String vs StringBuilder - Number of objects

Question

I came across two code snippets which were creating a query to be executed further:

StringBuilder stringBuilder = new StringBuilder(); 
stringBuilder.append("SELECT * FROM EMPLOYEE "); 
stringBuilder.append("WHERE SALARY > ? "); 
stringBuilder.append("GROUP BY DEPT");

And

String string = "SELECT * FROM EMPLOYEE " +
"WHERE SALARY > ? " +
"GROUP BY DEPT";

According to my analysis, both the snippets create 4 objects. First snippet creates a StringBuilder object and 3 string objects while the second snippet creates 4 String objects. Is my analysis correct?

How snippet one is more efficient than snippet 2?

and also [this](http://stackoverflow.com/questions/21526747/how-many-string-objects-will-be-created-in-strings-sachintendulkar#21526786) and [this](http://stackoverflow.com/questions/11180866/how-many-string-objects-will-be-created-in-memory) and [this](http://stackoverflow.com/questions/10045147/how-many-objects-are-being-created). Dude... — avalancha, Feb 05 '14 at 09:06

score 8 · Accepted Answer · answered Feb 05 '14 at 09:01

8

Your first analysis is correct, but second snippet creates only one String Object. Because, it's a compile time concatenation of Strings literals.

answered Feb 05 '14 at 09:01

Abimaran Kugathasan

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1

It is clear that the concatenation *can* be done at compile time. However, is the compiler *obliged* to do it? – NPE Feb 05 '14 at 09:03
3

Nowadays when people talk about "java" without an android tag, it is safe to assume its the hotspot compiler and then yes it is going to happen. If you build your own compiler you are no no way obliged to do compile time concatenation since leaving it as it is is still functionally correct. – Gimby Feb 05 '14 at 09:07
1

@NPE according to [JLS §15.18.1](http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.18.1) I don't think the compiler is obliged to. – Boris the Spider Feb 05 '14 at 09:07

score 5 · Answer 2 · answered Feb 05 '14 at 09:01

Not exactly...

The first version creates 1 StringBuilderevery execution and 3 String constants (created/interned once per JVM start). It will create another String object every execution when you go to use the value via stringBuilder.toString().

The second creates 1 String constant (created/interned once per JVM start) because the value of the entire concatenation is known at compile time. It is equivalent in effect to:

String string = "SELECT * FROM EMPLOYEE WHERE SALARY > ? GROUP BY DEPT";

Ankur Shanbhag · Answer 3 · 2014-02-05T10:26:14.553

The StringBuilder example will create 4 objects.

Whereas the second example will just create one String object. Even though you have concatenated 3 String literals, model Java compilers are smart enough to detect that you have concatenated static String literals and thus will append all of them into one and create a single String object.

Therefore,

String string = "SELECT * FROM EMPLOYEE " +
"WHERE SALARY > ? " +
"GROUP BY DEPT";

is same as

String string = "SELECT * FROM EMPLOYEE WHERE SALARY > ? GROUP BY DEPT";

score 0 · Answer 4 · answered Feb 05 '14 at 09:02

0

Strings are immutable so each time you do a concatenation (. operator) you're creating a new String object in memory. With StringBuilder you're adding the text to the same object so a new ones do not need to be created.

answered Feb 05 '14 at 09:02

Whoppa

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score 0 · Answer 5 · answered Feb 05 '14 at 09:17

The first snippet create 1 object. It's concatenate in compile-time.

StringBuilder has better performance when you are concatenating mutable strings. For example, with your example:

StringBuilder stringBuilder = new StringBuilder(); 
stringBuilder.append("SELECT * FROM EMPLOYEE "); 
if(salary > 0) {
    stringBuilder.append("WHERE SALARY > ? "); 
}
if(group == true) {
    stringBuilder.append("GROUP BY DEPT");
}

String string = "SELECT * FROM EMPLOYEE ";
if(salary > 0) {
    string = string + "WHERE SALARY > ? ";
}
if(group == true) {
    string = = string + "GROUP BY DEPT";
}

String vs StringBuilder - Number of objects

5 Answers5