Java Strings
Complete guide to Java Strings with examples. Learn String methods, manipulation, comparison, StringBuilder, StringBuffer, and string formatting.
In Java, strings are objects that represent sequences of characters. The String class is one of the most commonly used classes in Java programming.
1. What is a String in Java?
A string is a sequence of characters. In Java, strings are objects of the String class. Unlike primitive data types, strings are reference types.
// create strings
String type = "Java programming";Which class is used to create strings in Java?
Here, we have created a string variable named type that holds the value "Hello, World!".
Creating Strings in Java
There are two main ways to create strings in Java:
1. String Literal
String str1 = "Java Programming";
String str2 = "Javaistic";2. Using new Keyword
String str1 = new String("Java Programming");
String str2 = new String("Javaistic");Note: String literals are stored in the string pool for memory efficiency, while strings created with new keyword are stored in the heap memory.
2. String Methods
Java provides many useful methods for string manipulation:
1. length() Method
Returns the number of characters in the string.
class Main {
public static void main(String[] args) {
String str = "Java";
System.out.println("Length: " + str.length()); // Output: 4
}
}2. charAt() Method
Returns the character at the specified index.
class Main {
public static void main(String[] args) {
String str = "Java";
System.out.println("Character at index 1: " + str.charAt(1)); // Output: a
}
}3. substring() Method
Extracts a portion of the string.
class Main {
public static void main(String[] args) {
String str = "Java Programming";
System.out.println("Substring: " + str.substring(5)); // Output: Programming
System.out.println("Substring: " + str.substring(0, 4)); // Output: Java
}
}4. toLowerCase() and toUpperCase()
Convert string to lowercase or uppercase.
class Main {
public static void main(String[] args) {
String str = "Java Programming";
System.out.println("Lowercase: " + str.toLowerCase()); // java programming
System.out.println("Uppercase: " + str.toUpperCase()); // JAVA PROGRAMMING
}
}5. indexOf() Method
Returns the index of the first occurrence of a character or substring.
class Main {
public static void main(String[] args) {
String str = "Java Programming";
System.out.println("Index of 'a': " + str.indexOf('a')); // Output: 1
System.out.println("Index of 'Programming': " + str.indexOf("Programming")); // Output: 5
}
}3. String Comparison
1. Using equals() Method
class Main {
public static void main(String[] args) {
String str1 = "Java";
String str2 = "Java";
String str3 = new String("Java");
System.out.println(str1.equals(str2)); // true
System.out.println(str1.equals(str3)); // true
}
}2. Using equalsIgnoreCase() Method
class Main {
public static void main(String[] args) {
String str1 = "Java";
String str2 = "JAVA";
System.out.println(str1.equalsIgnoreCase(str2)); // true
}
}3. Using compareTo() Method
class Main {
public static void main(String[] args) {
String str1 = "Apple";
String str2 = "Banana";
System.out.println(str1.compareTo(str2)); // negative value (Apple < Banana)
}
}Important: Never use == operator to compare strings. It compares references, not content.
What is the output of 'Java'.substring(1, 3)?
4. String Concatenation
1. Using + Operator
class Main {
public static void main(String[] args) {
String firstName = "John";
String lastName = "Doe";
String fullName = firstName + " " + lastName;
System.out.println(fullName); // John Doe
}
}2. Using concat() Method
class Main {
public static void main(String[] args) {
String str1 = "Hello";
String str2 = " World";
String result = str1.concat(str2);
System.out.println(result); // Hello World
}
}5. StringBuilder and StringBuffer
When you need to perform many string operations, use StringBuilder or StringBuffer for better performance.
StringBuilder Example
class Main {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
sb.append("Java");
sb.append(" ");
sb.append("Programming");
System.out.println(sb.toString()); // Java Programming
// Insert at specific position
sb.insert(5, "is ");
System.out.println(sb.toString()); // Java is Programming
// Delete characters
sb.delete(5, 8);
System.out.println(sb.toString()); // Java Programming
}
}StringBuffer vs StringBuilder
| Feature | StringBuilder | StringBuffer |
|---|---|---|
| Thread Safety | Not thread-safe | Thread-safe |
| Performance | Faster | Slower |
| Usage | Single-threaded applications | Multi-threaded applications |
6. String Formatting
1. Using printf() Method
class Main {
public static void main(String[] args) {
String name = "John";
int age = 25;
System.out.printf("Name: %s, Age: %d%n", name, age);
// Output: Name: John, Age: 25
}
}2. Using String.format() Method
class Main {
public static void main(String[] args) {
String name = "Alice";
double score = 95.5;
String formatted = String.format("Student: %s, Score: %.1f", name, score);
System.out.println(formatted); // Student: Alice, Score: 95.5
}
}Key Takeaways
- Immutable: Strings cannot be changed. Methods like
.toUpperCase()return a new string. - Pool: String literals (
"Hello") are stored in a special memory pool for efficiency. - Comparison: ALWAYS use
.equals()to compare values.
Common Pitfalls
[!WARNING] The
==Trap:s1 == s2checks if they are the same object in memory, not if they have the same text. Uses1.equals(s2).
[!WARNING] Concatenation in Loops: Using
+in a loop (e.g.,s += "a") is very slow because it creates a new String object every time. UseStringBuilderinstead.
Challenge
Challenge
Task:
Create a string 'Java Programming' and print its length.