Object confusion

Question

I'm getting a little confused over the use of objects in general in code. I always considered an object in my code basically as a pointer, such that using it in a function or adding it into a list would retain the object itself not just a copy. However this doesn't seem to be the case.
This little code snippet shows where my confusion is.
When I add a String object to the list then alter the String, shouldn't that change translate to the object in the array? Or, does ArrayList.add automatically allocate a new object to store in itself?
When I pass an Integer into the function and change the value it is not reflected in the original Integer object I passed in. This is the behavior I would expect from an int primitive. Is there a way to pass an object like this by reference rather than value?
Lastly, when I pass the ArrayList in to the function, its changes are reflected in the original list. Do ArrayLists actually represent pointers like I assumed all objects did where as Integer objects are treated differently?
public static void main(String[] args) throws Exception {
     ArrayList<String> t = new ArrayList<String>();
     String s = "a";
     t.add(s);
     s = "b";
     System.out.println(t.get(0));
     System.out.println(s);
     Integer i = 1;
     test(t, i);
     System.out.println(i);
     System.out.println(t.get(0));
public static void test(ArrayList<String> list, Integer i){
     list.set(0, "c");
     i = 2;
}

Answer 1

jamesss wrote:
jverd wrote:
Not in this case, because String literals are cached. If you had done s = new String("a") or s = new anything or s = anything else that creates an object, then yes.By String literals being cached you mean the literal "a" is cached right? not whatever object s is currently pointing at?s points at a String object. In that line, that object is the interned ("cached") String object containing "a".
i.e. if I rewrote it as
String s;
for(int i=0; i<5; i++){
s = Integer.toString(i);
would this actually create 5 separate objects since it is using a different string value each time?Yes.
>
and with
for(int i=0; i<5; i++){
String s = "a";
does this imply new String("a") and thus would also create 5 separate objects?No. The "a" does not create a new String object. That object is placed into the constant pool when your class is loaded, and from then on, whenever "a" appears, in your code, it's just a reference to that already existing object. If you did new String("a"), that would create a new String object, but it would be pointless to do so.
Lastly, is there any real difference between the value held by a String reference variable and an Integer reference variable, obviously they must point at their own object type but do the reference values among types basically look/act the same?No difference. Any reference variable just holds a reference that points to an object of an appropriate type, or it holds null, meaning it doesn't point to any object. What's special about Strings is how "abc" literals get put into the constant pool so that you can avoid creating new string objects in some cases. The primitive wrappers--Integer, etc.--do something similar for autoboxing with numbers in the range -128..127.