Sample 1

String result = "";
for (int i=0;i<1000;i++) {
    result = result + "hello";
}

Sample 2

StringBuilder sb = new StringBuilder();
for (int i=0;i<1000;i++) {
    sb.append("hello");
}

In the above code, sample 1 runs faster or the sample 2 ? Or, are they same?

The sample 2 is actually much faster than sample 1. And also more memory efficient too. Here is why. In sample 1, for each iteration of loop a new String object is constructed with the concatenated result. This is essentially a a O(n^2) operation. In case of StringBuilder, the string “hello” is appended to the same StringBuilder object. Therefore its much faster and a O(1) operation. However, there is an overhead invovled in growing the capacity of internal character buffer as the string content increases in length.