The problem this explains

Two people share one shopping list in a notes app. When either edits it, both see the change, because there is one list and two links to it. Copy the link and you still have one list; copy the list and you have two. Java arrays work exactly this way: a variable holds a reference (the link), and assigning it to another variable copies the link, not the list. Knowing which you have is the difference between a working FRQ and a silent bug.

Two names, one array: references and aliasing

A primitive variable holds its value directly. A variable of an array (or object) type holds a reference: an arrow to where the real data lives on the heap. That one difference drives some of the most missed questions on the exam.

Primitives copy, references share

// primitive: b gets a COPY of the value
int x = 5;
int y = x;      // y is its own 5
y = 9;          // x is still 5

// reference: b gets a COPY of the arrow
int[] a = {1, 2, 3};
int[] b = a;    // same array!
b[0] = 9;       // a[0] is now 9 too

int[] b = a; does not build a second array. It copies the arrow, so a and b point at the same array. That shared state is called aliasing, and a change through either name is visible through the other.

== compares arrows, not contents

int[] a = {1, 2, 3};
int[] b = a;
int[] c = {1, 2, 3};

a == b   // true: same array
a == c   // false: different arrays,
         // even though contents match

Making a real copy

To change one array without touching the other, build a new array and copy the elements in a loop. Now they are separate objects, so mutating one leaves the other alone.

Vocabulary

Predict first, then check

See it in memory

Step through the aliasing scenario. Watch b copy the arrow (not the array), a mutation through b show up in a, and a fresh array become a separate object.

Live code editor

Eight problems, six scaffolded and two open-ended. Write Java, press Run to execute it, and Check to test against the expected output. Hints and the solution are here if you need them, but using them is tracked, so try first.

Alias or Copy?

Six quick rounds. Read the code and predict the result. For true/false, type 1 for true and 0 for false. This drills the aliasing traps the exam loves.

Multiple choice

Predict your answer before reading the options. Watch for the qualifier words. After you answer, the rationale explains every distractor.

FRQ connection: methods share the reference

On the exam, passing an array to a method passes the reference, so the method can change the caller's array. This is aliasing across a method boundary. Practice the copy-then-modify pattern that keeps the original safe.

Choose your lane