Question of the Day  Write valid mathematical equation using: one addition operator (‘+’) one equality operator (‘=’)  Should have equal values on both sides of equals sign  Not limited to what is possible in Java

Question of the Day  Write valid mathematical equation using: one addition operator (‘+’) one equality operator (‘=’)  Should have equal values on both sides of equals sign  Not limited to what is possible in Java = 3 2

Interfaces in Real World  Everyone should give me colored paper in wallets  Men pictured on these unimportant papers  In return, will show you trip pictures (when I get back)

Interfaces in Real World  Everyone should give me colored paper in wallets  Men pictured on these unimportant papers  In return, will show you trip pictures (when I get back)

Interfaces in Real World  Everyone should give me colored paper in wallets  Men pictured on these unimportant papers  In return, will show you trip pictures (when I get back)

Interfaces in Real World

Interfaces  Can only declare important constant fields  public static final must be used for fields  Interface declares public abstract methods  Methods callable anywhere by any class  But method’s body cannot be defined in interface

Adding to an Interface  Interfaces have sub-interfaces  Sub-interface extends super-interface  Super-interface’s methods included in sub-interface  Methods from super-super-interface also inherited  Sub-interface can extend multiple interfaces  Methods not defined so no problems possible

Declaring Interfaces public interface Drawable { public void setColor(Color c); public Color getColor(); public void draw(Graphics g); } public interface TranDraw extends Drawable{ public void setTransparency(int tLevel); } public interface MoveDraw extends Drawable{ public void setPosition(int x, int y); }

Interfaces implement  Classes implement interfaces  Implementing classes define interface’s methods  Any number of interfaces may be implemented  Multiple inheritance issues ignored -- methods empty  Unrelated to superclass chosen or used by a class  Classes must implement all interface’s methods  Includes methods inherited from super-interface

Implementing Interfaces public class Square implements Drawable { private Color c; private int x, y, side; public Square(Color col, int len) { c = col; side = len; } public void setColor(Color col){ c=col; } public Color getColor() { return c; } public void draw(Graphics g) { g.drawRect(x, y, side, side, c); } }

Implementing Interfaces public class Oval implements Drawable { private Color c; private int x, y, majRad, minRad; public Oval(Color co, int maj, int min){ c = co; majRad = maj; minRad = min; } public void setColor(Color col){ c=col; } public Color getColor() { return c; } public void draw(Graphics g) { g.drawOval(x, y, majRad, minRad, c); } }

Using Interfaces

Interface Overview  Classes can implement interfaces instances  Both used to mark instances define method(s)  All variables can use interface or class as type  Abstact methods defined by interface  More flexible: class can implement many interfaces  Can mark classes; do not have to declare methods

Typecasting int i = 13; Square s = ((Square)i);  Only exist to “assist” compiler with code  Changes variable’s type so compilation continues  Not in *.class file – does not affect instance at all  Only when KNOW instance & variables types differ  Errors at runtime instead of during compilation  Illegal code will compile, but still illegal

Typecasting int i = 13; Square s = ((Square)i);  Only exist to “assist” compiler with code  Changes variable’s type so compilation continues  Not in *.class file – does not affect instance at all  Only when KNOW instance & variables types differ  Errors at runtime instead of during compilation  Illegal code will compile, but still illegal

Narrowing Conversions  Java cannot compile narrowing conversions  Assign interface variable to implementing class variable  Compiler will not allow it, but could be legal  Typecasting required for these assignments Drawable obj = new Oval(...);

Narrowing Conversions  Java cannot compile narrowing conversions  Assign interface variable to implementing class variable  Compiler will not allow it, but could be legal  Typecasting required for these assignments Drawable obj = new Oval(...); Oval sad = obj; //  Does not work

Narrowing Conversions  Java cannot compile narrowing conversions  Assign interface variable to implementing class variable  Compiler will not allow it, but could be legal  Typecasting required for these assignments Drawable obj = new Oval(...); Oval sad = obj; //  Does not work Oval glad = (Oval)obj; // works!

Your Turn  Get into your groups and complete activity

For Next Lecture  Read B.13 for Wed. and be ready to discuss  What is inheritance? extends  How does the extends keyword work?  How does inheritance compare to interfaces?  As usual, there is weekly assignment on Angel Tuesday  Due by 5PM Tuesday via Assignment Submitter  Each problem graded using provided JUnit tests