Why StringBuilder when there is String?

String does not allow appending. Each method you invoke on a String creates a new object and returns it. This is because String is immutable - it cannot change its internal state.

On the other hand StringBuilder is mutable. When you call append(..) it alters the internal char array, rather than creating a new string object.

Thus it is more efficient to have:

StringBuilder sb = new StringBuilder();for (int i = 0; i < 500; i ++) {    sb.append(i);}

rather than str += i, which would create 500 new string objects.

Note that in the example I use a loop. As helios notes in the comments, the compiler automatically translates expressions like String d = a + b + c to something like

String d = new StringBuilder(a).append(b).append(c).toString();

Note also that there is StringBuffer in addition to StringBuilder. The difference is that the former has synchronized methods. If you use it as a local variable, use StringBuilder. If it happens that it's possible for it to be accessed by multiple threads, use StringBuffer (that's rarer)

Here is a concrete example on why -

int total = 50000;String s = ""; for (int i = 0; i < total; i++) { s += String.valueOf(i); } // 4828msStringBuilder sb = new StringBuilder(); for (int i = 0; i < total; i++) { sb.append(String.valueOf(i)); } // 4ms

As you can see the difference in performance is significant.

String class is immutable whereas StringBuilder is mutable.

String s = "Hello";s = s + "World";

Above code will create two object because String is immutable

StringBuilder sb = new StringBuilder("Hello");sb.append("World");

Above code will create only one object because StringBuilder is not immutable.

Lesson: Whenever there is a need to manipulate/update/append String many times go for StringBuilder as its efficient as compared to String.