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Unit 9 - Parameter Passing in Java

Unit 9 - Parameter Passing in Java. 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 );

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Unit 9 - Parameter Passing in Java

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  1. Unit 9- Parameter Passing in Java

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

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

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

  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. 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. Example: Suppose the method fun is defined as: 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);

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