Object References and Their Mechanisms
Definition
Object references
Variables that store the memory address of an object rather than the object itself.
Note
Object references are like pointers in languages such as C++ or Golang, where variables can hold the address of primitive value or an object in memory.
Example
- In Java, when you create an object using the new keyword, the variable holds a reference to the object:
Student student1 = new Student("Alice", 16);- Here, student1 is a reference to the Student object.
Sharing References and Modifying Objects
When you assign one object reference to another, both references point to the same object.
Example
public class Student{
public String name;
public int age;
public Student(String nameIn, int ageIn){
this.name = nameIn;
this.age = ageIn;
}
public static void main(String[] args){
Student student1 = new Student("Alice", 13); // initialise object
System.out.println("student1 initial name: " + student1.name + ", age: " + student1.age);
Student student2 = student1; // share reference
System.out.println("student2 initial name: " + student2.name + ", age: " + student2.age);
}
}
Changes made through one reference affect the object and are visible through other references.
Example
public class Student{
public String name;
public int age;
public Student(String nameIn, int ageIn){
this.name = nameIn;
this.age = ageIn;
}
public static void main(String[] args){
Student student1 = new Student("Alice", 13); // initialise object
System.out.println("student1 initial name: " + student1.name + ", age: " + student1.age);
Student student2 = student1; // share reference
System.out.println("student2 initial name: " + student2.name + ", age: " + student2.age);
// change student2 attributes
student2.age = 16;
student2.name = "Bob";
System.out.println("student2 new name: " + student2.name + ", age: " + student2.age);
System.out.println("student1 new name: " + student1.name + ", age: " + student1.age);
}
}
Null References
A reference that does not point to any object is called a null reference.
Example
Student student3 = null;Common Mistake
Accessing methods or properties on a null reference will result in a runtime error (e.g., NullPointerException in Java).
Passing References to Methods
When you pass an object reference to a method, the method receives a copy of the reference, not a copy of the object.
Example
public class Student{
private String name;
private int age;
public Student(String nameIn, int ageIn){
this.name = nameIn;
this.age = ageIn;
}
public String getName(){
return this.name;
}
public void setName(String nameIn){
this.name = nameIn;
}
public static void updateName(Student s, String newName)
{
s.setName(newName);
}
public static void main(String[] args){
Student student4 = new Student("Charlie", 17);
System.out.println("Initial name: " + student4.getName());
updateName(student4, "David"); // invoke method
System.out.println("New name: " + student4.getName());
}
}
Immutable Objects
- Some objects, such as strings in Java, are immutable, meaning their state cannot be changed.
- Modifying an immutable object creates a new object.
Example
public class Main{
public static void main(String[] args){
String str1 = "Hello";
String str2 = str1; // share reference
str2 += " World!"; // modify string
System.out.println("str1: " + str1);
System.out.println("str2: " + str2);
}
}
Example of Implementing Linked Lists with References
A linked list is a linear data structure where each element (node) contains a reference to the next node.
Example
One of the possible implementations (from the ground up) of a linked list (limited to the add method):
public class LinkedList{
public static class Node{ // ideally in seperate file
public int value;
public Node next;
}
private Node head; // default null
public void add(Node newNode){
newNode.next = null;
if (this.head == null){ // if list is empty
this.head = newNode; // move head pointer
return; // stop method execution
}
Node currentNode = this.head;
while (currentNode.next != null){ // iterate through until last
currentNode = currentNode.next; // move on to the next node
}
currentNode.next = newNode; // at this point currentNode is last node in the list
}
public void print(){ // iterate through list and output values
String res = "";
if (this.head == null){
System.out.println("List is empty");
return; // stop method execution
}
Node currentNode = this.head;
while (currentNode != null){ // iterate through
res += currentNode.value + " ";
currentNode = currentNode.next; // move on to the next node
}
System.out.println(res);
}
public static void main(String[] args){ // test the code
LinkedList myList = new LinkedList();
myList.print();
for (int i = 10; i <= 50; i+=10){ // populate list
Node newNode = new Node();
newNode.value = i;
myList.add(newNode);
}
myList.print(); // output list
}
}
- In this example, the add method updates the next reference of the last node to point to the new node.
- This demonstrates how references are used to link nodes together.
Hint
Swapping references is a common operation in algorithms like sorting.
Swapping nodes in linked list animation
Self review
- What happens when you assign one object reference to another?
- How do object references differ from primitive data types?
- Why is it important to understand object references when designing algorithms?
- Try implementing
- remove(int value) function for LinkedList
- a Binary Search Tree and its node insertion in Java.
