Final Keyword in Java: Variable, Method and Class
final variable = constant, final method = cannot override, final class = cannot extend. Examples of all three plus final vs finally vs finalize.
final at three levels
1. final variable
final int MAX_MARKS = 100;
MAX_MARKS = 90; // ERROR: cannot assign a value to final variable
final Student s = new Student("Aman");
s.marks = 95; // OK! object's fields can change
s = new Student("Priya"); // ERROR: the REFERENCE is locked, not the objectfinal List can still have items added — you just cannot point the variable to a different list. Exactly like int *const p in C.2. final method
class Payment {
final void verifyOTP() { // locked logic
System.out.println("OTP verified securely");
}
}
class UpiPayment extends Payment {
// void verifyOTP() { ... }
// ERROR: verifyOTP() in Payment is final, cannot be overridden
}Use when a method's logic is security- or correctness-critical and no subclass should be able to change it.
3. final class
final class Constants {
static final double GST = 0.18;
}
// class MyConstants extends Constants { }
// ERROR: cannot inherit from final class ConstantsFamous final classes in Java itself: String, Integer, Math. String is final so nobody can subclass it and break its immutability guarantee — a favourite follow-up question.
final vs finally vs finalize (the trio question)
| Term | What it is | Purpose |
|---|---|---|
final | Keyword | Lock variable / method / class |
finally | Block with try-catch | Cleanup code that always runs, exception or not |
finalize() | Method (deprecated) | Was called by GC before destroying object; deprecated since Java 9 |
try {
int x = 10 / 0;
} catch (ArithmeticException e) {
System.out.println("Error caught");
} finally {
System.out.println("Always runs - close files here");
}Final तीन levels पर
1. final variable
final int MAX_MARKS = 100;
MAX_MARKS = 90; // ERROR: cannot assign a value to final variable
final Student s = new Student("Aman");
s.marks = 95; // OK! object की fields बदल सकती हैं
s = new Student("Priya"); // ERROR: REFERENCE locked है, object नहींfinal List में items add हो सकते हैं — बस variable को दूसरी list पर point नहीं कर सकते. बिल्कुल C के int *const p जैसा.2. final method
class Payment {
final void verifyOTP() { // locked logic
System.out.println("OTP verified securely");
}
}
class UpiPayment extends Payment {
// void verifyOTP() { ... }
// ERROR: verifyOTP() in Payment is final, cannot be overridden
}तब use करें जब method की logic security- या correctness-critical हो और कोई subclass उसे बदल न पाए.
3. final class
final class Constants {
static final double GST = 0.18;
}
// class MyConstants extends Constants { }
// ERROR: cannot inherit from final class ConstantsJava की famous final classes: String, Integer, Math. String final है ताकि कोई subclass बनाकर उसकी immutability guarantee न तोड़ सके — favourite follow-up question.
final vs finally vs finalize (trio question)
| Term | क्या है | Purpose |
|---|---|---|
final | Keyword | Variable / method / class lock करना |
finally | try-catch के साथ block | Cleanup code जो हमेशा चलता है, exception हो या न हो |
finalize() | Method (deprecated) | GC object destroy करने से पहले call करता था; Java 9 से deprecated |
try {
int x = 10 / 0;
} catch (ArithmeticException e) {
System.out.println("Error caught");
} finally {
System.out.println("Always runs - files यहां close करें");
}Frequently Asked Questions
What does the final keyword do in Java?
On a variable it locks the value (or reference), on a method it prevents overriding, and on a class it prevents inheritance.
Can a final object be modified?
Its fields can change — final only locks the reference, so the variable cannot point to another object, but the object itself remains mutable.
What is the difference between final, finally and finalize?
final is a keyword for locking; finally is a try-catch block that always executes for cleanup; finalize() was a garbage-collection callback method, deprecated since Java 9.