Java
Object parameters
Learn to pass object references.
Object parameters
When you pass an object (say an ArrayList or an array) to a method, you actually pass a copy of the reference. The method reaches the same object.
static void addOne(ArrayList<Integer> list) {
list.add(1);
}
ArrayList<Integer> xs = new ArrayList<>();
addOne(xs);
// xs: [1]
When the method calls list.add(1), the outer xs changes too. Because both point to the same object. If you assign a new list (list = new ArrayList<>()), the outside is not affected.
Reassigning a reference does not affect the outside
If you reassign the parameter to a new object inside the method, the outside object does not change. Only the method's own reference starts pointing to the new object.
static void reset(ArrayList<Integer> list) {
list = new ArrayList<>(); // does not affect outside
}
ArrayList<Integer> xs = new ArrayList<>();
xs.add(9);
reset(xs);
// xs is still [9]
So changing the contents of the object affects the outside; reassigning the reference does not.
Exercise Add a value to the list
Define a static void method called addItem; parameters ArrayList<Integer> list, int value. The body should call list.add(value). In main, create a list, call addItem(list, 7), and print the list.
Show example solution
static void addItem(ArrayList<Integer> list, int value) {
list.add(value);
}
// inside main:
addItem(list, 7);
System.out.println(list);
Test yourself
-
Fill in the blank: add to the list
static void addOne(ArrayList<Integer> list) {
list.____(1);
}
Show answer
static void addOne(ArrayList<Integer> list) {
list.add(1);
}
-
Match scenario to result
- list.add(1)
- Outer list changes
- list = new ArrayList<>()
- Outer list stays the same
- list.clear()
- Outer list is emptied
- int n = 5; zero(n);
- n does not change
-
What happens if you pass an ArrayList to a method and call list.add(...) inside?
- The outer list changes
- It causes a compile error
- The outer list does not change
- A new copy is created
Show answer
The outer list does not change — The reference copy points to the same object; add changes the outside.
Next lesson: varargs
