Java String Expressions

Declaring strings

String name = "Alice";          // string literal
String empty = "";            // empty string
String nothing = null;        // no object at all

Strings in Java are objects, not primitives. They're stored in the "String pool".

Concatenation ( + )

String first = "Hello";
String second = "World";
String msg = first + " " + second;  // "Hello World"

int age = 17;
String s = "Age: " + age;            // "Age: 17"

If one side is a String, Java converts the other side automatically.

Immutability — important concept!

String s = "hello";
s.toUpperCase();        // does NOT change s!
s = s.toUpperCase();    // THIS works → "HELLO"

Strings are immutable. Every method returns a new string — you must save the result.

Escape characters

"\n"   → newline
"\t"   → tab
"\""   → double quote inside string
"\\"   → backslash

Length & characters

.length()

Number of characters

"hello".length() → 5

.charAt(i)

Character at index i

"hello".charAt(1) → 'e'

Searching

.indexOf(str)

First position of str, or -1

"hello".indexOf("ll") → 2

.contains(str)

true if str is found

"hello".contains("ell") → true

.startsWith(str)

true if starts with str

"hi".startsWith("h") → true

.endsWith(str)

true if ends with str

"hi".endsWith("i") → true

Slicing & transforming

.substring(start)

From start to end

"hello".substring(2) → "llo"

.substring(s, e)

From s up to (not including) e

"hello".substring(1,4) → "ell"

.toUpperCase()

All uppercase

"hi".toUpperCase() → "HI"

.toLowerCase()

All lowercase

"HI".toLowerCase() → "hi"

.trim()

Remove leading/trailing spaces

" hi ".trim() → "hi"

.replace(a, b)

Replace a with b (all occurrences)

"aaa".replace("a","b") → "bbb"

Index reminder (0-based!)

 h   e   l   l   o
 0   1   2   3   4    ← index

Java starts counting at 0. "hello".charAt(0) is 'h', not 'e'.

The big rule

// WRONG — compares memory addresses, not content
if (s1 == s2) { ... }

// CORRECT — compares actual characters
if (s1.equals(s2)) { ... }

Always use .equals() to compare string content. == checks if they're the same object in memory.

Case-insensitive compare

"Hello".equalsIgnoreCase("hello") → true

Alphabetical order — compareTo

s1.compareTo(s2)
// returns negative → s1 comes before s2
// returns 0        → s1 equals s2
// returns positive → s1 comes after s2

"apple".compareTo("banana") → negative (a < b)

Used for sorting strings alphabetically.

Checking empty

s.isEmpty()           // true if length == 0
s.isBlank()           // true if empty or only spaces (Java 11+)
s.equals("")         // same as isEmpty()

Number ↔ String

// int → String
String s = String.valueOf(42);          → "42"
String s = Integer.toString(42);        → "42"
String s = "" + 42;                    → "42"

// String → int
int n = Integer.parseInt("42");         → 42

// String → double
double d = Double.parseDouble("3.14"); → 3.14

Integer.parseInt() throws an exception if the string isn't a valid number.

char ↔ String

// char → String
char c = 'A';
String s = String.valueOf(c);   → "A"

// String → char (get one character)
char ch = "hello".charAt(0);   → 'h'

Looping through a string

String word = "hello";
for (int i = 0; i < word.length(); i++) {
    char ch = word.charAt(i);
    System.out.println(ch);
}

Classic Grade 11 pattern — loop with index, grab each char with .charAt(i).

String.format (printf-style)

String s = String.format("Name: %s, Age: %d", name, age);
// %s = String,  %d = int,  %f = double, %.2f = 2 decimal places