CS102 Data Types in Java CS 102 Java’s Central Casting

CS102 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.

CS102 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.

CS102 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

CS102 Inside a Primitive Type Actual values for integral types: byte: -128 to 127 short: to int: to long: to char: from '\u0000' to '\uffff’ (from 0 to 65535) Why use int instead of long ?

CS102 Boolean Type boolean type represents a logical quantity with two possible values, indicated by the literals true and false

CS102 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!”);

CS102 Building a Boolean from an Object Object reference obj can be converted (any reference other than null is true): obj! = null

CS102 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

CS102 A Graphical View int counter Airport midway The data of the midway object

CS102 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

CS102 Implementing an Interface An example from the Comparison applet: public class Comparison extends Applet implements ActionListener Comparison promises to do everything an ActionListener does

CS102 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(); }

CS102 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

CS102 Object is the Root of All Java Variable of type Object can hold: –Null reference –Reference to any object, whether class instance or array.

CS102 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

CS102 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

CS102 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...

CS102 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

CS102 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

CS102 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

CS102 Coming Attractions Arrays: Grouping related values together