final parameters

Learn immutability with final parameters.

final parameters

If you mark a parameter as final, it cannot be reassigned inside the method. This prevents accidental changes to the parameter and clearly shows the intent of the code.

static int doubleOf(final int x) {
  // x = 10; // compile error
  return x * 2;
}

Here, reassigning x is a compile error. final parameters become especially important in lambdas and inner classes; there, outer variables must be effectively final.

Practical use of final

final parameters are not always necessary; but they make intent clear. Especially when the method body is long, guaranteeing that the parameter does not change is valuable.

static void printArea(final int width, final int height) {
  int area = width * height;
  System.out.println("Area: " + area);
}

Here width and height are constant throughout the method. This gives confidence to whoever reads the code. On the call side, final has no effect.

Exercise Compute area (with final)

Define a static void method called printArea; parameters final int width, final int height. The body should compute the area and print it as Area: <area>. Call printArea(5, 3) from main.

Show example solution

static void printArea(final int width, final int height) {
  System.out.println("Area: " + (width * height));
}
// inside main:
printArea(5, 3);

Test yourself

Which is a compile error in a method with a final int x parameter?
- int y = x + 1;
- x = 10;
- return x * 2;
- System.out.println(x);
Show answer

x = 10; — A final parameter cannot be reassigned.

Fill in the blank: final parameter

static int doubleOf(____ int x) {
  return x * 2;
}

Show answer

static int doubleOf(final int x) {
  return x * 2;
}

Match keyword to role

static

Defines at class level
final

Prevents reassignment
void

Returns no value
return

Hands a value out

Next lesson: Summary - Method parameters

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