1. Copyright 2009 by Pearson Education Building Java Programs Chapter 8: Classes Lecture 8-3: More Critters, static

2. Copyright 2009 by Pearson Education 2 Encapsulation encapsulation: Hiding implementation details of an object from its clients. Encapsulation provides abstraction. separates external view (behavior) from internal view (state) Encapsulation protects the integrity of an object's data.

3. Copyright 2009 by Pearson Education 3 Private fields A field can be declared private. No code outside the class can access or change it. private type name; Examples: private int id; private String name; Client code sees an error when accessing private fields: PointMain.java:11: x has private access in Point System.out.println("p1 is (" + p1.x + ", " + p1.y + ")"); ^

4. Copyright 2009 by Pearson Education 4 Benefits of encapsulation Provides abstraction between an object and its clients. Protects an object from unwanted access by clients. A bank app forbids a client to change an Account 's balance. Allows you to change the class implementation. Point could be rewritten to use polar coordinates (radius r, angle θ), but with the same methods. Allows you to constrain objects' state (invariants). Example: A Hero’ s hitPoint s never goes below 0.

5. Copyright 2009 by Pearson Education Homework 8: Critters

6. Copyright 2009 by Pearson Education 6 How the simulator works When you press "Go", the simulator enters a loop: move each animal once ( getMove ), in random order if the animal has moved onto an occupied square, fight ! if the animal has moved onto food, ask it if it wants to eat Key concept: The simulator is in control, NOT your animal. Example: getMove can return only one move at a time. getMove can't use loops to return a sequence of moves. Your animal must keep state (as fields) so that it can make a single move, and know what moves to make later.

7. Copyright 2009 by Pearson Education 7 Ideas for state You must not only have the right state, but update that state properly when relevant actions occur. Counting is helpful: How many total moves has this animal made? How many times has it eaten? Fought? Remembering recent actions in fields is helpful: Which direction did the animal move last? How many times has it moved that way? Did the animal eat the last time it was asked? How many steps has the animal taken since last eating? How many fights has the animal been in since last eating?

8. Copyright 2009 by Pearson Education 8 Testing critters Focus on one specific Critter of one specific type Only spawn 1 of each Critter type Make sure your fields update properly Use println statements to see field values Look at the behavior one step at a time Use "Tick" rather than "Go"

9. Copyright 2009 by Pearson Education 9 A complex Critter: Snake MethodBehavior eat Never eats fight Randomly choose to roar or pounce getColor (red=20, green=50, blue=128) getMove 1 E, 1 S; 2 W, 1 S; 3 E, 1 S; 4 W, 1 S; 5 E,... toString Always returns "S"

10. Copyright 2009 by Pearson Education 10 Determining necessary fields Information required to decide what move to make? Direction to go in Length of current cycle Number of moves made in current cycle Information required to decide how to fight? A Random object

11. Copyright 2009 by Pearson Education 11 Snake solution import java.awt.*; // for Color import java.util.*; // for Random public class Snake extends Critter { private int length; // # steps in current horizontal cycle private int step; // # of cycle's steps already taken private Random rand; // for fighting public Snake() { length = 1; step = 0; rand = new Random(); } public Direction getMove() { step++; if (step > length) { // cycle was just completed length++; step = 0; return Direction.SOUTH; } else if (length % 2 == 1) { return Direction.EAST; } else { return Direction.WEST; }...

12. Copyright 2009 by Pearson Education 12 Snake solution 2... public Attack fight(String opponent) { int attack = rand.nextInt(2); if (attack == 0) { return Attack.POUNCE; } else { return Attack.ROAR; } public String toString() { return "S"; } public Color getColor() { return new Color(20, 50, 128); } // We don't need to write an eat method; // We can just keep the default eat behavior (returning false) }

13. Copyright 2009 by Pearson Education Static fields and methods

14. Copyright 2009 by Pearson Education 14 Critter: Drunken Frat Guy All DFG Critters are trying to get to the same party The party is at a randomly-generated location On a 60px wide by 50px tall world They stumble north then east until they reach the party

15. Copyright 2009 by Pearson Education 15 DFG: a flawed solution import java.util.*; public class DrunkenFratGuy extends Critter { private int partyX; private int partyY; public DrunkenFratGuy() { Random r = new Random(); partyX = r.nextInt(60); partyY = r.nextInt(50); } public Direction getMove() { if(partyY != getY()) { return Direction.NORTH; } else if(partyX != getX()) { return Direction.EAST; } else { return Direction.CENTER; }

16. Copyright 2009 by Pearson Education 16 DFG: Where did they all go? Each DFG is heading to its own party! We need a way for Critters of a type to share information Tournament-winning Huskies do this Hunt in packs Don't kill each other Share location of opponents

17. Copyright 2009 by Pearson Education 17 Static fields vs. fields static: Part of a class, rather than part of an object. A single static field is shared by all objects of that class static field, general syntax: private static type name ; or, private static type name = value ; Example: private static int count = 0;

18. Copyright 2009 by Pearson Education 18 Static field example Count the number of Husky objects created: public class Husky implements Critter { // count of Huskies created so far private static int objectCount = 0; private int number; // each Husky has a number public Husky() { objectCount++; number = objectCount; }... public String toString() { return "I am Husky #" + number + "out of " + objectCount; }

19. Copyright 2009 by Pearson Education 19 Static methods static method: part of a class, not part of an object. shared by all objects of that class does not understand the implicit parameter; therefore, cannot access fields directly if public, can be called from inside or outside the class Declaration syntax: (same as we have seen before) public static return type name ( params ) { statements ; } Methods that do not access/modify the object’s state should be made static

20. Copyright 2009 by Pearson Education 20 Static method example 1 Java's built-in Math class has code that looks like this: public class Math {... public static int abs(int a) { if (a >= 0) { return a; } else { return -a; } public static int max(int a, int b) { if (a >= b) { return a; } else { return b; }

21. Copyright 2009 by Pearson Education 21 Calling static methods, outside Static method call syntax (outside the class): class name. method name ( values ); This is the syntax client code uses to call a static method. Examples: int absVal = Math.max(5, 7); double[] a1 = {2.4, 2.5, 60.3, -4.1}; double[] a2 = Arrays.copyOf(a1, a1.length);

22. Copyright 2009 by Pearson Education 22 Calling static methods, inside Static method call syntax (inside the class): method name ( values ); This is the syntax the class uses to call its own static method. Example: public class Math { // other methods such as ceil, floor, abs, etc. //... public static int round(double d) { if (d - (int) d >= 0.5) { return ceil(d); } else { return floor(d); }

23. Copyright 2009 by Pearson Education 23 DFG: all go to the same party import java.util.*; public class DrunkenFratGuy extends Critter { private static int partyX; private static int partyY; public DrunkenFratGuy() { Random r = new Random(); partyX = r.nextInt(60); partyY = r.nextInt(50); } public Direction getMove() { if(partyY != getY()) { return Direction.NORTH; } else if(partyX != getX()) { return Direction.EAST; } else { return Direction.CENTER; }