1. AP Computer Science A – Healdsburg High School 1 Unit 9 - Parameter Passing in Java

2. AP Computer Science A – Healdsburg High School 2 Passing Parameters to Constructors and Methods Any expression that has an appropriate data type can serve as a parameter: double u = 3, v = - 4;... Polynomial p = new Polynomial (1.0, - (u + v), u * v); double y = p.getValue (2 * v - u); public class Polynomial { public Polynomial (double a, double b, double c) {... } public double getValue (double x) {... }...

3. AP Computer Science A – Healdsburg High School 3 Passing Parameters (cont’d) int is promoted to double when necessary: The same as: (3.0)... Polynomial p = new Polynomial (1, - 5, 6); double y = p.getValue (3); The same as: (1.0, -5.0, 6.0)

4. AP Computer Science A – Healdsburg High School 4 Passing Parameters (cont’d) Primitive data types are always passed “by value”: the value is copied into the parameter. double x = 3.0; double y = p.getValue ( x ); public class Polynomial {... public double getValue (double u) { double v;... } x: 3.0 u: 3.0 copy u acts like a local variable in getValue

5. AP Computer Science A – Healdsburg High School 5 Passing Parameters (cont’d) public class Test { public double square (double x) { x *= x; return x; } public static void main(String[ ] args) { Test calc = new Test (); double x = 3.0; double y = calc.square (x); System.out.println (x + " " + y); } x here is a copy of the parameter passed to square. The copy is changed, but...... the original x is unchanged. Output: 3 9

6. AP Computer Science A – Healdsburg High School 6 Passing Parameters (cont’d) Objects are always passed as references: the reference is copied, not the object. Fraction f1 = new Fraction (1, 2); Fraction f2 = new Fraction (5, 17); Fraction f3 = f1.add (f2); public class Fraction {... public Fraction add (Fraction f) {... } copy reference A Fraction object: num = 5 denom = 17 refers to the same object refers to

7. AP Computer Science A – Healdsburg High School 7 Passing Parameters (cont’d) A method can change an object passed to it as a parameter (because the method gets a reference to the original object). A method can change the object for which it was called (this object acts like an implicit parameter):

8. AP Computer Science A – Healdsburg High School 8 Passing Parameters (cont’d) Inside a method, this refers to the object for which the method was called. this can be passed to other constructors and methods as a parameter: public class ChessGame {... Player player1 = new Player (this);... A reference to this ChessGame object

9. AP Computer Science A – Healdsburg High School 9 public int fun(int x, int y) { y -= x; return y; } What is printed to the screen after the following code is executed? int a = 3, b = 7; b = fun(a, b); a = fun(b, a); System.out.println(a + " " + b); Example: Suppose the method fun is defined as: