Building Java Programs Chapter 8 Lecture 8-2: More Objects: null, toString, Constructors, this, Encapsulation reading: 8.2 – 8.4

The null reference reading: 8.3

The null reference null : A value that does not refer to any object. The elements of an array of objects are initialized to null. String[] words = new String[5]; DrawingPanel[] windows = new DrawingPanel[3]; index 1 2 3 4 value null words index 1 2 value null windows

Things you can do w/ null store null in a variable or an array element String s = null; words[2] = null; print a null reference System.out.println(s); // null ask whether a variable or array element is null if (words[2] == null) { ... pass null as a parameter to a method System.out.println(null); // null return null from a method (often to indicate failure) return null;

Null pointer exception dereference: To access data or methods of an object with the dot notation, such as s.length() . It is illegal to dereference null (causes an exception). null is not an object, so it has no methods or data. String[] words = new String[5]; words[0] = "hello"; words[2] = "goodbye"; // words[1], [3], [4] are null for (int i = 0; i < words.length; i++) { System.out.println("word is: " + words[i]); words[i] = words[i].toUpperCase(); // ERROR } Output: word is: hello word is: null Exception in thread "main" java.lang.NullPointerException index 1 2 3 4 value "HELLO" null "goodbye"

Looking before you leap You can check for null before calling an object's methods. String[] words = new String[5]; words[0] = "hello"; words[2] = "goodbye"; // words[1], [3], [4] are null for (int i = 0; i < words.length; i++) { if (words[i] != null) { words[i] = words[i].toUpperCase(); } index 1 2 3 4 value "HELLO" null "GOODBYE" words

Two-phase initialization 1) initialize the array itself (each element is initially null) 2) initialize each element of the array to be a new object String[] words = new String[4]; // phase 1 for (int i = 0; i < words.length; i++) { coords[i] = "word" + i; // phase 2 } index 1 2 3 value "word0" "word1" "word2" "word3" words

The toString method reading: 8.2

Any redundancies? public class PointMain { public static void main(String[] args) { // create two Point objects Point p1 = new Point(); p1.x = 5; p1.y = 2; Point p2 = new Point(); p2.x = 4; p2.x = 3; // print each point System.out.println("p1: (" + p1.x + ", " + p1.y + ")"); System.out.println("p2: (" + p2.x + ", " + p2.y + ")"); // move p2 and then print it again p2.translate(2, 4); } OUTPUT: p1: (5, 2) p2: (4, 3) p2: (6, 7)

Printing objects By default, Java doesn't know how to print objects: Point p = new Point(); p.x = 10; p.y = 7; System.out.println("p is " + p); // p is Point@9e8c34 // better, but cumbersome; p is (10, 7) System.out.println("p is (" + p.x + ", " + p.y + ")"); // desired behavior System.out.println("p is " + p); // p is (10, 7)

The toString method A method that tells Java how to convert an object into a string Implicitly called when you use an object in a String context: Point p1 = new Point(7, 2); System.out.println("p1: " + p1); The above code is really doing the following: System.out.println("p1: " + p1.toString()); Every class has a toString, even if you don’t define one. Default: class's name @ object's memory address Point@9e8c34

toString syntax public String toString() { <code that returns a String representation>; } Method header must match exactly. Example: // Returns a String representing this Point return "(" + x + ", " + y + ")";

Quarantine client, v3 public class Quarantine3 { ... public static void quarantineCities(Graphics g, Point3[] cities, Point3 quarPoint, int quarRad) { g.setColor(Color.RED); System.out.println("Quarantined cities: "); g.drawOval(quarPoint.x - quarRad, quarPoint.y - quarRad, 2 * quarRad, 2 * quarRad); for (int i = 0; i < cities.length; i++) { if (quarPoint.distance(cities[i]) <= quarRad) { cities[i].draw(g); System.out.println("\t" + cities[i]); } } } }

Object initialization: constructors reading: 8.3

Initializing objects Currently it takes 3 lines to create a Point and initialize it: Point p = new Point(); p.x = 3; p.y = 8; // tedious We'd rather specify the fields' initial values at the start: Point p = new Point(3, 8); // better! We are able to do this with most types of objects in Java.

Constructors constructor: Initializes the state of new objects. public <type>(<parameters>) { <statement(s)>; } runs when the client uses the new keyword How does this differ from previous methods? no return type is specified; it implicitly "returns" the new object being created This is not the same as having a void return type!! If a class has no constructor, Java gives it a default constructor with no parameters that sets all fields to 0 (or equivalent).

Constructor example public class Point { int x; int y; // Constructs a Point at the given x/y location. public Point(int initialX, int initialY) { x = initialX; y = initialY; } public void translate(int dx, int dy) { x = x + dx; y = y + dy; ...

