april 6, 1998cs102-02lecture 2-1 java operators cs 102-02 lecture 2-1 being a smooth operator

April 6, 1998 CS102-02 Lecture 2-1

Java Operators

CS 102-02

Lecture 2-1

Being a Smooth Operator


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


Kinds of OperatorsCategory What it does… ExamplesArithmetic Addition, subtraction +, -, /Assignment Set a value to an expression =, +=, &=Conditional Choose one of two values ? :Logical Logical comparisons &&, ||Relational Compare values ==, >=Bitwise Move bits within a number <<, >>


Operator Precedence

• Usually things go left-to-right, but there are precedence rules

• Nutshell reading lists operators by precedence

• Override precedence with ()’s


Arithmetic Operators

• The usual suspects: plus, minus, blah, blah, blah

• Modulo/remainder operator


Modulo Operator• Modulo (or remainder) operator: what’s left

over after division7%3 = 1198%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


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


Testing Out Short-Hand

After X Y Whole thingX++ * Y++X + Y++X / Y++

Suppose we start with:X = 7;Y = 9;

What’s the difference between:X++;++X;


Are You My Type?

• What’s the type of a result?

Expression Result typeint * int intfloat * float ??int * float ??int / int ??

• Conversion & promotion


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 Y Y = 8;

Add 1 to Y Y++;

Assign Y+10 to Y X += 10;


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.”;


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…


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)


And, But and Or will get you pretty far..

• Logical operators combine simple expressions to form complex ones

• Boolean logicExpression One Expression Two One AND Two One OR Two One XOR TwoFalse False False False FalseFalse True False True TrueTrue False False True TrueTrue True True True False


Boolean Types

• True or false are real values in Java

• Some languages just use 0 and not 0if (y = 7) then …

• In Java result of a comparison is Boolean8 != 9 ??8 != 8 ??


Logical Operators in Java

• Translating logic into JavaAND &&

OR ||

XOR ^

NOT !


Boolean Expressions

• De Morgan’s Laws with Expressions One & Two

One OR Two == One AND Two

One AND Two == One OR Two

• Some handy relationsOne XOR One == False

One OR One == True


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?


Sideline on Side Effects

• Side effects are results of expression evaluation other than the expression’s value

• ExamplesX++;

– Output:System.out.println(“Howdy!”);


Short-Circuiting Side Effects

• Short-circuiting could prevent a side effect

• How do you force the compiler to evaluate a second expression?


No Short-Circuit Here

• Guarantee that the second expression is evaluatedAND &

OR |

XOR ^

(Why is ^ listed here?)


Relational Operators

• Determine the relationship between values

• Equality & inequality

• Less than, greater than


(In)Equality

• Equality is different from assignment

== != =

• Most keyboards just have =– Use == for equality– And != for inequality


Bitwise Operators

• Computers are binary creatures: everything’s on or off

• For example, computers can’t store decimal numbers so


Binary Arithmetic

• Everything’s in powers of two

• Turn 78 into:

128 64 32 16 8 4 2 10 1 0 0 1 1 1 0

64 8 4 2


Accentuate the positive

• Computers don’t know about negative numbers

• Use the first (leftmost) bit as a sign bit:1 if negative: -5 is 11111101

0 if positive: +5 is 00000011


Bitwise is Binary

• Work with the bits inside the values

• Only good for integral values (integer numbers, bytes and characters)Operator Name Description& AND AND the corresponding bits in the two

operands| OR OR the corresponding bits in the two

operands^ XOR XOR the corresponding bits in the two

operands<< Left shift Shift the bits from right to left>> Right shift with sign

extensionShift the bits right and preserve the sign

>>> Right shift with zeroextension

Shift the bits right and always fill in 0’s

~ Complement Switch 0’s and 1’s


And Shift Your Bits ‘Round and ‘Round

• Bitwise AND of 78 and 34

128 64 32 16 8 4 2 178 0 1 0 0 1 1 1 034 0 0 1 0 0 0 1 02 0 0 0 0 0 0 1 0


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


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

april 6, 1998cs102-02lecture 2-1 java operators cs 102-02 lecture 2-1 being a smooth operator

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