Java — Strings

Creating strings

A String in Java is an object — not a primitive. You create one with double quotes:

String greeting = "Hello";
String empty = "";
String withSpace = "Hello World";

You can also use the constructor (rarely needed):

String constructed = new String("Hello");

The direct form is preferred — it's shorter and lets Java reuse the same string in memory.

String length

String name = "Ada";
System.out.println(name.length());  // 3

Unlike arrays where .length is a property, strings use .length() as a method. Forgetting the parentheses is a compile error.

Accessing characters

String word = "Hello";

System.out.println(word.charAt(0));   // H
System.out.println(word.charAt(4));   // o

Characters are zero-indexed — position 0 is the first character. Java throws a StringIndexOutOfBoundsException if you go past the end.

Finding substrings and positions

String sentence = "Java is fun and powerful";

System.out.println(sentence.indexOf("fun"));       // 9
System.out.println(sentence.indexOf("Python"));    // -1 (not found)
System.out.println(sentence.substring(9));          // fun and powerful
System.out.println(sentence.substring(9, 12));     // fun

indexOf returns the starting position (or -1). substring(start) takes everything from that position. substring(start, end) takes the slice between the two positions (end is exclusive).

Case conversion

String name = "Ada Lovelace";

System.out.println(name.toUpperCase());  // ADA LOVELACE
System.out.println(name.toLowerCase());  // ada lovelace

Both return new strings — the original stays unchanged. Strings are immutable in Java.

Trimming and replacing

String messy = "   Hello, World!   ";

System.out.println(messy.trim());                    // "Hello, World!"
System.out.println(messy.strip());                   // "Hello, World!" (Java 11+)
System.out.println("Hello".replace("l", "L"));      // HeLLo
System.out.println("aabbcc".replaceAll("a", "x"));  // xxbbcc

trim() removes leading and trailing whitespace. strip() is the modern version that also handles Unicode spaces.

Checking string content

String email = "user@example.com";

System.out.println(email.isEmpty());              // false
System.out.println(email.contains("@"));          // true
System.out.println(email.startsWith("user"));     // true
System.out.println(email.endsWith(".com"));        // true

These methods return booleans, making them perfect for conditions:

if (email.contains("@") && email.contains(".")) {
    System.out.println("Looks like an email.");
}

Comparing strings

Never use == to compare string content:

String a = "hello";
String b = "hello";
String c = new String("hello");

System.out.println(a == b);       // true (Java reused the same object)
System.out.println(a == c);       // false (different objects!)
System.out.println(a.equals(c));  // true  (same content)

== checks if two references point to the same object in memory. .equals() checks if the content is identical. Always use .equals() for string comparison.

For case-insensitive comparison:

String name = "Ada";
System.out.println(name.equalsIgnoreCase("ada"));  // true

Splitting and joining

String csv = "apple,banana,cherry";
String[] fruits = csv.split(",");
System.out.println(fruits.length);  // 3
System.out.println(fruits[1]);      // banana

String joined = String.join(" - ", fruits);
System.out.println(joined);  // apple - banana - cherry

split() breaks a string into an array using a delimiter. String.join() reassembles them with a new separator. These are essential for parsing data.

String concatenation

String first = "Hello";
String second = " World";
String result = first + second;
System.out.println(result);  // Hello World

For building strings in loops, StringBuilder is more efficient:

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 5; i++) {
    sb.append(i).append(" ");
}
System.out.println(sb.toString().trim());  // 0 1 2 3 4

Each + on strings creates a new String object. In a loop, that's wasteful. StringBuilder builds one string, modifying it internally until you call toString().

String formatting

String name = "Ada";
int age = 36;

// Concatenation
String v1 = "Name: " + name + ", Age: " + age;

// printf
String v2 = String.format("Name: %s, Age: %d", name, age);

// Both produce: Name: Ada, Age: 36

String.format() is the String equivalent of System.out.printf — it returns the formatted string instead of printing it. Use it when you need to store or pass formatted text.

Common string mistakes

// Mistake 1: comparing with ==
String a = new String("hello");
String b = new String("hello");
if (a == b) { /* this won't run! */ }

// Mistake 2: null check before methods
String s = null;
if (s.length() > 0) { }  // NullPointerException!

// Correct approach:
if (s != null && s.length() > 0) { }  // safe

NullPointerException on strings is the most common runtime error in Java. Always check for null before calling methods.

Mini Practice

1. Create a string with your full name — print its length and each character
2. Reverse a string: "Hello" → "olleH" using charAt() and a loop
3. Check if a string is a palindrome (reads the same forwards and backwards)
4. Split a sentence into words and print each word on a new line
5. Build a story by concatenating strings from an array using a for loop

Next: making decisions with if-else →