Final Variable

Final Instance Variable

• If value changes object to object → instance variable
• Each object has its own copy
• Gets default value from JVM

class Test {
  int x;
  public static void main(String[] args) {
    Test t = new Test();
    System.out.print(t.x); // 0
  }
}

Rule: Final instance variable must be initialized explicitly.

• At declaration → final int x = 10;
• Instance block
• Constructor

// Declaration
final int x = 10;

// Instance block
{
  x = 10;
}

// Constructor
Test() {
  x = 10;
}

Invalid: assigning outside constructor/instance block → compile error

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Final Static Variables

Static → shared among all objects

class Test {
  static int x;
  public static void main(String[] args) {
    System.out.print(x); // 0
  }
}

Rule: Final static must be initialized before class loads.

• At declaration
• Static block

// Valid
final static int X = 10;

// Static block
static {
  X = 10;
}

// Invalid
public void m() {
  X = 10; // error
}

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Final Local Variables

• Declared inside method
• No default value
• Must initialize before use
• Only final allowed

// Error
int x;
System.out.print(x);

// Valid
int x;
System.out.print("Hello");

// Final local
final int x;
System.out.print("Hello");

// Final parameters
void m(final int x) {
  // x = 10; // error
}

Example

Summary

• Final Instance → constructor / block / declaration
• Final Static → declaration / static block
• Final Local → must assign before use