Why string concatenation takes so long time? [duplicate]

Answer 1

Use a String Builder to append to strings.

When you concatenate, Java is actually creating a new String with the results of the concatenation. Do it multiple times and you are creating gazillion of strings for nothing.

Try:

StringBuilder sb = new StringBuilder();
for (String object : jsonData) { 
    counter++; 
    sb.append(object.toString());  //this does the concatenation internally
                                   //but is very efficient
}

finalJsonDataStr = sb.toString(); //this gives you back the whole string 

Remark:

When you do stuff like

myString = "hello " + someStringVariable + " World!" + " My name is " + name;

The compiler is smart enough to replace all that with a single StringBuilder, like:

myString = new StringBuilder("hello ")
                      .append(someStringVariable)
                      .append(" World!")
                      .append(" My name is ")
                      .append(name).toString();

But for some reason I don't know, it doesn't do it when the concatenation happens inside a loop.