1. April 13, 1998CS102-02Lecture 3-1 Data Types in Java CS 102-02 Lecture 3-1 Java’s Central Casting

2. April 13, 1998CS102-02Lecture 3-1 Are You Java’s Type? Type: a set of values that are semantically similar Java is a strongly typed language –Every variable and every expression has a type that is known at compile time. –Strong typing helps detect errors at compile time.

3. April 13, 1998CS102-02Lecture 3-1 What’s the Role of Types? Types limit the: –Values that a variable can hold or that an expression can produce –Limit the operations supported on those values –Determine the meaning of the operations.

4. April 13, 1998CS102-02Lecture 3-1 Java Types Primitive types –boolean –numeric Integral: byte, short, int, long, and char Floating-point: float and double Variables of primitive types hold the actual value

5. April 13, 1998CS102-02Lecture 3-1 Inside a Primitive Type Actual values for integral types: byte: -128 to 127 short: -32768 to 32767 int: -2147483648 to 2147483647 long: -9223372036854775808 to 9223372036854775807 char: from '\u0000' to '\uffff’ (from 0 to 65535) Why use int instead of long ?

6. April 13, 1998CS102-02Lecture 3-1 Boolean Type boolean type represents a logical quantity with two possible values, indicated by the literals true and false

7. April 13, 1998CS102-02Lecture 3-1 Building a Boolean from a Number Can’t say (why not?): if (x) System.out.println(“Congratulations, it’s a Boole!”); Convert an integer x (following the C language convention that any nonzero value is true): if (x != 0) System.out.println(“Congratulations, it’s a Boole!”);

8. April 13, 1998CS102-02Lecture 3-1 Building a Boolean from an Object Object reference obj can be converted (any reference other than null is true): obj! = null

9. April 13, 1998CS102-02Lecture 3-1 The Other Kind of Type Reference types –Variables of reference types don’t hold values, but references to values –Classes, interfaces and arrays are all reference types

10. April 13, 1998CS102-02Lecture 3-1 A Graphical View 0010010 1110010 int counter Airport midway The data of the midway object

11. April 13, 1998CS102-02Lecture 3-1 Classes, Interfaces and Arrays, oh my! Classes we’ve already seen Interfaces are programming contracts –An interface is a set of constants and methods –In Java, a class implements an interface

12. April 13, 1998CS102-02Lecture 3-1 Implementing an Interface An example from the Comparison applet: public class Comparison extends Applet implements ActionListener Comparison promises to do everything an ActionListener does

13. April 13, 1998CS102-02Lecture 3-1 Comparison Does What It Has To What does an ActionListener have to do? actionPerformed public abstract void actionPerformed(ActionEvent e)ActionEvent Provides an implementation for the interface: public void actionPerformed(ActionEvent e) { number1 = Integer.parseInt(input1.getText() ); number2 = Integer.parseInt(input2.getText() ); repaint(); }

14. April 13, 1998CS102-02Lecture 3-1 Comparison Object has Two Types Comparison is a class that implements an interface: public class Comparison extends Applet implements ActionListener

15. April 13, 1998CS102-02Lecture 3-1 Interface Type Example From the Comparison applet: input2 = new TextField( 10 ); input2.addActionListener( this ); add( input2 ); // put input2 on applet What is addActionListener()? public synchronized void addActionListener(ActionListener l)

16. April 13, 1998CS102-02Lecture 3-1 Sideline Events Events are things that happen to programs (other than errors) // Process user's action on the input2 // text field public void actionPerformed(ActionEvent e) { number1 = Integer.parseInt( input1.getText() ); number2 = Integer.parseInt( input2.getText() ); repaint(); }

17. April 13, 1998CS102-02Lecture 3-1 Classes & Interfaces Class type T can hold: –Null reference –Reference to an instance of class T or of any class that is a subclass of T. Interface type can hold: –Null reference –Reference to any instance of any class that implements the interface.

18. April 13, 1998CS102-02Lecture 3-1 Array Types If T is a primitive type, then a variable of type "array of T"can hold: –Null reference –Reference to any array of type "array of T" If T is a reference type, then a variable of type "array of T" can hold: –Null reference –Reference to any array of type "array of S" such that type S is assignable to type T

19. April 13, 1998CS102-02Lecture 3-1 Object is the Root of All Java Variable of type Object can hold: –Null reference –Reference to any object, whether class instance or array.

20. April 13, 1998CS102-02Lecture 3-1 Class  Type Variables have types Objects (and arrays) don’t have a type, but belong to a class Usually we’ll consider them the same

21. April 13, 1998CS102-02Lecture 3-1 Casting Against Type A value could be two different types –Is 12 an int or a float? Compiler isn’t smart, so it’s conservative (signals an error) Override the compiler with a cast –Cast says: Treat this variable as the type I say –To cast in Java, write: (newType) variable

22. April 13, 1998CS102-02Lecture 3-1 Examples of casting // Casting a float literal to a type int. // Without the cast operator, this would be a // compile-time error, because it’s a narrowing // conversion: int i = (int)12.5f; // From class average applet (Figure 2.9) if ( counter != 0 ) { average = (double) total / counter; System.out.println( "Class average is " + average ); } else...

23. April 13, 1998CS102-02Lecture 3-1 Can’t Always Cast Can’t do this: if ((boolean) x) System.out.println(“Congratulations, it’s a Boole!”); Sometimes casts are automatic, and are called conversions

24. April 13, 1998CS102-02Lecture 3-1 One of Two Ways Create an expression in a context where the type of the expression is not appropriate and either: –Error at compile time ( if statement has any type other than boolean ) –May be able to accept a type that is related to the type of the expression

25. April 13, 1998CS102-02Lecture 3-1 Automatic Conversion For convenience, Java performs an implicit conversion –From the type of the expression to a type acceptable for its surrounding context Kinds of conversions: –Identity, Widening primitive, Narrowing primitive, Widening reference, Narrowing reference, String

26. April 13, 1998CS102-02Lecture 3-1 Funky Conversions What does this print? class Test { public static void main(String[] args) { int big = 1234567890; float approx = big; System.out.println(big -(int)approx); }

27. April 13, 1998CS102-02Lecture 3-1 Coming Attractions Arrays: Grouping related values together Back to objects for a bit