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Java Operators

Java Operators. Being a Smooth Operator. CS 102-02 Lecture 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

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Java Operators

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  1. Java Operators Being a Smooth Operator CS 102-02 Lecture 2-1

  2. 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

  3. Kinds of Operators

  4. Operator Precedence • Usually things go left-to-right, but there are precedence rules • Nutshell reading lists operators by precedence • Override precedence with ()’s

  5. Arithmetic Operators • The usual suspects: plus, minus, blah, blah, blah • Modulo/remainder operator

  6. 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

  7. 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

  8. Testing Out Short-Hand • Suppose we start with: • X = 7; • Y = 9; What’s the difference between: X++; ++X;

  9. Are You My Type? • What’s the type of a result? Expression Result type int * int int float * float ?? int * float ?? int / int ?? • Conversion & promotion

  10. 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;

  11. 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.”;

  12. 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…

  13. 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)

  14. And, But and Or will get you pretty far.. • Logical operators combine simple expressions to form complex ones • Boolean logic

  15. 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 ??

  16. Logical Operators in Java • Translating logic into Java AND && OR || XOR ^ NOT !

  17. 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

  18. 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?

  19. Sideline on Side Effects • Side effects are results of expression evaluation other than the expression’s value • Examples X++; • Output: System.out.println(“Howdy!”);

  20. Short-Circuiting Side Effects • Short-circuiting could prevent a side effect • How do you force the compiler to evaluate a second expression?

  21. No Short-Circuit Here • Guarantee that the second expression is evaluated AND & OR | XOR ^ (Why is ^ listed here?)

  22. Relational Operators • Determine the relationship between values • Equality & inequality • Less than, greater than

  23. (In)Equality • Equality is different from assignment == != = • Most keyboards just have = • Use == for equality • And != for inequality

  24. Bitwise Operators • Computers are binary creatures: everything’s on or off • For example, computers can’t store decimal numbers so

  25. Binary Arithmetic • Everything’s in powers of two • Turn 78 into:

  26. 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

  27. Bitwise is Binary • Work with the bits inside the values • Only good for integral values (integer numbers, bytes and characters)

  28. And Shift Your Bits ‘Round and ‘Round • Bitwise AND of 78 and 34

  29. 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

  30. 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

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