CSC 212 – Data Structures Lecture 12: Java Review

Fields Fields defined in class, but not in method  Instances share fields; have different values Definition must also include a data type  Can be primitive or reference type  Fields behave like variables of that type would Fields should be private or protected  In general, should be private  protected allows subclasses to access field

javadoc Example /** This class represents individual cars that people drive. */ public class Car { /** * Name of company that made the car and name of the car model */ String makeAndModel; /** Color of the car. */ String color; /** How full the gas tank is, expressed as * a percentage. */ float tankLevel; /** Number of miles recorded on the odometer */ int odometerReading;

javadoc Example /** Reset the fuel tank to be full. */ public void fillTank() { tankLevel = 1.0; } /** * Change the amount of fuel by some means levelDelta Change in fuel level (as % of tank capacity) */ private void adjustFuel(float levelDelta) { tankLevel += levelAdded; /* Check that the tank level makes sense. */ if (tankLevel > 1.0) { tankLevel = 1.0; } else if (tankLevel < 0.0) { tankLevel = 0.0; } }

/** * Drive around town and look for the hot spots to hit. distance Number of miles traveled gasUsed Amount of fuel used on this trip */ public void crusin(int distance, float gasUsed) { int newDistance = odometerReading + distance; adjustFuel(gasUsed); odometerReading = newDistance; } /** * Find out if the car will start. Assumes that you will not hotwire the car. haveKey True if we the key to this car; else it is false True if the engine would start, false otherwise */ public boolean willStart(boolean haveKey) { if (!haveKey || tankLevel == 0.0) { return false; } else if (makeAndModel.startsWith(“Jaguar”)) { java.util.Random rnd = new Random(); return rnd.nextBoolean(); } else { return true; } }

Visibility Modifiers Going to class naked is common nightmare  Try keeping certain details private  Want control over our state  Fields represent state of an object Fields normally declared private private int fieldName; Only access the field from class methods Use protected when subclass also accesses

Accessor Methods Field used in other classes’ methods  Done via accessor methods (“getters”) Getter usually named getField public field_type getField() {... }  Other classes must use accessor method  Includes code that can limit access to field  Do not copy code to each field use!

Mutator Methods Set field’s value in other classes’ methods  Use mutator methods (“setters”) for this Setter usually named setField public void setField(field_type param) {... }  Changes to field normally via mutators  Check that only legal values assigned to field  Makes changing or updating checks easier

Visibility of Methods Methods normally act on or with object  Use active verb as name  Declare as public Replace multiple copies of code with single method  Perform common test, update, or calculation  These should be private or protected  Simplifies using and editing the class

Fields Each class instance has fields  Field values in separate instances may * differ  Static fields shared by all instances Objects’ fields initialized at instantiation  Initial value is 0, false, ‘’, or null Should define as few fields as possible  Do not include fields that can be computed  Unneeded fields makes coding harder

Parameters Variables named in method description  Created & initialized each time method called  “Live” until method completes Initialized with value of argument  Primitives  copy value into parameter  References  parameter aliases argument Documented as part of method comments  Done paramName paramDescript

Locals Variables declared inside a method  Required to assigned value before using  “Live” only for code block in which declared Cannot be included in javadoc  Local variables help a method complete  Only comment as part of describing how method works

Tracing Parameters public void badTransfer(BankAccount source, float amount) { if (amount >= source.balance) { amount = source.balance(); source = null; } else { source.balance -= amount; } balance += amount; } check.badTransfer(richOne, ); owner balance  0.05 identifier  “2408” check haxx0r owner balance  identifier  “2408” richOne this source amount  balance  

Tracing Parameters public void badTransfer(BankAccount source, float amount) { if (amount >= source.balance) { amount = source.balance(); source = null; } else { source.balance -= amount; } balance += amount; } check.badTransfer(richOne, Float.MAX_VALUE); owner balance  identifier  “2408” check haxx0r owner balance  identifier  “2408” richOne this source amount  ~3.4·10 38  

Inherited Fields & Methods Subclass inherits superclass’s methods and fields  Non-private fields and methods used as if in subclass Can reuse names in subclasses  Fields are hidden  Methods are overriden Method called according to instance type Fields accessed by variable type

this & super Keywords this keyword is rarely used in Java  Access field with same name as parameter or local  From constructor call another defined in class Usage of super within constructor  Must be first statement in constructor  Calls constructor defined in superclass Usage of super elsewhere in class  Call overriden method as defined in superclass: super.methodName(…);  Use hidden field as defined in superclass: super.fieldName;

Before Next Lecture… Start week #5 assignment Continue programming assignment #1  It is due Friday Start reviewing for Midterm #1  It will be 1 week from today Wednesday’s lecture talks about linked lists  Read Sections of the book