Java中String和List的引用

public static void f(ArrayList<Integer> list){
list.add(1);
}
public static void main(String[] a){
ArrayList<Integer> list = new ArrayList<Integer>();
System.out.println(list.size());
f(list);
System.out.println(list.size());
}

static void f(String s){
s = s+ s;
}
public static void main(String[] args){
String s = "zhang";
System.out.println("调用前 s = "+s );
f(s);
System.out.println("调用后 s = "+s );
}