StringBuilder sb = new StringBuilder("mainWord"); sb.append("secondWord" + "thirdWord"); System.out.println(sb); The append method here accepts a String as a parameter. My question is, how does this not allocate memory for this String in the heap?
Our difference between String and StringBuilder is that a new object is created in memory for String, but not for StringBuilder. However, the append method still accepts a String as a parameter, which logically should be stored in memory somewhere.
StringBuilder sb = new StringBuilder("mainWord"); sb.append("secondWord" + "thirdWord"); System.out.println(sb); When compiling, for "mainWord" the compiler blocks memory that is statically initialized. It is not on the heap. It would do the same for "secondWord" and "thirdWord" but nowadays is smart enough to realize that adding two constants will always produce the same result. So it performs the calculation directly and blocks memory for "secondWordthrirdWord". Again it is not on the heap.
At runtime a StringBuilder is initialized with "mainWord". At this time the reference is passed, and Java is smart enough to not copy the whole String. When calling append with the other string, again a reference is passed. But now StringBuilder realizes it needs to copy both the data. So it reserves some memory on the heap and copies the content of both strings in there - creating the first buffer object on the heap.
Stringis already allocated.11: ldc #14 // String secondWordthirdWordStringBuilderoverString(especially in loops) is to prevent the creation of uneccessary amounts ofString:("one" + "two") + "three"(via a loop) will create the intermediate string representations, whereasStringBuilderwill hold the data and compile it into a string at the end (usually in an exponentially growing character array, akin toArrayList) via#toString, which#printlnimplicitly calls onsb."one" + "two"(as literal code) will get compiled down to"onetwo": the compiler sees two constants and optimizes away the extra addition."onetwo"is actually stored in "static memory": it exists within the compiled class file itself, and can be accessed as such. When you want to dynamically concatenate strings (in a way the compiler can't optimize), you will create a newStringobject when you finish adding them together. So loops (hard to pre-optimize) will create tens if not hundreds of intermediate strings.