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📦 Variables & Types · Beginner

Primitives vs references in Java

Primitives hold values; reference variables hold a pointer to an object or null.

🧩 The mysteryCopy an int and change the copy: the original stays safe. Copy an array and change the copy: the original changes too! What's going on?

Cash vs a locker note

A primitive variable holds its value directly, like cash in your hand. A reference variable (any class, interface or array type, like String or int[]) holds the address of an object stored elsewhere, like a note with a locker number.

🔮 Predict it

Your turn

What does this print?

int a = 3;
int b = a;
b = 8;
System.out.println(a);
System.out.println(b);
  1. 3 8
  2. 8 8
  3. 3 3
Show the answer

int b = a; copies the value 3 into b. Changing b afterwards doesn't touch a. Output: 3 then 8.

Copying a reference shares the object

Arrays are objects. int[] b = a; copies the address, not the array. Now a and b point to the same array, so a change made through one is visible through the other.

int[] a = {5};
int[] b = a;   // same array!
b[0] = 9;
// a[0] is now 9 too
🔮 Predict it

Your turn

What does this print?

int[] x = {1, 2};
int[] y = x;
y[1] = 50;
System.out.println(x[1]);
  1. 2
  2. 50
  3. Compile error
Show the answer

50. x and y refer to the very same array object, so y[1] = 50 changes what x[1] sees.

null: the blank note

A reference can be null, meaning it points to no object. Only reference types can be null. An int, double or boolean always contains a value.

String name = null;  // ok
int count = null;    // error
⚠️ The trap

Calling a method on null

There's no object to ask for its length, so the JVM throws NullPointerException. It compiles fine and blows up at runtime.

String name = null;
System.out.println(name.length());
// NullPointerException
💼 In the real world

In real projects

NullPointerException is the most famous crash in Java. Tony Hoare, who introduced null references in 1965, later called them his "billion-dollar mistake". Since Java 14, NPE messages name exactly which variable was null.

Key takeaways

  1. int b = a; copies the number
  2. int[] b = a; copies the reference: same array
  3. Only reference variables can be null
  4. Calling a method on null → NullPointerException

💡 A primitive is cash in your hand; a reference is a note with the address of a locker.

🤯 Did you know?

Java references can't be used for pointer arithmetic: you can't add to them or turn them into numbers. That rule removes whole families of memory bugs common in C.

Practice questions

What does this print?

int a = 5;
int b = a;
b = 9;
System.out.println(a);
  1. 9
  2. 14
  3. 5
  4. 0
Check your answer

5. b = a copies the value 5 into b. Changing b afterwards doesn't affect a.

What does this print?

int[] a = {5};
int[] b = a;
b[0] = 9;
System.out.println(a[0]);
  1. 9
  2. 5
  3. 0
  4. Compile error
Check your answer

9. Arrays are objects. b = a copies the reference, so a and b point at the same array, and the change is visible through both.

Next: every primitive has an object twin: Integer, Double, Boolean... Why would anyone want an int dressed up as an object?