Java Reference Data Types – Core Notes#

Concise points about non-primitive (reference) types and related concepts.

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Table of Contents#

• 1. What Is a Reference Type?
• 2. Object References and Mutability
• 3. Passing Objects to Methods
• 4. Strings as Reference Types
• 5. Interfaces as Reference Types
• 6. Primitive vs Reference (Snapshot)
• 7. Wrapper Classes
• 8. Autoboxing & Unboxing
• 9. Wrapper Caching (Favorite)
• 10. Constants (final / static final)
• 11. Collections Require Wrappers
• 12. Performance Note
• 13. Primitive vs Wrapper (Quick Compare)
• 14. Carry-Forward Points

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1. What Is a Reference Type?#

• Stores an address to an object (not the data).
• Examples: class objects, strings, arrays, interfaces.
• Mental model: variable = remote control; object = TV.

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2. Object References and Mutability#

Employee empObj = new Employee();
empObj.empId = 110010;

• empObj holds a reference; fields live inside the object.
• Multiple references can point to the same object.

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3. Passing Objects to Methods#

modify(empObj);

void modify(Employee e) {
    e.empId = 110025;
}

• Java is pass-by-value; the reference value is copied.
• Mutations via the copy are visible to all references.
• You cannot rebind the caller’s reference from inside the method.

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4. Strings as Reference Types#

String s1 = "hello";
String s2 = "hello";

• Strings are reference types but immutable.
• Pooling allows s1 and s2 to share the same object.
• == compares references; equals() compares content (always use equals()).

4.1 Literal vs new String()#

String s1 = "hello";            // pooled
String s3 = new String("hello"); // new heap object

• Same content, different references; new String() always creates a new object.

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5. Interfaces as Reference Types#

Person p = new Engineer();

• Interface reference can point to any implementing class; dispatch is runtime.
• Cannot instantiate an interface directly (new Person() ❌).
• Principle: program to an interface, not an implementation.

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6. Primitive vs Reference (Snapshot)#

Concept	Primitive	Reference
Stores	Value	Address
Method passing	Copy of value	Copy of reference
Can be null	❌	✅
Mutation visible	❌	✅

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7. Wrapper Classes#

• Convert primitives to objects.

Primitive	Wrapper
int	Integer
double	Double
char	Character
boolean	Boolean

Why: collections/APIs expect objects; wrappers add methods; allow null.

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8. Autoboxing & Unboxing#

int a = 10;
Integer obj = a;      // autoboxing -> Integer.valueOf(a)
int b = obj;          // unboxing   -> obj.intValue()

• Compiler inserts conversions.
• Unboxing a null wrapper throws NullPointerException.

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9. Wrapper Caching (Favorite)#

Integer a = 100, b = 100; // cached → a == b is true
Integer x = 200, y = 200; // new objects → x == y is false

• Cache range: -128 to 127.
• Use equals() for comparison.

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10. Constants (final / static final)#

final int MAX = 100;
public static final int MAX_USERS = 1000;

• final: cannot be reassigned; encourages immutability.
• static final: one shared, compile-time constant; name in UPPER_CASE_WITH_UNDERSCORES.

public static final Integer LIMIT = 10; // wrapper constant (avoid null)

Prefer primitives for constants unless an object is required.

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11. Collections Require Wrappers#

// List<int> list = new ArrayList<>(); // ❌
List<Integer> list = new ArrayList<>(); // ✅

list.add(10);        // autoboxing
int x = list.get(0); // unboxing

• Collections store objects; wrappers bridge the gap.

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12. Performance Note#

• Autoboxing allocates; avoid in tight numeric loops.
• Rule: use primitives for computation; wrappers for collections/APIs.

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13. Primitive vs Wrapper (Quick Compare)#

Feature	Primitive	Wrapper
Object	❌	✅
Can be null	❌	✅
Collections	❌	✅
Performance	Faster	Slower
Methods	❌	✅

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14. Carry-Forward Points#

• References point to objects; mutations are shared across references.
• Java passes references by value.
• Strings are immutable; compare with equals().
• Wrappers enable collections/APIs and null; mind autoboxing cost.
• Use equals() for wrappers; use static final for true constants.

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