In Java, object variables don’t store the object itself — they store a reference (memory address) to where the object lives on the heap. This distinction causes some of the trickiest AP exam questions.
Primitives vs References
Primitive (int, double, boolean)
Reference (Object, String, array)
Stores the actual value
Stores a memory address
int x = 5; — x holds 5
Dog d = new Dog(); — d holds address
Copied by value
Copied by reference
Aliasing: Two Variables, One Object
When you assign one object variable to another, both variables point to the same object. Changing the object through one variable changes it for the other too.
int[] a = {1, 2, 3};
int[] b = a; // b is an ALIAS for a -- same array
b[0] = 99;
System.out.println(a[0]); // 99 -- a was changed too!
⚠ Alias Trap: A very common AP question shows two variables modified and asks what the original contains. If an object was assigned (not copied), both variables see the change.
null References
A reference variable that hasn’t been assigned an object holds null. Calling a method on null causes a NullPointerException at runtime.
String s = null;
System.out.println(s.length()); // NullPointerException!
📝 Practice Question 1
Consider the following code:
int[] x = {5, 10, 15};
int[] y = x;
y[2] = 0;
System.out.println(x[2]);
What is printed?
📝 Practice Question 2
Which of the following statements about object references in Java is correct?
I. Two variables can refer to the same object. II. Assigning one object variable to another creates a copy of the object. III. A reference variable set to null can still have methods called on it.
✅ Exam Tip: Draw a box-and-arrow diagram when tracing reference questions. Each variable is a box; arrows point to the object on the heap. Aliasing = two arrows pointing to the same box.
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