Java Data Types: Complete Guide to Primitive and Non‑Primitive Types

Selecting the proper data type is the first step to writing efficient, bug‑free Java code. The data type tells the compiler how much memory to allocate and what operations are legal.

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1️⃣ Primitive Data Types

Primitive types are built into the JVM and store actual values in stack memory. They have fixed size and no additional methods.

Type	Size (bits)	Default	Min Value	Max Value	Typical Use
boolean	JVM‑specific	false	false	true	Flags, status
char	16	\u0000	\u0000	\uFFFF	Single Unicode character
byte	8	0	‑128	127	I/O streams, raw data
short	16	0	‑32 768	32 767	Memory‑critical arrays
int	32	0	‑2 147 483 648	2 147 483 647	Default integer
long	64	0L	‑2⁶³	2⁶³‑1	Large counters, timestamps
float	32	0.0f	‑3.4E38	3.4E38	Low‑precision decimals
double	64	0.0d	‑1.7E308	1.7E308	High‑precision decimals

int age = 30;
double price = 19.99;
boolean isActive = true;

Performance Note

Arithmetic on primitives is hardware‑level—much faster than object wrappers like Integer or Double.

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2️⃣ Non‑Primitive (Reference) Data Types

Non‑primitive types store references to objects in heap memory. Size is dynamic and depends on content.

Category	Description	Example
String	Sequence of characters (immutable)	"Hello"
Array	Ordered collection of primitives/objects	int[] nums = {1,2,3};
Class	Blueprint for objects	class Student { ... }
Others	Interface, Enum, Annotation	Runnable, enum Color

Why No Fixed Size?

The JVM can’t predict how many elements a user will store in an Array or how many fields a custom Class will contain.

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3️⃣ Choosing the Right Type (Real‑World Scenario)

Problem: Store 1 000 000 sensor readings with decimals.

• float saves memory (4 MB) but risks precision loss.
• double doubles memory (8 MB) but retains accuracy.
• BigDecimal (non‑primitive) offers arbitrary precision but is 10–20× slower.

Rule of Thumb: Start with double. Optimize to float or BigDecimal only when profiling proves the need.

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4️⃣ Common Mistakes & Anti‑Patterns

Mistake	Impact	Fix
Using float for currency	Rounding errors	Use BigDecimal
Mixing int and double in arithmetic	Implicit cast may surprise	Cast explicitly
Using wrapper types in tight loops	Autoboxing overhead	Stick to primitives

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5️⃣ Interview Nuggets

1. Q: Why is String not a primitive?
   A: Because it offers methods and dynamic sizing, implemented as an object.
2. Q: Difference between == on primitives vs objects?
   A: Primitives compare values; objects compare references unless overridden.

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🧭 Version Relevance

Feature	Java Version	Notes
strictfp keyword	1.2	Guarantees FP consistency
var (local type inference)	10	Still infers to a primitive or reference type underneath
Records (compact classes)	16	Non‑primitive, concise data carriers

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📝 Summary

• Primitives = fixed size, fast, no methods.
• Non‑Primitives = reference types, variable size, rich APIs.
• Right sizing improves performance and memory efficiency.

Next: Build a small program that benchmarks primitive arrays vs ArrayList<Integer> for 10 million additions.