Effective JAVA2 reading notes-classes and Interfaces (iv)

19th: interfaces are only used to define types

This article provides a counter-example, saying that some interfaces contain only constants. This is a bad use of the interface. To implement the same functionality, you should use a tool class that cannot be instantiated (4th).

 Public classphysicalconstants {Privatephysicalconstants () {}//Prevents instantiation//Avogadro ' s number (1/mol)     Public Static Final DoubleAvogadros_number = 6.02214199e23; //Boltzmann constant (j/k)     Public Static Final DoubleBoltzmann_constant = 1.3806503e-23; //Mass of the Electron (kg)     Public Static Final DoubleElectron_mass = 9.10938188e-31;}

In addition, we have learned that if you use it a lot, you need to always call it in a similar physicalconstants.avogadros_number way. At this point, you can use the static import mechanism of Java (just a little trick). is to use the import static Com.effectivejava. physicalconstants.* the way the class is imported, you can then use the constant avogadros_number directly, without having to add the class name.

Reference http://www.cnblogs.com/mengdd/archive/2013/01/23/2873312.html

20th: Class hierarchy is better than label class

This is also an example of a counter (label class).

classFigure {enumShape {RECTANGLE, CIRCLE}; Finalshape shape; //rectangles have long and wide    Doublelength; Doublewidth; //round with Radius    Doubleradius; Figure (Doubleradius) {Shape=shape.circle;  This. Radius =radius; } figure (DoubleLengthDoublewidth) {Shape=Shape.rectangle;  This. length =length;  This. width =width; }    DoubleArea () {Switch(shape) { CaseRECTANGLE:returnLength *width;  CaseCIRCLE:returnMath.PI * (RADIUS *radius); default:            Throw NewAssertionerror (); }    }}

View Code

This class can represent a circle or a rectangle. But the level is not clear, very messy.

The correct implementation, that is, our common implementation, should be to write a graph class as a base class, and then the Circle class and the rectangle class are implemented separately.

Abstract classFigure {Abstract DoubleArea ();}classCircleextendsFigure {Final Doubleradius; Circle (Doubleradius) {         This. Radius =radius; }    DoubleArea () {returnMath.PI * (RADIUS *radius); }}classRectangleextendsFigure {Final Doublelength; Final Doublewidth; Rectangle (DoubleLengthDoublewidth) {         This. length =length;  This. width =width; }    DoubleArea () {returnLength *width; }}

Effective JAVA2 reading notes-classes and Interfaces (iv)