Calling and Returning From a Method. Control Flow

Transcription

1 Calling and Returning From a Method Program execution normally proceeds from one statement to the next. When the call instruction executes, it causes the method body to begin executing. (transfer of ) When a method s body is completely executed, execution returns to the call instruction that invoked the method Pearson Addison-Wesley. All rights reserved import java.awt.color; import javax.swing.jframe; private JFrame window; Control Flow window = new JFrame( Example Frame ); addblackdot(); private void addblackdot() { blackdot = new Oval(10, 10, 20, 20); window.add(blackdot, 0); private void addblackdot() { blackdot = new Oval(10, 10, 20, 20); window.add(blackdot, 0); 2006 Pearson Addison-Wesley. All rights reserved

2 Passing Parameters Parameters are used to pass data from the calling code to the called method. The data to be passed is given in the form of an argument. The method names this data as a formal parameter. The formal parameter is assigned the actual argument at call time. The formal parameter behaves like a local variable of the method. Multiple parameters are positional Pearson Addison-Wesley. All rights reserved private JFrame window, win2; Parameter Passage addblackdot( window, 20 ); addblackdot( win2, 10 ); private void addblackdot(jframe f, int d) { f.add(blackdot,0); private void addblackdot( JFrame f, int d) { f.add(blackdot, 0); 2006 Pearson Addison-Wesley. All rights reserved

3 private JFrame window, win2; Parameter Passage addblackdot( window, 20 ); addblackdot( win2, 10 ); private void addblackdot(jframe f, int d) { f.add(blackdot,0); private void addblackdot( JFrame f, int d) { f.add(blackdot, 0); 2006 Pearson Addison-Wesley. All rights reserved Local Variables Each local variable of a method begins anew each time the method is called. Remember that a variable is a reference to an object Pearson Addison-Wesley. All rights reserved

4 private JFrame window, win2; Local Variables addblackdot( window, 20 ); addblackdot( win2, 10 ); private void addblackdot(jframe f, int d) { f.add(blackdot,0); private void addblackdot( JFrame f, int d) { f.add(blackdot, 0); 2006 Pearson Addison-Wesley. All rights reserved Anonymous Objects private JFrame window; window = new JFrame("Only Window ); window.setbounds(0, 0, 400, 200); adddotat( 50, 50); adddotat( 100, 100); adddotat( 100, 105); Consider a different method. private void adddotat( int x, int y) { Oval blackdot; window.add(blackdot, 0); 2006 Pearson Addison-Wesley. All rights reserved

5 Non-void Methods Parameters are used to pass data from the calling code to the called method. but what if the method needs to return data to the caller? A method returns an object to the location from which it is called Pearson Addison-Wesley. All rights reserved Private Non-void Methodss Parms private Type MethodName ( ) { InstructionSequence Parms Type, ParameterName Type denotes any valid data type (such as int or the name of some class) Pearson Addison-Wesley. All rights reserved

6 JFrame window, win2; Oval dot; dot = ablackdot( window, 20 ); window.add( dot, 0 ); dot = ablackdot( win2, 10 ); win2.add( dot, 0 ); Non-void Return f = window; d = 20; private Oval ablackdot(jframe f, int d) { private Oval ablackdot( JFrame f, int d) { 2006 Pearson Addison-Wesley. All rights reserved JFrame window, win2; window.add( ablackdot(20) ); win2.add( ablackdot(10) ); Anonymous Why or why not? private Oval ablackdot( int d) { 2006 Pearson Addison-Wesley. All rights reserved

7 JFrame window, win2; Oval dot, dot2; dot = ablackdot( 20 ); window.add( dot, 0 ); dot2 = ablackdot( 10 ); win2.add( dot2, 0 ); Separate Variables Why or why not? private Oval ablackdot( int d) { 2006 Pearson Addison-Wesley. All rights reserved