类(Class)和实体(instance)的关系

如图所示

package com.web;public class Test23 {public static void main(String[] args) {Person person1 &＃61; new Person("Kate", "Jones", 27, 1.6, 50.0);person1.printData();Person person2 &＃61; new Person("John", "Christopher", "Smith", 65, 1.75, 80.0);person2.printData();Person.printCount();}
}class Person {public static int count &＃61; 0;public String firstName;public String middleName;public String lastName;public int age;public double height, weight;Person(String firstName, String lastName, int age, double height, double weight) {Person.count&＃43;&＃43;;this.firstName &＃61; firstName;this.lastName &＃61; lastName;this.age &＃61; age;this.height &＃61; height;this.weight &＃61; weight;}Person(String firstName, String middleName, String lastName, int age, double height, double weight) {// this()を用いて、コンストラクタを呼び出してくださいthis(firstName, lastName, age, height, weight);//不加类型已经是实参了this(String firstName, String lastName, int age, double height, double weight)错的;this.middleName &＃61; middleName;}public String fullName() {if (this.middleName !&＃61; null) {return this.firstName &＃43; " " &＃43;this.middleName &＃43; " " &＃43;this.lastName;} else {return this.firstName &＃43; " " &＃43; this.lastName;}}public void printData() {System.out.println("私の名前は" &＃43; this.fullName() &＃43; "です");System.out.println("年齢は" &＃43; this.age &＃43; "歳です");System.out.println("BMIは" &＃43; Math.round(this.bmi()) &＃43; "です");}public double bmi() {return this.weight / this.height / this.height;}public static void printCount() {System.out.println("合計" &＃43; Person.count &＃43; "人です");}
}