Parameter Passing - Java and the Web - Lecture Slides, Slides of Java Programming

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Parameter Passing

Parameter Passing – Primitive

Type

• Java passes all parameters to a method by value.
• The actual parameters (the values that are passed in) are assigned to the formal parameters (declared in the method header).
• This means that the value of the actual parameter is copied into the formal parameter.

Parameter Passing – Primitive Type

Example

public class PrimitiveParameter { public void changeValue (int parameter) {parameter = 20;} public static void main (String args[]) { int parameter = 10; PrimitiveParameter test = new PrimitiveParameter(); test.changeValue(parameter); } }

Parameter Passing – Primitive Type

Example

• In the main method:
  • The “parameter” is declared and assigned the value 10 (the actual parameter).
  • After the new class instance creation the method changeValue is called. As a result of the call a copy of formal parameter is made and sent to the method.

Parameter Passing – Reference

Type

• When an object is passed to a method, Java passes a reference to that object.
• The value that is copied is the address of the actual object.
• Therefore it is important to understand that when a formal parameter object is used it modifies the actual object state permanently.

Parameter Passing – Reference

Type Example

public class RefParam { public void changeObject (Car parameter) {parameter.changeParameter();} ... RefParam change = new RefParam(); parameter = new Car(); change.changeParameter(parameter); }

Parameter Passing – Reference

Type Example

• In code section:
  • The formal parameter does not exist anymore, but the object still exists in its changed state.
  • In case the value of the formal parameter is changed it points then to a different object.

Parameter Passing – Applet

• Parameters are also passed from HTML files to Java applets. - The HTML file passes parameters as string types, therefore they need to be converted if used as another type. - The parameters are initialized in applet tag section of HTML file and retrieved in init() method of Java applet.

Parameter Passing – Applet

Example

• The Java applet code section: init() { String FontName = getParameter(“fontName”); String FontSize = getParameter(“fontSize”); String Leading = getParameter(“leading”); String PaidUp = getParameter(“accountEn”); ... }

Methods and Parameters

• Methods are the cornerstone of OO programming, the basic unit of action
• Parameters allow the caller of the Method to affect its outcome by providing a value or values when it is invoked
• Without parameters, adjusting the Method outcome based upon invoker state would be quite cumbersome, if not impossible

Array Parameters

• Java has native support for Arrays as parameters
• A Java array is self-aware of its own length, unlike C/C++. All array information array can be passed as a single parameter, no length parameter is needed.
• Arrays are passed in the same way as Objects

Object Parameters

• Anything that is not a primitive or array falls under the umbrella of Object
• Objects can be passed natively, just like primitives or arrays
• In Java, handles to anything that is not primitive or an array is a reference.
• Does this mean Java passes Object parameters by Reference?

Modify a Parameter Value

… Point p = new Point(10, 20); addTen(p); System.out.println(p.x + “,“ + p.y); … void addTen(Point p) { p.x = p.x + 10; p.y = p.y + 10; }

Modify a Parameter Value

• The output of the System.out.println will be 20, 30
• What if you do not want an outside method modifying your parameter? Ways to handle this will be explored later