ITEC 120 2010fall ibarland

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lect13b
passing references by value

Tomorrow, (a) bring a camera to lab, and (b) don't wear green. (I'll try to get a green-screen, and next week you can use that image to superimpose yourself on another image [of the exact same dimensions].)

You can't write a function to swap your own local variables

  void tryAndSwap( int a, int b ) {
    int tmp;
    tmp = a;
    a = b;     // Assigning to a parameter -- bad style!
    b = tmp;   // Assigning to a parameter -- bad style!
    }

int myFavoriteInt = 5;
int yourFavoriteInt = 23;

tryAndSwap( myFavoriteInt, yourFavoriteInt );

myFavoriteInt == 5     // True, or false?
yourFavoriteInt == 5   // True, or false?

...but you can write a function to swap the shapes of two Robots

  // In class Robot:

  public static void swapRobotShape( Robot r1, Robot r2 ) {
    // What to put here?




    }



// From the code pad, say:
//
Robot optimus = new Robot( "X47", "Daisy", false, false );
Robot optimusPrime = new Robot( "X47'", "Mack Truck", false, false );

optimus.toString();
Robot.swapRobotShape(optimus, optimusPrime);
optimus.toString();

We say: “Java always passes arguments by value”: it takes the actual argument's value, and uses (a copy of) that value to initalize the parameter. The thing to realize is that (in java) “references are values”.

Some languages allow something different: they let you “pass arguments by reference”, a special language-feature that would let you write something like swapInts(int,int). However, this has fallen out of favor recently, and is commonly viewed as something which can add obscure bugs without really giving the programmer more power. Just write methods which return a (reference to) the new items, and assign your variables explicitly.

• passing params: swapping locals, vs. trying to have a method void swap(int,int), vs. having a method void swapShapes(Robot,Robot).
  A function can never modify your local variable. However, it might modify a field of an object your local variable refers to.
• Discuss the Hideout lab:
  
  • To call a method, “object.methodName(args…)” or
    to call a static method, “ClassName.methodName(args…)”.

    So as we're calling toString, we might write this.pres.toString(), which fits that syntax.
    We might also write this.crew[i].toString().

  • How to write toString.
  • How to write deepEquals?
    Why doesn't java.util.Arrays#equals work?
  • How to write elect(int)? (Tmp variables already discuss; skip.)
  • How to write coolestRobot?
  • How to write holdElection()? Unfortuatnely, we can't use elect directly. For you to figure out the exact re-factoring.
  • How to write isInRevolt()?
• static fields (non-final)

home—info—lects—labs—exams—hws
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Object120 + its docs—java.lang docs—java.util docs

©2010, Ian Barland, Radford University Last modified 2010.Dec.01 (Wed)

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