How many objects are created

Question

I was having a discussion about usage of Strings and StringBuffers in Java. How many objects are created in each of these two examples?

Ex 1:

String s = "a";
s = s + "b";
s = s + "c";        

Ex 2:

StringBuilder sb = new StringBuilder("a");
sb.append("b");
sb.append("c");

In my opinion, Ex 1 will create 5 and Ex 2 will create 4 objects.

Answer 1

I've used a memory profiler to get the exact counts.

On my machine, the first example creates 8 objects:

String s = "a";
s = s + "b";
s = s + "c";

• two objects of type String;
• two objects of type StringBuilder;
• four objects of type char[].

On the other hand, the second example:

StringBuffer sb = new StringBuffer("a");
sb.append("b");
sb.append("c");

creates 2 objects:

• one object of type StringBuilder;
• one object of type char[].

This is using JDK 1.6u30.

P.S. To the make the comparison fair, you probably ought to call sb.toString() at the end of the second example.

Answer 2

In terms of objects created:

Example 1 creates 8 objects:

String s = "a"; // No object created
s = s + "b"; // 1 StringBuilder/StringBuffer + 1 String + 2 char[] (1 for SB and 1 for String)
s = s + "c"; // 1 StringBuilder/StringBuffer + 1 String + 2 char[] (1 for SB and 1 for String)

Example 2 creates 2 object:

StringBuffer sb = new StringBuffer("a"); // 1 StringBuffer + 1 char[] (in SB)
sb.append("b"); // 0
sb.append("c"); // 0

To be fair, I did not know that new char[] actually created an Object in Java (but I knew they were created). Thanks to aix for pointing that out.