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CSE 8A Lecture 18

CSE 8A Lecture 18. Reading for next class: 11.6-11.8 Today’s goals: More practice with designing classes Tracing code and creating memory models PSA 9 (classes) due next Monday (12/3) Individual (no partner) PSA8 Interview due tonight

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CSE 8A Lecture 18

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  1. CSE 8A Lecture 18 • Reading for next class: 11.6-11.8 • Today’s goals: • More practice with designing classes • Tracing code and creating memory models • PSA 9 (classes) due next Monday (12/3) • Individual (no partner) • PSA8 Interview due tonight • WIC event: Yahoo! Demos. Immediately after class in the Qualcomm conference room in Jacobs Hall.

  2. Solo: (60 sec) • Discuss: (2 min) • Group: (20 sec) A Point class The Point of Java: objects and classes public class Point { private int x; private int y; public Point(intx_in, inty_in) { this.x = x_in; this.y = y_in; } public static void main( String[] args ) { Point r = new Point(10, 20); Point p = new Point(3, r.x); Point q = p; } Draw a picture of the memory model at the end of main (vote on next slide)

  3. x 10 x 10 r r y 20 y 20 A. C. x 3 x 3 p p y 10 y q x 3 q y 10 x 10 r D. None of these E. I don’t know y 20 B. x 3 p y 10 q

  4. Solo: (60 sec) • Discuss: (2 min) • Group: (20 sec) A Point class The Point of Java: objects and classes public class Point { private int x; private int y; public Point(intx_in, inty_in) { this.x = x_in; this.y = y_in; } public static void main( String[] args ) { Point r = new Point(10, 20); Point p = new Point(3, r.x); Point q = p; r.x = 1; q.y = r.x + p.x; } What are the values of r, p, and q when this code completes? r p q A. (1, 20) (3, 1) (3, 4) B. (1, 20) (1, 1) (3, 2) C. (10, 20) (3, 13) (3, 13) D. (1, 20) (1, 13) (1, 13) E. None of these

  5. Class, Field, or Method? • A class is a type of thing • A field (instance variable) is a property, that might possibly have different values at different times • A method is an action that can be performed • So usually class and field names are nouns, method names are verbs…

  6. Another example of a class • Class: Species • Instance Variables: • name, a String • population on 7 continents, an array of 7 ints • growthRate, a double • Constructor: one that takes no arguments, and: • initializes name to “No Name Yet” • initializes population to all 0 • Initializes growthRate to 33.3

  7. Solo: (60 sec) • Discuss: (2 min) • Group: (20 sec) How many errors are there in this code (and what are they) • 2 • 3 • 4 • 5 • >=6 public class Species private String name; { public static void main(String[] args) { double[] population; double growthRate; } public Species() { String name = "No Name Yet"; double[] population = {0,0,0,0,0,0,0}; growthRate = 33.3; } }

  8. How many errors are there in this code (and what are they) public class Species { private String name; { public static void main(String[] args) { privateint[] population; private double growthRate; } public Species() { String name = "No Name Yet"; int[] population = {0,0,0,0,0,0,0}; growthRate = 33.3; } } All instance variables and methods have to go inside the class { } All instance variables have to go outside of method definitions Instance variables should be private Refer to instance variables in a constructor; do not redeclare them

  9. Visibility of Instance Variables • Class design rule of thumb: make all instance variables private • “private” means: visible only inside this class • So a private instance variable or instance method cannot be seen from outside the class • Making an instance variable private prevents incorrectly setting its value by malicious or careless users of the class

  10. Private instance variables in Species public class Species { ///////// fields //////////// private String name; private int[] population; private double growthRate; /////// constructors /////////// public Species() { String name = "No Name Yet"; population = {0,0,0,0,0,0,0}; growthRate = 33.3; } /////// methods //////////////// }

  11. Getter and Setter methods • Q: Instance variables correspond to properties of an object… if they are private and hidden inside, how can they interact with other objects? • A: Define public instance methods which give controlled, safe access to the private instance variables • If the method can change an instance variable, it is a “mutator” or “modifier” or “setter” method • If it only returns the value of an instance variable, it is an “accessor” or “getter” method

  12. Solo: (45 sec) • Discuss: (2 min) • Group: (20 sec) Which of following would you select for “getter” method signatures for Species class? public void getName(); public void getPopulation(); public void getGrowthRate(); public String getName(); public int[] getPopulation(); public double getGrowthRate(); public void getName(String newName); public void getPopulation(intnewPop); public void getGrowthRate(double newGrowthRate); private String getName(); private int[] getPopulation(); private double getGrowthRate();

  13. Solo: (45 sec) • Discuss: (2 min) • Group: (20 sec) Which of following would you select for “setter” method declarations for Species class? public void setName(); public void setPopulation(); public void setGrowthRate(); public String setName(); public int[] setPopulation(); public double setGrowthRate(); public void setName(String newName); public void setPopulation(int[] newPop); public void setGrowthRate(double newGrowthRate); public void setName(String newName); public booleansetPopulation(int[] newPop); public void setGrowthRate(double newGrowthRate);

  14. Solo: (30 sec) • Discuss: (2 min) • Group: (20 sec) Return type for Setters • A getter method should have a non-void return type • A setter can be designed in several ways: • void: just change the values of the instance variable(s), don’t return anything • boolean: return true if the setting was successful and false if not (for example if setting would be ‘illegal’) • The type of the value that is being changed: return the previous value

  15. Solo: (45 sec) • Discuss: (2 min) • Group: (20 sec) Overloading: Which are legal overloads? • 1 • 2 • 3 • 1 and 3 • 1 and 2 public Species() public Species(String newName); public booleansetGrowthRate(double gr) public void setGrowthRate(double gr) public void setPopulation( intnorthAmerica, intsouthAmerica, inteurope, intasia, intafrica, intaustralia, intantarctica) public void setPopulation(int[] a)

  16. Solo: (45 sec) • Discuss: (2 min) • Group: (20 sec) Terminology Check • Declaration • Instantiation • Initialization foo = new double[5]; for (inti = 0; i < foo.length; i++) { foo[i] = -11.5; } double [] foo;

  17. Draw a memory model for this code: public Species(String newName, int[] newPop, double newGR) { name = newName; population = new int[newPop.length]; for (inti=0; i< this.population.length;i++) population[i] = newPop[i]; growthRate = newGR; }

  18. TODO • Reading for next class: 11.6-11.8 • Interview for PSA8 (if you haven’t done it already) • Finish PSA9 • Do this one individually (no team programming)

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