Lecture Set 4 Evaluation Order Today More assignment operators Precedence and shortcircuiting CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr Expressions Java phrases that yield values e g x x 1 y x y z 0 foo equals cat Expressions have values int boolean etc Expressions can be assigned to variables appear inside other expressions etc CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 1 Expressions and Side Effects Some expressions can also alter the values of variables x 1 is an expression e g x 1 Yes Value is result of evaluation right hand side of It also alters the value of x Such alterations are called side effects CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 2 1 Are the Following Legal int x y x y 1 Yes Result assigns 1 to x and to y int x 0 y 1 boolean b false if b x y x y Yes Result assigns true to b and 1 to x CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 3 Other Expressions with Side Effects Java includes abbreviations for common forms of assignment Example increment operations Basically equivalent to x x 1 x Pre increment Increments x returns the new value of x x Post increment Increments x returns the old value of x Same or Different x x x x Compare x x x x always true never true y y y y CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 4 Other Assignment Operators Example decrement operations Basically equivalent to x x 1 x Pre decrement Decrements x returns the new value of x x Post decrement Decrements x returns the old value of x General modification by constant General form var op with constant Examples x 2 equivalent to x x 2 x 2 equivalent to x x 2 x 2 equivalent to x x 2 x 2 equivalent to x x 2 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 5 2 Precedence Explains how to evaluate expressions What is value of 1 2 3 4 Precedence rules answer this question Higher precedence operators evaluated first Example from math Please Excuse my Dear Aunt Sally or PEMDAS Multiple and divide higher precedence before you add and subtract lower precedence Java follows Aunt Sally s Rules but what about other operators CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 6 Java Precedence Rules parentheses unary ops multiply divide add subtract comparisons equality logical and logical or assignments x x x x x x x only these are right to left associative CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr increasing precedence 7 Examples x y z Equivalent to x y z x y y z w z Equivalent to x y y z w z What is value of 1 2 3 4 1 2 3 4 1 2 3 4 1 2 12 1 12 11 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 8 3 Should You Rely on Precedence No The only ones people can remember are Please Excuse My Dear Aunt Sally PEMDAS Bad if 2 x 5 z 3 w x 2 Better if 2 x 5 z 3 w x 2 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 9 Short circuiting As soon as Java knows an answer it quits evaluating the expression What does Java print int x 0 y 1 if y 1 x 0 y System out println x 0 Why y 1 is false The result of will be false regardless of second expression Java therefore does not evaluate second expression of This treatment of is called short circuiting Subexpressions evaluated from left to right Evaluation stops when value of over all expression is determined CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 10 Examples What does Java print int x 0 y 1 if y 1 x 0 y System out println x 1 What does Java print int x 0 y 1 if y 1 x 0 y 1 x 0 y System out println y System out println x 0 CMSC 131 Spring 2007 1 Jan Plane adapted from Bonnie Dorr 11 4 Examples cont What does Java print int x 0 y 0 while x 4 y x System out println y 15 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 12 Programming with Side Effects Generally Side effects in conditions are hard to understand Good programming practice Conditions should be side effect free Side effects should be in stand alone statements Major Goal Strive to create the most readable and maintainable code CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 13 Primitive Types and their Hierarchy double float long int short byte int x 7 2 double y 6 Changing to something else Further Up this list is acceptable called Widening Conversion Changing to Something else Further Down this list is not acceptable Explicit casting needed for when you want a downcast called Narrowing Conversion CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 14 5 Type Casting Which of the following are legal int x 3 5 Illegal 3 5 is not an int float x 3 Legal 3 is an int which is also a float long i 3 Legal 3 is an int which is also a long byte x 155 Illlegal 155 is to big to be a byte 127 double d 3 14159F Legal 3 14159F is a float which is also a double CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 15 Mixed Expressions What is result of float x 3 4 x assigned value 0 0F Why 3 4 are ints So integer operation is used yielding 0 before upcasting is performed To get floating point result use explicit casting float x float 3 float 4 Assigns x the value 0 75F Can also do following float x float 3 4 Why float 3 returns a value type float 3 0F 4 is an int In this case Java compiler uses widening conversion on lower type here int to obtain values in same type before computing operation CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 16 6
View Full Document
Unlocking...