1 Understanding String Indexing
Before we learn String methods, remember that Strings are indexed starting from 0:
String Index Example: "Hello World"
0
H1
e2
l3
l4
o5
6
W7
o8
r9
l10
dLength = 11 | Valid indices: 0 to 10
2 Basic String Methods
Basic Information Methods
length()
Returns the number of characters in the String.
"Hello".length()
→ 5
charAt(int index)
Returns the character at the specified index.
"Hello".charAt(1)
→ 'e'
isEmpty()
Returns true if the String has length 0.
"".isEmpty()
→ true
String text = "Hello World";
System.out.println(text.length()); // 11
System.out.println(text.charAt(0)); // H
System.out.println(text.charAt(6)); // W
System.out.println(text.isEmpty()); // false
System.out.println("".isEmpty()); // true
3 String Concatenation
There are multiple ways to combine Strings in Java:
String first = "Hello";
String second = "World";
// Method 1: Using + operator
String result1 = first + " " + second;
System.out.println(result1); // Hello World
// Method 2: Using concat() method
String result2 = first.concat(" ").concat(second);
System.out.println(result2); // Hello World
// Concatenating with other types
int number = 42;
String message = "The answer is: " + number;
System.out.println(message); // The answer is: 42
Remember: Strings are Immutable!
Each concatenation creates a NEW String object. The original Strings are not modified!
4 Extracting Substrings
Substring Methods
substring(int beginIndex)
Returns substring from beginIndex to end.
"Hello World".substring(6)
→ "World"
substring(int begin, int end)
Returns substring from begin to end-1.
"Hello World".substring(0, 5)
→ "Hello"
Visual: substring(2, 7) on "Hello World"
0
H1
e2
l3
l4
o5
6
W7
o8
r9
l10
dResult: "llo W" (from index 2 up to but NOT including 7)
String text = "Hello World";
System.out.println(text.substring(6)); // World
System.out.println(text.substring(0, 5)); // Hello
System.out.println(text.substring(2, 7)); // llo W
5 String Comparison
Comparison Methods
| Method | Description | Example | Result |
|---|---|---|---|
equals(String) |
Case-sensitive comparison | "Java".equals("java") |
false |
equalsIgnoreCase(String) |
Case-insensitive comparison | "Java".equalsIgnoreCase("java") |
true |
compareTo(String) |
Lexicographic comparison | "Apple".compareTo("Banana") |
negative (-1) |
startsWith(String) |
Checks if starts with prefix | "Hello".startsWith("He") |
true |
endsWith(String) |
Checks if ends with suffix | "Hello".endsWith("lo") |
true |
contains(CharSequence) |
Checks if contains substring | "Hello".contains("ell") |
true |
String s1 = "Java";
String s2 = "java";
// equals vs equalsIgnoreCase
System.out.println(s1.equals(s2)); // false
System.out.println(s1.equalsIgnoreCase(s2)); // true
// compareTo returns: negative, 0, or positive
System.out.println("Apple".compareTo("Banana")); // -1 (A comes before B)
System.out.println("Banana".compareTo("Apple")); // 1 (B comes after A)
System.out.println("Apple".compareTo("Apple")); // 0 (equal)
Always use .equals() for String comparison!
Never use == to compare String content. Use == only to check if two variables reference the same object.
6 Searching in Strings
Search Methods
indexOf(char/String)
Returns first occurrence index, or -1 if not found.
"Hello".indexOf('l')
→ 2
lastIndexOf(char/String)
Returns last occurrence index, or -1 if not found.
"Hello".lastIndexOf('l')
→ 3
Visual: indexOf('l') vs lastIndexOf('l') on "Hello"
0
H1
e2 ←first
l3 ←last
l4
o
String text = "Hello World";
System.out.println(text.indexOf('o')); // 4 (first 'o')
System.out.println(text.lastIndexOf('o')); // 7 (last 'o')
System.out.println(text.indexOf("World")); // 6
System.out.println(text.indexOf('x')); // -1 (not found)
// Search starting from specific index
System.out.println(text.indexOf('o', 5)); // 7 (start searching from index 5)
7 String Transformation
Transformation Methods
toUpperCase()
Converts all characters to uppercase.
"Hello".toUpperCase()
→ "HELLO"
toLowerCase()
Converts all characters to lowercase.
"Hello".toLowerCase()
→ "hello"
trim()
Removes leading and trailing whitespace.
" Hello ".trim()
→ "Hello"
replace(old, new)
Replaces all occurrences of old with new.
"Hello".replace('l', 'p')
→ "Heppo"
String text = " Hello World ";
System.out.println(text.toUpperCase()); // " HELLO WORLD "
System.out.println(text.toLowerCase()); // " hello world "
System.out.println(text.trim()); // "Hello World"
System.out.println(text.replace(" ", "-")); // "--Hello-World--"
// Remember: Original String is NOT changed!
System.out.println(text); // " Hello World " (unchanged)
8 Conversion: String ↔ char[] ↔ Numbers
Conversion Methods
String to char[]
String text = "Hello";
// Convert String to char array
char[] chars = text.toCharArray();
// Now we can modify individual characters
chars[0] = 'J'; // Change 'H' to 'J'
// Convert back to String
String newText = new String(chars);
System.out.println(newText); // "Jello"
Numbers to String
int number = 42;
double pi = 3.14159;
// Method 1: String.valueOf()
String s1 = String.valueOf(number); // "42"
String s2 = String.valueOf(pi); // "3.14159"
// Method 2: Concatenation with empty String
String s3 = "" + number; // "42"
String to Numbers
String numStr = "42";
String piStr = "3.14159";
// Parse String to number
int num = Integer.parseInt(numStr); // 42
double pi = Double.parseDouble(piStr); // 3.14159
// Be careful with invalid input!
// Integer.parseInt("abc") throws NumberFormatException
9 Summary: Quick Reference
| Category | Methods |
|---|---|
| Basic | length(), charAt(), isEmpty() |
| Concatenation | + operator, concat() |
| Extraction | substring() |
| Comparison | equals(), equalsIgnoreCase(), compareTo(), contains(), startsWith(), endsWith() |
| Search | indexOf(), lastIndexOf() |
| Transform | toUpperCase(), toLowerCase(), trim(), replace() |
| Conversion | toCharArray(), String.valueOf(), Integer.parseInt() |
What's Next?
In the next lesson, we'll learn about StringBuilder and StringBuffer - mutable alternatives that are much more efficient for frequent String modifications!