Lecture 07-Inside-Out, Super-This

Lecture - CS1110 21 Sept 2010 Inside-out rule use of this super Developing methods(using Strings. You can do A3 in groups of 2, Read sec

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Unformatted text preview: CS1110 21 Sept 2010 Inside-out rule; use of this, super Developing methods (using Strings). You can do A3 in groups of 2, Read sec. 2.5, stepwise refinement BUT GROUP EARLY ON CMS Listen to Plive, 2.5.1–2.5.4. Reading for next lecture: the same Office hours are being held Today: Pick up: A3 Today’s slides A1 A3: Adding functionality to A1 • Keeping class invariant true • Use already-written functions • Boolean expressions • Use of null and testing for it • Use of static variables Due Wednesday, 29 September Rsrecah on spleilng Aoccdrnig to a rscheearch at Cmabirgde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is that the frsit and lsat ltteer be at the rghit pclae. The rset can be a total mses and you can sitll raed it wouthit porbelm.Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe. 1 2 Remember frame boxes and figuring out variable references? The inside-out rule (see p. 83) Code in a construct can reference any of the names declared or defined in that construct, as well as names that appear in enclosing constructs. (If a name is declared twice, the closer one prevails.) PersonPop a0 name Person a1 name Person Method parameters participate in the inside-out rule: remember the frame. a0 name Person a1 name Doesn’t work right Person setName(String n) { name= n; } setName(String name) { name= name; } getNameAndPop() { return name + PersonPop; } getNameAndPop() { return name + PersonPop; } File drawer for class Person 3 Parameter n would be found in the frame for the method call. Parameter name “blocks” the reference to the field name. 4 A solution: this and super In folder a1, this refers to a1 a1 About super Within an object, this evaluates to the name of the object. Within a subclass object, super refers to the partition above the one that contains super. In folder a0,  this refers to a0 PersonPop a1 Person name Object method equals() method toString() Person Elephant toString() { … } a0 name setName(String name) { this.name= name; } setName(String name) { this.name= name; } File drawer for class Person 5 Because of the keyword super, this calls toString in the Object partition. otherMethod { … … super.toString() … } 6 1 Strings are (important) objects that come with useful methods. String s= "abc d"; Note the “index (number) from 0” scheme: Principles and strategies embodied in stepwise refinement Develop algorithm step by step, using principles and strategies embodied in “stepwise refinement” or “top-down programming. READ Sec. 2.5 and Plive p. 2-5. • Take small steps. Do a little at a time • Refine. Replace an English statement (what to do) by a sequence of statements to do it (how to do it). • Refine. Introduce a local variable —but only with a reason • Compile often • Intersperse programming and testing • Write a method specification —before writing its body • Separate concerns: focus on one issue at a time • Mañana principle: next slide 8 Text pp. 175–181 discusses Strings Look in CD ProgramLive Look at API specs for String s.length() is 5 (number of chars) s.charAt(2) is 'c' (char at index 2) s.substring(2,4) is "c " (NOT "c d") s.substring(2) is "c d" " bcd ".trim() is "bcd" (trim beginning and ending blanks) s.indexOf(s1) –index or position of first occurrence of s1 in s (-1 if none) 01234 abc d DO NOT USE == TO TEST  STRING EQUALITY! s == t tests whether s and t contain the name of the same object, not whether the objects contain the same string. Use s.equals(t) 7 Principles and strategies • Mañana Principle. During programming, you may see the need for a new method. A good way to proceed in many cases is to: 1. Write the specification of the method. 2. Write just enough of the body so that the program can be compiled and so that the method body does something reasonable, but no the complete task. So you put off completing this method until another time —mañana (tomorrow) —but you have a good spec for it. 3. Return to what you were doing and continue developing at that place, presumably writing a call on the method that was just “stubbed in”, as we say. Anglicizing an Integer anglicize(1) is "one"! anglicize(15) is "fifteen"! anglicize(123) is "one hundred twenty three"! anglicize(10570) is "ten thousand five hundred seventy"! /** = the anglicization of n. Precondition: 0 < n < 1,000,000 */ public static String anglicize(int n) { } We develop this function, in DrJava, using the principles and strategies of stepwise refinement (also called topdown programming). 10 9 2 ...
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