Tracing a constructor call What happens when the following call is made? Point p1 = new Point(7, 2); public Point(int initialX, int initialY) { x = initialX; y = initialY; } public void translate(int dx, int dy) { x += dx; y += dy; x 7 y 2 x y p1

Common constructor bugs 1. Re-declaring fields as local variables ("shadowing"): public Point(int initialX, int initialY) { int x = initialX; int y = initialY; } This declares local variables with the same name as the fields, rather than storing values into the fields. The fields remain 0. 2. Giving parameters the same name as fields: public Point(int x, int y) { x = x; y = y; This is another form of shadowing. We’ll see how to fix this soon.

Common constructor bugs 3. Accidentally giving the constructor a return type: public void Point(int initialX, int initialY) { x = initialX; y = initialY; } This is actually not a constructor, but a method named Point

Multiple constructors A class can have multiple constructors. Each one must accept a unique set of parameters. Exercise: Write a Point constructor with no parameters that initializes the point to (0, 0). // Constructs a new point at (0, 0). public Point() { x = 0; y = 0; }

Quarantine client, v4 public class Quarantine4 { public static void main(String[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("cities.txt")); Scanner console = new Scanner(System.in); // read in city locations int cityCount = input.nextInt(); Point4[] cities = new Point4[cityCount]; for (int i = 0; i < cityCount; i++) { cities[i] = new Point4(input.nextInt(), input.nextInt()); } ... // get quarantine location/radius System.out.print("Quarantine site x and y? "); Point4 quarPoint = new Point4(console.nextInt(), console.nextInt()); ... }

The keyword this reading: 8.3

The implicit parameter implicit parameter: The object on which an instance method is called. During the call p1.draw(g); the object referred to by p1 is the implicit parameter. During the call p2.draw(g); the object referred to by p2 is the implicit parameter. The instance method can refer to that object's fields. We say that it executes in the context of a particular object. draw can refer to the x and y of the object it was called on.

The this keyword this : Refers to the implicit parameter inside your class. (a variable that stores the object on which a method is called) Refer to a field: this.<field> Call a method: this.<method>(<parameters>); One constructor this(<parameters>); can call another:

Variable shadowing shadowing: 2 variables with same name in same scope. Normally illegal, except when one variable is a field. public class Point { int x; int y; ... // this is legal public void setLocation(int x, int y) { } In most of the class, x and y refer to the fields. In setLocation, x and y refer to the method's parameters.

Fixing shadowing Inside setLocation, public class Point { int x; int y; ... public void setLocation(int x, int y) { this.x = x; this.y = y; } Inside setLocation, To refer to the data field x, say this.x To refer to the parameter x, say x

Calling another constructor public class Point { int x; int y; public Point() { this(0, 0); // calls (x, y) constructor } public Point(int x, int y) { this.x = x; this.y = y; ... Avoids redundancy between constructors Only a constructor (not a method) can call another constructor

Encapsulation reading: 8.4

Encapsulation encapsulation: Hiding implementation details from clients. Encapsulation forces abstraction. separates external view (behavior) from internal view (state) protects the integrity of an object's data

A field that cannot be accessed from outside the class Private fields A field that cannot be accessed from outside the class private <type> <name>; Examples: private int id; private String name; Client code won't compile if it accesses private fields: PointMain.java:11: x has private access in Point System.out.println(p1.x); ^

Accessing private state // A "read-only" access to the x field ("accessor") public int getX() { return x; } // Allows clients to change the x field ("mutator") public void setX(int newX) { x = newX; Client code will look more like this: System.out.println(p1.getX()); p1.setX(14);

Point class // A Point object represents an (x, y) location. public class Point5 { private int x; private int y; // Constructs a Point at the given x/y location. public Point5(int x, int y) { this.x = x; this.y = y; } // Constructs a Point at the origin. public Point5() { this(0, 0); } public void setX(int x) { this.x = x; } public void setY(int y) { this.y = y; } public int getX() { return x; } public int getY() { return y; } ...

Point class ... // Draw this Point. public void draw(Graphics g) { g.fillOval(x, y, 3, 3); g.drawString(toString(), x, y); } // Changes the location of this Point object. public void setLocation(int x, int y) { setX(x); setY(y); } // Shift this Point the given amounts. public void translate(int dx, int dy) { setLocation(x + dx, y + dy); } // Calculate the distance between this Point and another one. public double distance(Point5 other) { int dx = x - other.getX(); int dy = y - other.getY(); return Math.sqrt(dx * dx + dy * dy); } // Print a string representation of this Point public String toString() { return "(" + x + ", " + y + ")"; } }

Benefits of encapsulation Abstraction between object and clients Protects object from unwanted access Example: Can't fraudulently increase an Account's balance. Can change the class implementation later Example: Point could be rewritten in polar coordinates (r, θ) with the same methods. Can constrain objects' state (invariants) Example: Only allow Accounts with non-negative balance. Example: Only allow Dates with a month from 1-12.