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StringBuilder - Mutable Strings

StringBuilder is a mutable sequence of characters. Unlike String, it can be modified without creating new objects, making it ideal for string manipulation.

Key Characteristics

Mutable: Can be modified in place
NOT thread-safe: Faster but not synchronized
Dynamic capacity: Grows automatically
No String Pool: Always on heap

✓ When to Use: String concatenation in loops, building strings dynamically, or any scenario with many string modifications.

💡 Performance: StringBuilder is ~100x faster than String concatenation in loops!

Code Examples

Creating and basic append operations

java

1// Creating StringBuilder
2StringBuilder sb1 = new StringBuilder();           // Empty, capacity 16
3StringBuilder sb2 = new StringBuilder(100);        // Empty, capacity 100
4StringBuilder sb3 = new StringBuilder("Hello");    // From String
5
6// Basic operations
7sb1.append("Hello");
8sb1.append(" ");
9sb1.append("World");
10System.out.println(sb1);  // "Hello World"
11
12// Chaining (methods return same StringBuilder)
13StringBuilder sb = new StringBuilder()
14    .append("Hello")
15    .append(" ")
16    .append("World")
17    .append("!");
18System.out.println(sb);  // "Hello World!"
19
20// Convert to String
21String result = sb.toString();

Common StringBuilder methods

java

1// StringBuilder methods
2StringBuilder sb = new StringBuilder("Hello World");
3
4// append() - Add to end
5sb.append("!");              // "Hello World!"
6
7// insert() - Insert at position
8sb.insert(5, ",");           // "Hello, World!"
9
10// delete() - Remove range
11sb.delete(5, 7);             // "Hello World!" (removes ", ")
12
13// deleteCharAt() - Remove single char
14sb.deleteCharAt(10);         // "Hello World" (removes "!")
15
16// replace() - Replace range with string
17sb.replace(6, 11, "Java");   // "Hello Java"
18
19// reverse() - Reverse string
20sb.reverse();                // "avaJ olleH"
21sb.reverse();                // "Hello Java" (reverse back)
22
23// setCharAt() - Modify single char
24sb.setCharAt(0, 'h');        // "hello Java"
25
26// length and capacity
27System.out.println(sb.length());      // Current length
28System.out.println(sb.capacity());    // Internal buffer size

Performance comparison with String

java

1// Performance comparison
2int iterations = 100000;
3
4// BAD: String concatenation - SLOW!
5long start1 = System.currentTimeMillis();
6String result1 = "";
7for (int i = 0; i < iterations; i++) {
8    result1 += i;  // Creates new String each time!
9}
10long time1 = System.currentTimeMillis() - start1;
11System.out.println("String time: " + time1 + "ms");
12
13// GOOD: StringBuilder - FAST!
14long start2 = System.currentTimeMillis();
15StringBuilder sb = new StringBuilder();
16for (int i = 0; i < iterations; i++) {
17    sb.append(i);  // Modifies same object
18}
19String result2 = sb.toString();
20long time2 = System.currentTimeMillis() - start2;
21System.out.println("StringBuilder time: " + time2 + "ms");
22
23// Result: StringBuilder is ~100x faster!

Practical StringBuilder use cases

java

1// Practical examples
2
3// 1. Building CSV line
4StringBuilder csv = new StringBuilder();
5String[] values = {"Alice", "25", "Developer"};
6for (int i = 0; i < values.length; i++) {
7    csv.append(values[i]);
8    if (i < values.length - 1) csv.append(",");
9}
10System.out.println(csv);  // "Alice,25,Developer"
11
12// 2. Reversing a string
13public static String reverse(String str) {
14    return new StringBuilder(str).reverse().toString();
15}
16
17// 3. Building HTML
18StringBuilder html = new StringBuilder();
19html.append("<ul>\n");
20for (String item : Arrays.asList("Item 1", "Item 2", "Item 3")) {
21    html.append("  <li>").append(item).append("</li>\n");
22}
23html.append("</ul>");
24
25// 4. Pre-allocate capacity for known size
26int estimatedSize = 1000;
27StringBuilder optimized = new StringBuilder(estimatedSize);
28// Avoids internal array resizing
29
30// 5. Palindrome check
31public static boolean isPalindrome(String str) {
32    String reversed = new StringBuilder(str).reverse().toString();
33    return str.equals(reversed);
34}

Use Cases

String concatenation in loops
Building dynamic output (HTML, JSON, CSV)
String reversal
Parsing and building strings
Buffer for text processing
Log message construction

Common Mistakes to Avoid

Using String concatenation in loops instead
Not converting to String with toString()
Using StringBuilder in multithreaded code (use StringBuffer)
Creating new StringBuilder for each operation
Not pre-sizing when final length is known
Forgetting that StringBuilder is mutable