Java: StringBuilder
StringBuilder is a mutable character sequence, ideal for scenarios that require frequent string modifications.
1. Why StringBuilder is Needed
String is immutable—every modification creates a new object, which is inefficient.
▶ Example: String vs StringBuilder
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// String concatenation (inefficient)
String s = "";
for (int i = 0; i < 10000; i++) {
s += "a"; // Creates a new object each time
}
// StringBuilder concatenation (efficient)
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 10000; i++) {
sb.append("a"); // Modifies the same object
}
String result = sb.toString();
💡 Performance difference: For 10,000 loop concatenations, StringBuilder is over 100x faster than String.
2. Creating StringBuilder
JAVA
// Empty StringBuilder
StringBuilder sb1 = new StringBuilder();
// With initial capacity
StringBuilder sb2 = new StringBuilder(100);
// With initial content
StringBuilder sb3 = new StringBuilder("Hello");
3. Common Methods
(1) append(): Add to End
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StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append(" ");
sb.append("World");
sb.append(123);
System.out.println(sb); // HelloWorld123
(2) insert(): Insert at Position
JAVA
StringBuilder sb = new StringBuilder("Hello");
sb.insert(5, " World");
System.out.println(sb); // Hello World
(3) delete(): Delete Characters
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StringBuilder sb = new StringBuilder("Hello World");
sb.delete(5, 6); // Delete character at index 5 (space)
System.out.println(sb); // HelloWorld
(4) replace(): Replace Characters
JAVA
StringBuilder sb = new StringBuilder("Hello World");
sb.replace(6, 11, "Java");
System.out.println(sb); // Hello Java
(5) reverse(): Reverse String
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StringBuilder sb = new StringBuilder("Hello");
sb.reverse();
System.out.println(sb); // olleH
(6) Other Methods
| Method | Description | Example |
|---|---|---|
charAt() |
Get character at index | sb.charAt(0) |
length() |
Current length | sb.length() |
capacity() |
Current capacity | sb.capacity() |
setCharAt() |
Set character at index | sb.setCharAt(0,'h') |
substring() |
Get substring | sb.substring(0,3) |
4. Method Chaining
StringBuilder methods return this, supporting method chaining.
JAVA
String result = new StringBuilder()
.append("Hello")
.append(" ")
.append("World")
.append("!")
.toString();
System.out.println(result); // Hello World!
▶ Example: Building SQL Statements
JAVA
public class SqlBuilder {
public static String buildSelect(String table, String[] columns, String where) {
StringBuilder sql = new StringBuilder("SELECT ");
// Column names
sql.append(String.join(", ", columns));
sql.append(" FROM ");
sql.append(table);
// Condition
if (where != null && !where.isEmpty()) {
sql.append(" WHERE ");
sql.append(where);
}
return sql.toString();
}
public static void main(String[] args) {
String sql = buildSelect("users",
new String[]{"id", "name", "email"},
"age > 18");
System.out.println(sql);
// SELECT id, name, email FROM users WHERE age > 18
}
}
▶ Example: Building HTML
JAVA
public class HtmlBuilder {
public static String buildTable(String[][] data) {
StringBuilder html = new StringBuilder();
html.append("<table>\n");
for (String[] row : data) {
html.append(" <tr>\n");
for (String cell : row) {
html.append(" <td>").append(cell).append("</td>\n");
}
html.append(" </tr>\n");
}
html.append("</table>");
return html.toString();
}
public static void main(String[] args) {
String[][] data = {
{"Alice", "25", "Engineer"},
{"Bob", "30", "Designer"}
};
System.out.println(buildTable(data));
}
}
5. StringBuilder vs StringBuffer
| Feature | StringBuilder | StringBuffer |
|---|---|---|
| Thread-safe | No | Yes |
| Performance | Faster | Slower |
| Use case | Single-threaded | Multi-threaded |
💡 Recommendation: Use StringBuilder for single-threaded code. Use StringBuffer for multi-threaded code. Generally, StringBuilder is preferred.
6. Capacity and Growth
StringBuilder has a capacity concept and automatically grows when exceeded.
JAVA
StringBuilder sb = new StringBuilder();
System.out.println("Initial capacity: " + sb.capacity()); // 16
// Append content, grows when capacity exceeded
for (int i = 0; i < 20; i++) {
sb.append("a");
}
System.out.println("Current capacity: " + sb.capacity()); // 34
System.out.println("Current length: " + sb.length()); // 20
💡 Growth rule: New capacity = old capacity * 2 + 2
7. ❓ Frequently Asked Questions
Q When should I use StringBuilder?
A When you need to modify strings frequently, such as in loop concatenations or building complex strings.
Q How do I choose between StringBuilder and String?
A Use String for few operations (simpler). Use StringBuilder for many operations (faster).
Q What's the default capacity of StringBuilder?
A 16 by default. You can specify it in the constructor.
❓ FAQ
Q Are strings mutable in Java?
A No. String objects are immutable. Every modification creates a new String.
Q Should I use == or equals for string comparison?
A Always use equals(). == compares references, not content.
Q How to efficiently concatenate many strings?
A Use StringBuilder instead of + in loops. It avoids creating many intermediate objects.
📖 Summary
- StringBuilder is a mutable character sequence, ideal for frequent modifications
- Common methods: append/insert/delete/replace/reverse
- Supports method chaining
- Use StringBuilder for single-threaded code, StringBuffer for multi-threaded
- Default capacity is 16, grows automatically when exceeded
📝 Exercises
- String concatenation: Use StringBuilder to concatenate numbers 1 to 100, separated by commas
- HTML building: Use StringBuilder to build an unordered list (ul/li)
- Performance test: Compare the time difference between String and StringBuilder for 10,000 concatenations
8. Next Lesson
In the next lesson, we'll learn about Math and Dates — the Math class and date/time API.
