when to use string builder, string buffer

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In java String Builder Should be Used in case you need to perform concatenate more string together.

i.e

 public String toString()
 {
    return  a + b + c ;
 }


For the above code using + will be converted to


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



For the above case you can use concat as below but since + will be converted as String Builder its better to use + rather than concat.


 public String toString()
 {
    return  a.concat(b).concat(c);
 }



The key is whether you are writing a single concatenation all in one place or accumulating it over time.


There's no point in explicitly using StringBuilder. 


But if you are building a string e.g. inside a loop, use StringBuilder.


To clarify, assuming that hugeArray contains thousands of strings, code like this:


...
String result = "";
for (String s : hugeArray) {
    result = result + s;
}



It should be as below


 ...
StringBuilder sb = new StringBuilder();

for (String s : hugeArray) {
    sb.append(s);
}
String result = sb.toString();

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