Object References and Aliasing in AP CSA

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

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!

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!

int[] x = {5, 10, 15};
int[] y = x;
y[2] = 0;
System.out.println(x[2]);