Java编程入门,英语教材.ppt

Lecture 3 – Programming 2 Object Oriented Programming In Week 2 we covered Methods arguments, parameters return void activation stack / object heap accessing members via dot operator ( . ) This Week Constructs if, switch for, while Class development Constructors Get/Set methods Access specification Next couple of weeks . . . Inheritance Dynamic binding Abstract classes/methods Polymorphism Lecture Test ! ! ! New functionality Our program currently processes a Square, then a Rectangle and then a Circle. Let’s modify it so that the user can indicate which shape he or she wants to work with. The new program will ask the user to enter 1 - for a Square 2 - for a Rectangle 3 - for a Circle If Statement Like Python, Java has an if statement The if statement allows a choice between different execution paths It checks a boolean conditional expression (true/false) If the expression is true The if path is followed If the expression is false The else path (if it exists) is followed Unlike Python there is no elif keyword We can use else if however The modified main method System.out.print (Enter 1 for Square, 2 for Rectangle, 3 for Circle: ); int choice = scan.nextInt( ); if (choice == 1) processSquare (scan); else if (choice == 2) processRectangle (scan); else if (choice == 3) processCircle (scan); else System.out.println (Wrong number ); Notes When we run the above program only one shape will be processed – not 3 The shape processed will be determined by the user – by entering 1, 2 or 3 If the user enters something other than 1, 2 or 3 the program will give an error message More than one statement If the program is required to execute more than one statement on any particular path, they need to be enclosed in braces { } if (choice == 1) { System.out.println (“You have chosen 1”); processSquare ( ); } If the braces are omitted only the first statement will belong to the if processSquare will be called no matter what value choice has Another method - ifT