String vs StringBuilder - Number of objects [duplicate]

Question

This question already has answers here:

How many Java string objects created in the code String s="abc"+"xyz";? (3 answers)

Closed 8 years ago.

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?

Answer 1

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

Answer 2

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";

Answer 3

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";

Answer 4

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.

Answer 5

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";
}