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 = new Person("Kate", "Jones", 27, 1.6, 50.0);
        person1.printData();
        Person person2 = new Person("John", "Christopher", "Smith", 65, 1.75, 80.0);
        person2.printData();
        Person.printCount();
    }
}

class Person {
    
    
    public static int count = 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++;
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
        this.height = height;
        this.weight = 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 = middleName;
    }

    public String fullName() {
    
    
         if (this.middleName != null) {
    
    
            return this.firstName + " " +this.middleName + " " +this.lastName;
        } else {
    
    
            return this.firstName + " " + this.lastName;
        }
    }

    public void printData() {
    
    
        System.out.println("私の名前は" + this.fullName() + "です");
        System.out.println("年齢は" + this.age + "歳です");
        System.out.println("BMIは" + Math.round(this.bmi()) + "です");
    }

    public double bmi() {
    
    
        return this.weight / this.height / this.height;
    }

    public static void printCount() {
    
    
        System.out.println("合計" + Person.count + "人です");
    }
}