Types, Classes, and Interfaces in Java

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4126 Types, Classes, and Interfaces
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byte r, g, b;
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TYPES, VALUES, AND VARIABLES
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public void setColor(byte rv, byte gv, byte bv) { r = rv; g = gv; b = bv; } } class Test { public static void main(String[] args) { Point p = new Point(); ColoredPoint cp = new ColoredPoint(); p = cp; Colorable c = cp; } }
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In this example:
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The local variable p of the method main of class Test has type Point and is initially assigned a reference to a new instance of class Point
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The assignment of the value of cp to the variable p causes p to hold a reference to a ColoredPoint object This is permitted because ColoredPoint is a subclass of Point, so the class ColoredPoint is assignment compatible ( 52) with the type Point A ColoredPoint object includes support for all the methods of a Point In addition to its particular elds r, g, and b, it has the elds of class Point, namely x and y The local variable c has as its type the interface type Colorable, so it can hold a reference to any object whose class implements Colorable; speci cally, it can hold a reference to a ColoredPoint
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DISCUSSION
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Note that an expression such as new Colorable() is not valid because it is not possible to create an instance of an interface, only of a class
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Every array also has a class; the method getClass, when invoked for an array object, will return a class object (of class Class) that represents the class of the array
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The local variable cp similarly has as its type ColoredPoint, and is initially assigned a reference to a new instance of class ColoredPoint
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TYPES, VALUES, AND VARIABLES
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Types, Classes, and Interfaces
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The classes for arrays have strange names that are not valid identi ers; for example, the class for an array of int components has the name [I and so the value of the expression:
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new int[10]getClass()getName()
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is the string "[I"; see the speci cation of ClassgetName for details
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Oft on the dappled turf at ease I sit, and play with similes, Loose types of things through all degrees the Same Flower 75
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4126 Types, Classes, and Interfaces
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TYPES, VALUES, AND VARIABLES
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C H A P T E R
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Conversions and Promotions
EVERY expression written in the Java programming language has a type that
A conversion from type Object to type Thread requires a run-time check to make sure that the run-time value is actually an instance of class Thread or one of its subclasses; if it is not, an exception is thrown
A conversion from type Thread to type Object requires no run-time action; Thread is a subclass of Object, so any reference produced by an expression of type Thread is a valid reference value of type Object A conversion from type int to type long requires run-time sign-extension of a 32-bit integer value to the 64-bit long representation No information is lost
can be deduced from the structure of the expression and the types of the literals, variables, and methods mentioned in the expression It is possible, however, to write an expression in a context where the type of the expression is not appropriate In some cases, this leads to an error at compile time In other cases, the context may be able to accept a type that is related to the type of the expression; as a convenience, rather than requiring the programmer to indicate a type conversion explicitly, the language performs an implicit conversion from the type of the expression to a type acceptable for its surrounding context A speci c conversion from type S to type T allows an expression of type S to be treated at compile time as if it had type T instead In some cases this will require a corresponding action at run time to check the validity of the conversion or to translate the run-time value of the expression into a form appropriate for the new type T For example: