Useful Tips : StringBuilder and Java String Concatenation

As you certainly now, string concatenation in Java can be achieved using the “+” operator like that :

String parText = "This " + "is " + "a full text";
parText = parText + " in a new String object";

But each time you try to concat the string in a new line a new String object is created, therefor the String concatenation may end up as being a performance issue.

To fix this we use two types of objects : StringBuilder and StringBuffer. Then you can translate your String (e.g. a SQL Query) into :

StringBuilder stb = new StringBuilder();
stb.append("This ");
stb.append("is ");
stb.append("a full text");
stb.append(" in a new String object");

The main differences between StringBuilder and StringBuffer is that the last one is synchronized and may be used in a multi-threaded context while the other one won’t have any type of synchronization. Luckily when you try to concatenate multiple string items, Eclipse IDE, (if you ask it by typing Ctrl + 1), will ask you if you need to translate it to a StringBuilder object :

But nowadays, if you're careful and know what you're doing, you can avoid typing this kind of “ultra verbose” way of dealing with strings. Actually since Java 1.5 String concatenations when they're done on a whole line, are directly translated by Java Compiler, but if you jump onto another line and keep on concatenating the same String, optimization will be lost. E.g. :

// this way
StringBuilder stb = new StringBuilder();
stb.append("This ");
stb.append("is ");
stb.append("a full text");
stb.append(" in a new String object");
// and this one are equivalent
String parText = "This " + "is " + "a full text" + " in a new String object";
// but this one isn’t :
String parText2 = “This " + “is " + “a full text”;
parText2 = parText2 + " in a new String object”;