What is the difference between String, StringBuilder, and StringBuffer?

Question

Accepted Answer

**String** — immutable. **StringBuilder** — mutable, fast, single-threaded. **StringBuffer** — mutable, thread-safe, slower.

Analogy: **String** = printed book (can't edit). **StringBuilder** = whiteboard (edit freely). **StringBuffer** = whiteboard with lock (safe but slow).

**String:**
- Immutable — each change creates new object.
- String pool optimization.
- Thread-safe (immutable).
- Bad for loops (O(n^2)).

```java
String s = "Hello";
s = s + " World"; // New object created

// BAD in loops:
String result = "";
for (int i = 0; i < 10000; i++) {
    result += i + " "; // O(n^2) — very slow!
}
```

**StringBuilder:**
- Mutable — modifies in place.
- Fastest for single-threaded.
- Not thread-safe.

```java
StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append(" World");
String result = sb.toString();

// Good in loops:
StringBuilder sb2 = new StringBuilder();
for (int i = 0; i < 10000; i++) {
    sb2.append(i).append(" ");
}
String result2 = sb2.toString(); // O(n) — fast!

sb.insert(5, " ");  // Insert
sb.delete(0, 6);    // Delete
sb.reverse();       // Reverse
sb.charAt(0);       // Access
sb.length();        // Length
sb.capacity();      // Capacity
```

**StringBuffer:**
- Mutable — like StringBuilder.
- Thread-safe — synchronized methods.
- Slower than StringBuilder.

```java
StringBuffer sbf = new StringBuffer();
sbf.append("Hello");
sbf.append(" World");
String result = sbf.toString();

// Thread-safe shared buffer:
class SharedBuffer {
    private StringBuffer buffer = new StringBuffer();
    public synchronized void add(String msg) {
        buffer.append(msg).append("
");
    }
    public String build() { return buffer.toString(); }
}
```

**Performance:** String concat in loops: SLOW. StringBuilder: FASTEST. StringBuffer: slower but safe.

**When to use:** String = constants. StringBuilder = loops (99% of cases). StringBuffer = multi-threaded (rare).

Java: What is the difference between String, StringBuilder, and StringBuffer?