Download Java Expressions and Operator Precedence - Prof. Bonnie J. Dorr and more Assignments Computer Science in PDF only on Docsity!
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr)
Lecture Set 4:
Evaluation Order
Today:
More assignment operators
Precedence and short-
circuiting
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr) 1
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.
Jan Plane (adapted from Bonnie Dorr)^ CMSC 131 Spring 2007^2
Expressions and Side Effects
Some expressions can also alter the values
of variables
e.g. x=
x=1 is an expression?
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) 3
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) 4
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” In crements x, returns the old value of x Same or Different
x == x++
x == ++x
Compare x++ * y++ ++x * ++y ++x * y++ x++ * ++y
always true
never true
Jan Plane (adapted from Bonnie Dorr)^ CMSC 131 Spring 2007^5
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” De crements x, returns the old value of x General modification by constant General form: <op with=> Examples x += 2 equivalent to x = x+ x -= 2 equivalent to x = x- x = 2 equivalent to x = x x /= 2 equivalent to x = x/
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr) 9
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) 10
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
Jan Plane (adapted from Bonnie Dorr)^ CMSC 131 Spring 2007^11
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 1
CMSC 131 Spring 2007 Jan Plane (adapted from Bonnie Dorr) 12
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) 13
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.
Jan Plane (adapted from Bonnie Dorr)^ CMSC 131 Spring 2007^14
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 called “Narrowing Conversion” Explicit casting needed for when you want a downcast
