Lecture 17 Ternary Operator Switch Break Continue Last time 1 Example class development Rational Numbers Today 1 ternary operator the conditional operator 2 switch 3 break CMSC 131 Fall 2006 Jan Plane adapted from Bonnie Dorr The Conditional Operator The only ternary operator has 3 operands Format boolean expression expression1 expression2 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 true CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 1 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 2 The switch Statement General Form The control expression is one of the following types char int short byte switch control expression Our text says it cannot case case label 1 be a byte or short statement sequence 1 This is wrong break Each case label must be a value in case case label 2 type of control expression statement sequence 2 break You may have any number of statements including if else and loops case case label n statement sequence n The break statement jumps out of the switch statement break default default statement sequence The optional default case is break executed if no other case matches CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 3 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 4 The default Case default is optional If omitted and no case matches then the switch statement does nothing However you should always include a default case even if you want nothing to be done if no case matches you should never rely on implicit behavior Although cases are not required to be in order following is legal switch option case 2 case 9 default case 1 it is much better to list cases in increasing order with default last CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 5 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 6 More about break 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 process n Loop only terminates when break executed This only happens when n 0 CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 7 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 CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 8 continue Statement continue can also be used to affect loops break halts loops continue jumps to bottom of loop body Following prints even numbers between 0 and 10 for int i 0 i 10 i if i 2 1 continue System out println i continue should be avoided Effect of continue statement is to jump to bottom of loop immediately when i is odd This bypasses println Confusing Easy equivalents exist e g if else Included in Java mainly for historical reasons When you use it make sure it has a good comment explaining what is happening CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr 9
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