Java").add("Python").add("C++");
System.out.println(joiner.toString()); // "[Java, Python, C++]"
// Using Streams
List<String> list = Arrays.asList("A", "B", "C");
String s = list.stream().collect(Collectors.joining(", "));
System.out.println(s); // "A, B, C"

How to convert primitive or object values to string (String.valueOf(), toString())?

String.valueOf() is a static method that converts any type to String (handles null safely). toString() is an instance method on objects (throws NullPointerException if null).

Interview Example:

// String.valueOf() - static method, null-safe
int num = 100;
String s1 = String.valueOf(num);
System.out.println(s1); // "100"
boolean flag = true;
String s2 = String.valueOf(flag);
System.out.println(s2); // "true"
Object obj = null;
String s3 = String.valueOf(obj);
System.out.println(s3); // "null" (safe)
// toString() - instance method
Integer i = 100;
String s4 = i.toString();
System.out.println(s4); // "100"
Integer nullInt = null;
// String s5 = nullInt.toString(); // NullPointerException
// Concatenation (implicit valueOf)
String s6 = "" + 100;
System.out.println(s6); // "100"

How to remove all spaces from a string?

Use replace(), replaceAll(), or modern replaceAll() with regex to remove all spaces (or specific whitespace characters).

Interview Example:

String str = "Java Programming Interview";
// Method 1: replace() - removes only regular spaces
String noSpaces1 = str.replace(" ", "");
System.out.println(noSpaces1); // "JavaProgrammingInterview"
// Method 2: replaceAll() - removes all whitespace (space, tab, newline)
String text = "Java \t Programming \n Interview";
String noSpaces2 = text.replaceAll("\\s+", "");
System.out.println(noSpaces2); // "JavaProgrammingInterview"
// Method 3: replaceAll() - removes only spaces
String noSpaces3 = str.replaceAll(" ", "");
System.out.println(noSpaces3); // "JavaProgrammingInterview"
// To replace multiple spaces with single space
String multiSpace = "Java    Programming";
String single = multiSpace.replaceAll("\\s+", " ");
System.out.println(single); // "Java Programming"

How to check if a string starts or ends with a given substring?

Use startsWith() to check if string begins with a prefix and endsWith() to check if it ends with a suffix. Both return boolean and are case-sensitive.

Interview Example:

String fileName = "document.pdf";
// startsWith()
System.out.println(fileName.startsWith("doc")); // true
System.out.println(fileName.startsWith("Doc")); // false (case-sensitive)
System.out.println(fileName.startsWith("pdf")); // false
// endsWith()
System.out.println(fileName.endsWith(".pdf")); // true
System.out.println(fileName.endsWith(".PDF")); // false
System.out.println(fileName.endsWith("ent.pdf")); // true
// startsWith() with offset
String url = "https://example.com";
System.out.println(url.startsWith("://", 5)); // true (starts at index 5)
// Practical use case - file validation
if (fileName.endsWith(".pdf") || fileName.endsWith(".doc")) {
  System.out.println("Valid document");
}

How to convert a string into a char[] and back?

Use toCharArray() to convert String to char array and String constructor or String.valueOf() to convert char array back to String.

Interview Example:

String str = "Java";
// String to char[]
char[] charArray = str.toCharArray();
for (char c : charArray) {
  System.out.print(c + " "); // J a v a
}
// Modify char array
charArray[0] = 'j';
System.out.println(charArray); // java
// char[] to String - Method 1: Constructor
String newStr1 = new String(charArray);
System.out.println(newStr1); // "java"
// char[] to String - Method 2: valueOf()
String newStr2 = String.valueOf(charArray);
System.out.println(newStr2); // "java"
// Partial char[] to String
char[] arr = {'H', 'e', 'l', 'l', 'o'};
String partial = new String(arr, 0, 3); // offset=0, count=3
System.out.println(partial); // "Hel"
// Reverse a string example
String reverse = new String(new StringBuilder(str).reverse().toString().toCharArray());