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Data Types in Java

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

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Data Types in Java

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  1. Data Types in Java Java’s Central Casting CS 102

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

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

  9. 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. A Graphical View 0010010 int counter Airport midway 1110010 The data of the midway object

  11. 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. 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. Comparison Does What It Has To • What does an ActionListener have to do? actionPerformed public abstract void actionPerformed(ActionEvent e) • Provides an implementation for the interface: public void actionPerformed(ActionEvent e) { number1 = Integer.parseInt(input1.getText() ); number2 = Integer.parseInt(input2.getText() ); repaint(); }

  14. 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

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

  16. 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

  17. 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

  18. 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 ...

  19. 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

  20. 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

  21. 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

  22. Coming Attractions • Arrays: Grouping related values together

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