Java编程入门-英语教材

The keyword for A group of three expressions enclosed in brackets
The first expression is the initialiser It is called only once – when the for loop is entered We can declare a new variable at the same time Furthermore, we can give it a value So we create an int variable called i, and initialise it to 0
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
Like Python, Java provides two different looping structures (there are others but we won’t cover them)
for loop
- definite loop
while loop - indefinite loop
Again to avoid cluttering up the main method we’ll put the new code into a method called ifTest
This method will be void (it won’t return a value) and will take one parameter (of class Scanner)
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)
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
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
CODE DEMONSTRATING WHILE LOOP
void whileTest (Scanner scan) { int i = 0; while (i < 5) { ifTest(scan); i++; }
}
Note that the loop consists of three major elements
It then examines a list of cases
The case that matches (has the same value) as the expression is chosen and the code associated with it is executed.
The code on the next slide shows the code in ifTest being rewritten as a switch statement
METHOD TO TEST THE FOR LOOP
void forTest (Scanner scan) { for (int i = 0; i < 5; i++) { ifTest(scan); }
}
Note that the first expression declares i and intialises it at the same time
The loop iterates The expression is evaluated again
The loop will continue to iterate until the expression evaluates to false
Be careful of infinite loops – usually a bad thing
}
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 - IFTEST
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
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.
A statement that initialises the counter variable (i) An expression that tests whether i is less than 5 A statement to add 1 to i – otherwise infinite loop (why?)
The second expression is a conditional expression The for loop will iterate only if this expression is true
The third expression updates the for loop counter (i) 1 is added to i after each iteration
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 ( . )
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
SWITCH STATEMENT
Not covered in Programming 1 (so pay attention☺)
A decision construct like the if statement
It evaluates an int (or byte, char, short, String) expression
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 ( );
The code for this method is shown on the next slide
The code for the modified main method will be shown on the slide after thatIFTEST CODE
void ifTest (Scanner scan) {
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)