Primitive vs reference storage in Java - Visual Side-by-Side Comparison

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Concept Flow - Primitive vs reference storage

Declare primitive variable

↓

Store value directly in variable

↓

Declare reference variable

↓

Create object in heap memory

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Store reference (address) in variable

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Access object via reference

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Modify object data affects all references

Primitive variables hold actual values directly. Reference variables hold addresses pointing to objects in memory.

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Execution Sample

Java

int a = 5;
int b = a;
b = 10;

String s1 = new String("Hi");
String s2 = s1;
s2 = "Hello";

Shows how primitives copy values and references copy addresses to objects.

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Execution Table

Step	Action	Variable States	Memory	Notes
1	int a = 5;	a=5	Stack: a=5	Primitive value 5 stored in 'a'
2	int b = a;	a=5, b=5	Stack: a=5, b=5	Value of 'a' copied to 'b'
3	b = 10;	a=5, b=10	Stack: a=5, b=10	'b' changed, 'a' unchanged
4	String s1 = new String("Hi");	s1->obj1	Heap: obj1="Hi" Stack: s1->obj1	Object created in heap, s1 points to it
5	String s2 = s1;	s1->obj1, s2->obj1	Heap: obj1="Hi" Stack: s1->obj1, s2->obj1	s2 references same object as s1
6	s2 = "Hello";	s1->obj1, s2->obj2	Heap: obj1="Hi", obj2="Hello" Stack: s1->obj1, s2->obj2	s2 now points to new object, s1 unchanged
7	End	a=5, b=10, s1->obj1("Hi"), s2->obj2("Hello")	Heap: obj1="Hi", obj2="Hello" Stack: a=5, b=10, s1->obj1, s2->obj2	Execution finished

💡 All assignments done, variables hold expected values and references

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Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
a	undefined	5	5	5	5	5	5
b	undefined	5	10	10	10	10	10
s1	undefined	undefined	undefined	obj1	obj1	obj1	obj1
s2	undefined	undefined	undefined	undefined	obj1	obj2	obj2

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Key Moments - 3 Insights

▶Why does changing 'b' not affect 'a' after copying primitive values?

▶Why do 's1' and 's2' point to the same object after 's2 = s1;'?

▶What happens when 's2' is assigned a new string literal?

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Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of 'a'?

A 10

B 5

C undefined

D 0

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Concept Snapshot

Primitive variables store actual values directly.
Reference variables store addresses pointing to objects in heap memory.
Assigning primitives copies values; assigning references copies addresses.
Changing a primitive copy does not affect original.
Changing a reference variable can point to a new object without affecting others.

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Full Transcript

This lesson shows the difference between primitive and reference storage in Java. Primitive variables like 'int' hold actual values directly in memory. When you assign one primitive variable to another, the value is copied, so changing one does not affect the other. Reference variables like 'String' hold addresses pointing to objects stored in heap memory. Assigning one reference variable to another copies the address, so both point to the same object. Changing the reference to point to a new object does not affect the other references. The execution table traces these steps with variable states and memory changes, helping visualize how Java handles primitives and references.