Here is the test code:

public class StringStuff {
    private String test = "test";
    public String testBuilder(){
        return new StringBuilder(test).append("stuff").toString();
    }
    public String testCat(){
        return test+"stuff";
    }
    public static void main(String[] args) {

    }
}

test builder does this:
 0 new #4 <java/lang/StringBuilder>
 3 dup
 4 aload_0
 5 getfield #3 <com/elmwarez/StringStuff.test>
 8 invokespecial #5 <java/lang/StringBuilder.<init>>
11 ldc #6 <stuff>
13 invokevirtual #7 <java/lang/StringBuilder.append>
16 invokevirtual #8 <java/lang/StringBuilder.toString>
19 areturn

test cat does this:
 0 new #4 <java/lang/StringBuilder>
 3 dup
 4 invokespecial #9 <java/lang/StringBuilder.<init>>
 7 aload_0
 8 getfield #3 <com/elmwarez/StringStuff.test>
11 invokevirtual #7 <java/lang/StringBuilder.append>
14 ldc #6 <stuff>
16 invokevirtual #7 <java/lang/StringBuilder.append>
19 invokevirtual #8 <java/lang/StringBuilder.toString>
22 areturn