April 6, 1998CS102-02Lecture 2-1 Java Operators CS Lecture 2-1 Being a Smooth Operator

April 6, 1998CS102-02Lecture 2-1 Operating with Java Most programming languages have operators –Operators are short-hand symbols for actions = Assign right to left + Add two numbers (or concatenate two strings) –Operators in Java have fixed meaning No operator overloading Can’t say: List = List + Item;// Add item to list

April 6, 1998CS102-02Lecture 2-1 Kinds of Operators

April 6, 1998CS102-02Lecture 2-1 Operator Precedence Usually things go left-to-right, but there are precedence rules Nutshell reading lists operators by precedence Override precedence with ()’s

April 6, 1998CS102-02Lecture 2-1 Arithmetic Operators The usual suspects: plus, minus, blah, blah, blah Modulo/remainder operator

April 6, 1998CS102-02Lecture 2-1 Modulo Operator Modulo (or remainder) operator: what’s left over after division 7%3 = 1 198%3 = ?? 6.0%4.0 = 2 Is it odd or even? Looping with clock arithmetic –Appointment at 5pm everyday –Baking 217 cakes: step 3 of 7 same as 24 of 28

April 6, 1998CS102-02Lecture 2-1 Short-Hand Operators Increment and decrement: ++ and -- –Often need to add or subtract 1 –Pre: Add (subtract) first –Post: Add (subtract) afterwards Compiler can sometimes optimize

April 6, 1998CS102-02Lecture 2-1 Testing Out Short-Hand Suppose we start with: X = 7; Y = 9; What’s the difference between: X++; ++X;

April 6, 1998CS102-02Lecture 2-1 Are You My Type? What’s the type of a result? ExpressionResult type int * int int float * float?? int * float?? int / int?? Conversion & promotion

April 6, 1998CS102-02Lecture 2-1 Assignment Operators Change the value on the left to the value of the expression on the right If you want to:Try: Assign 8 to YY = 8; Add 1 to YY++; Assign Y+10 to YX += 10;

April 6, 1998CS102-02Lecture 2-1 Works for Strings Too Strings are “added” (concatenated) with + What is Name after the third line? Name = “Simpson”; First = “Lisa”; Name += First; What’s the result here? Age = 11; Message = “He’s “ + Age + “ years old.”;

April 6, 1998CS102-02Lecture 2-1 Conditional Operator Instead of If..Then..Else, use ?: Takes three arguments in the form: Boolean condition ? If-true : If-false If (Simpson == “Lisa”) { Message = “She’s our favorite!”; } else { Message= “Doh!”; } System.out.println(Message); is the same as…

April 6, 1998CS102-02Lecture 2-1 Using the Conditional Operator System.out.println(Simpson==“Lisa” ? ”She’s our favorite” :“Doh!”); (The above should be on one line in a real program)

April 6, 1998CS102-02Lecture 2-1 And, But and Or will get you pretty far.. Logical operators combine simple expressions to form complex ones Boolean logic

April 6, 1998CS102-02Lecture 2-1 Boolean Types True or false are real values in Java Some languages just use 0 and not 0 if (y = 7) then … In Java result of a comparison is Boolean 8 != 9?? 8 != 8??

April 6, 1998CS102-02Lecture 2-1 Logical Operators in Java Translating logic into Java AND&& OR|| XOR^ NOT!

April 6, 1998CS102-02Lecture 2-1 Boolean Expressions De Morgan’s Laws with Expressions One & Two One OR Two == One AND Two One AND Two == One OR Two Some handy relations One XOR One == False One OR One == True

April 6, 1998CS102-02Lecture 2-1 Short-Circuit Remember: False AND Anything == False True OR Anything == True Sometimes compiler can short-circuit and skip evaluation of second expression What if there are side effects?

April 6, 1998CS102-02Lecture 2-1 Sideline on Side Effects Side effects are results of expression evaluation other than the expression’s value Examples X++; –Output: System.out.println(“Howdy!”);

April 6, 1998CS102-02Lecture 2-1 Short-Circuiting Side Effects Short-circuiting could prevent a side effect How do you force the compiler to evaluate a second expression?

April 6, 1998CS102-02Lecture 2-1 No Short-Circuit Here Guarantee that the second expression is evaluated AND& OR| XOR^ (Why is ^ listed here?)

April 6, 1998CS102-02Lecture 2-1 Relational Operators Determine the relationship between values Equality & inequality Less than, greater than

April 6, 1998CS102-02Lecture 2-1 (In)Equality Equality is different from assignment == != = Most keyboards just have = –Use == for equality –And != for inequality

April 6, 1998CS102-02Lecture 2-1 Bitwise Operators Computers are binary creatures: everything’s on or off For example, computers can’t store decimal numbers so

April 6, 1998CS102-02Lecture 2-1 Binary Arithmetic Everything’s in powers of two Turn 78 into:

April 6, 1998CS102-02Lecture 2-1 Accentuate the positive Computers don’t know about negative numbers Use the first (leftmost) bit as a sign bit: 1 if negative:-5 is if positive:+5 is

April 6, 1998CS102-02Lecture 2-1 Bitwise is Binary Work with the bits inside the values Only good for integral values (integer numbers, bytes and characters)

April 6, 1998CS102-02Lecture 2-1 And Shift Your Bits ‘Round and ‘Round Bitwise AND of 78 and 34

April 6, 1998CS102-02Lecture 2-1 Why Bother with Bitwise? Use numbers not for themselves but for their internal representations Example: A tic-tac-toe grid might have 0’s for O’s and 1’s for X’s Just need 9 bits to do the whole table and only 27 bits for 3-D tic-tac-toe

April 6, 1998CS102-02Lecture 2-1 That’s It for Operators Operators are key to building large expressions in Java Know operator precedence (or at least where to look it up) Next time: Use operators to build expressions for control structures