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CMSC 131 Fall 2006 Jan Plane (adapted from Bonnie Dorr)
Lecture Set #12:
Ternary Operator,
Switch, Break, Continue
1. Method Overloading Warning
2. ternary operator: The ?:
(conditional operator)
3. switch
4. break/continue
Method Overloading
prototype:
public static void f(int x, float y)
signature:
f(int , float )
You can only overload methods if they have
different signatures.
Implicit widening conversions
Beware of subtle problems with widening
conversions
CMSC 131 Fall 2007 Jan Plane (adapted from Bonnie Dorr)
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CMSC 131 Fall 2007 Jan Plane (adapted from Bonnie Dorr)
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The Conditional Operator
The only ternary operator (has 3 operands)
Format:
boolean-expression?expression1:expression
Purpose:
test to see if boolean-expression is true or false
whole expression takes on the value of expression1 when
boolean-expression was true
whole expression takes on the value of expression2 when
boolean-expression was false
See examples
CMSC 131 Fall 2007 Jan Plane (adapted from Bonnie Dorr)
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What is another way to write
this if-else-if statement?
if (grade == ‘A’)
System.out.println (“I’m very happy”);
else if (grade == ‘B’)
System.out.println (“I’m relatively happy”);
else if (grade == ‘C’)
System.out.println (“At least I get credit”);
else
System.out.println (“Check with the professor”);
CMSC 131 Fall 2007 Jan Plane (adapted from Bonnie Dorr)
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Case Continuation
The control expression can have one of the following types: char, int, short, byte not float, double, boolean, long not a String or other object Case continuation also called “cascading case behavior”, “falling through to the next case”, etc.
It is occasionally handy for combining of cases e.g. case-insensitivity switch (grade) { case ‘a’: case ‘A’: System.out.println (“I’m very happy”); break; … }
Be very careful about using this cascading behavior!
Always insert break statements after every case Then remove ones you do not want
CMSC 131 Fall 2007 Jan Plane (adapted from Bonnie Dorr)
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Why Use switch?
switch can also be implemented using if-else
switch also restricted in terms of data types in
control statements
Including break statements is a pain
However
switch often more efficient (compiler generates better
code)
Code can be more compact because of case-continuation
behavior
Sometimes case analysis is clearer using switch
CMSC 131 Fall 2007 Jan Plane (adapted from Bonnie Dorr)
8
More about break for loops
break can also be used to exit immediately from any loop
while do-while for
e.g. “Read numbers from input until negative number encountered” Scanner sc = new Scanner (System.in); int n; while (true) { n = sc.nextInt (); if (n < 0) break; else ; } Loop only terminates when break executed This only happens when n < 0
CMSC 131 Fall 2007 Jan Plane (adapted from Bonnie Dorr)
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Warning about break
Undisciplined use of break can make loops
impossible to understand
Termination of loops without break can be understood
purely by looking while, for parts
When break included, arbitrary termination behavior can be
introduced
Rule of thumb: use break only when loop condition
is always true (i.e. break is only way to terminate
loop)
When you use it, make sure it has a good comment
explaining what is happening
