Object parameters
- Java course
- Beginner
- Lesson 78/116
- 4 min
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.
Practice 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.
Starter code
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>();
// Call the addItem method with 7
// Print the list
}
// Define the addItem method here
}Sample solution
static void addItem(ArrayList<Integer> list, int value) {
list.add(value);
}
// inside main:
addItem(list, 7);
System.out.println(list);Expected output
[7]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Fill in the blank: add to the list
static void addOne(ArrayList<Integer> list) {
list.____(1);
}static void addOne(ArrayList<Integer> list) {
list.add(1);
}Explanation: list.add(1) changes the list both in the method and outside.
2 Match scenario to result
| Term | Meaning |
|---|---|
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 |
3 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 Correct answer
- A new copy is created
Explanation: The reference copy points to the same object; add changes the outside.