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

beginner

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String is immutable, while StringBuilder and StringBuffer are mutable buffers you change in place. The two buffers differ in thread safety. StringBuffer synchronizes its methods, and StringBuilder does not.

	`String`	`StringBuilder`	`StringBuffer`
Mutable	No	Yes	Yes
Thread-safe	Yes, because it never changes	No	Yes, through synchronized methods
Cost of building text	A new object for every change	Lowest	Higher, it pays for locking

Every change to a String creates a new object, so += inside a loop copies the text again and again. A builder appends into one growing buffer instead.

StringBuilder is the default choice for building text in one thread. It has the same API as StringBuffer without the synchronization overhead. The Java documentation recommends it wherever possible, because it is faster under most implementations. StringBuffer only makes sense when several threads really append to the same buffer.

String str = "Hello";
str += " World"; // Creates new String object
StringBuilder sb = new StringBuilder("Hello");
sb.append(" World"); // Modifies existing buffer

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