21. java面向对象

一、定义

​ JavaBean是一种Java语言写成的可重用组件，所谓JavaBean是指符合如下要求的java类：①类是公共的 ②有一个无参的公共构造器 ③有属性，且有对应的get、set方法。用户可以使用JavaBean将功能、处理、值、数据库访问和其他任何可以用Java代码创造的对象进行打包，并且其他的开发者可以通过内部的JSP页面、Servlet、其他JavaBean、applet程序或应用来使用这些对象。用户可以认为JavaBean提供了一种随时随地的复制粘贴的功能，而不要关心任何改变。

二、this关键字

• this可以修饰属性、方法、构造器
  • this理解为当前对象
  • 在类方法中，用"this.属性"或"this.方法"是调用对象的属性或方法，一般我们都省this

public class Person {
    private String name;
    private int age;

    //构造器
    public Person(){
        this.eat();
    }

    public Person(String name){
        this.name = name;
    }

    public Person(String name, int age){
        this.name = name;
        this.age = age;
    }


    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
    //如果不加this，则设置是就是空字符串，因为name就是形参就近原则
    //加上后，就可区分this.name是属性；后面name是形参
    public void setName(String name) {
        this.name = name;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public void eat(){
        System.out.println("people eat fruit");
        this.study();
    }

    public void study(){
        System.out.println("pepole study english");
    }
}

class PersonTest{
    public static void main(String[] args) {
        Person p1 = new Person();
        p1.eat();
    }
}

• this调构造器
  • 我们在构造器中，可以显示使用"this(形参列表)"方式调用指定本类中指定其他构造器
  • 构造器中，不能使用"this(形参列表)"方式调用自己
  • 如果一个类中有n个构造器，最多有n-1个调用，不能成环
  • 构造器中只能声明一个"this(形参列表)"

public class Person {
    private String name;
    private int age;

    //构造器;若没创建对象都要去实现一段代码为了减少冗余，使用this调构造器
    public Person(){
        System.out.println("person 需要创建很多...,现行创建");
    }

    public Person(String name){
        // 调用空参构造器
        this();
        this.name = name;
    }

    public Person(String name, int age){
        // 调用带参构造器，此构造起中有name参数，就不需要自己创建了
        this(name);
        //this.name = name;
        this.age = age;
    }

    // 封装性体现
    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public void eat(){
        System.out.println("people eat fruit");
        this.study();
    }

    public void study(){
        System.out.println("pepole study english");
    }
}

class PersonTest{
    public static void main(String[] args) {
        Person p1 = new Person("tom", 18);
        System.out.println(p1.getAge());
    }
}

三、实例

1. account

public class Account {
    /*
     * id:账号
     * balance:余额
     * annualInterestRate:年利率
     * */
    private int id;
    private double balance;
    private double annualInterestRate;

    //构造器
    public Account(int id, double balance, double annualInterestRate) {
        this.id = id;
        this.balance = balance;
        this.annualInterestRate = annualInterestRate;
    }

    //get和set方法
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public double getBalance() {
        return balance;
    }

    public void setBalance(double balance) {
        this.balance = balance;
    }

    public double getAnnualInterestRate() {
        return annualInterestRate;
    }

    public void setAnnualInterestRate(double annualInterestRate) {
        this.annualInterestRate = annualInterestRate;
    }

    //取钱
    public void withdraw(double amount) {
        //余额不足提示
        if (balance < amount) {
            System.out.println("sorry, Insufficient balance");
            return;
        }
        balance -= amount;
        System.out.println("Successfully withdraw" + " " + amount + " yuan!");
    }

    //存钱
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            System.out.println("Successfully deposited" + " " + amount + " yuan!");
        }
    }
}

2. customer

public class Customer {
    private String firstName;
    private String lastName;
    //对象属性
    private Account account;

    public Customer(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Account getAccount() {
        return account;
    }

    public void setAccount(Account account) {
        this.account = account;
    }
}

3. customerTest

public class CustomerTest {
    public static void main(String[] args) {
        /*
        * 一个用户Jane Smith，账号1001，余额2000，利率1.23%
        * */
        Customer customerObj = new Customer("Jone","Smith");
        Account accountObj = new Account(1001, 2000, 0.0123);
        //在银行开个户，设置id=1001，金额2000
        customerObj.setAccount(accountObj);
        //先通过用户获取账户，存100元，取960元
        customerObj.getAccount().deposit(100);
        customerObj.getAccount().withdraw(960);
        customerObj.getAccount().withdraw(2000);
    }
}

4. bank

import sun.print.CUPSPrinter;

public class Bank {
    //银行有多个用户,创建一个数组
    private Customer[] customers;
    //客户个数
    private int numberOfCustomer;

    public Bank() {
        //需要先初始化数组
        customers = new Customer[10];
    }

    //添加客户,添加一个后，许将numberOfCustomer自增1
    public void addCustomer(String f, String l) {
        Customer customerObjTwo = new Customer(f, l);
        customers[numberOfCustomer] = customerObjTwo;
        numberOfCustomer++;
        //或
        customers[numberOfCustomer++] = customerObjTwo;
    }

    //获取客户数量
    public int getNumeberOfCustomer() {
        return numberOfCustomer;
    }

    //获取指定位置上的客户
    public Customer getCustomer(int index) {
        if (index >= 0 && index < numberOfCustomer) {
            return customers[index];
        }
        return null;
    }
}

5. bankTest

public class BankTest {
    public static void main(String[] args) {
        /*
        * 创建一个银行，增加一个客户tom hu
        * 获取该客户，为他创建一个账户102，余额10000
        * 先通过客户，获取他账户，在取2000元
        * */
        Bank bankobj = new Bank();
        bankobj.addCustomer("tom","hu");
        bankobj.getCustomer(0).setAccount(new Account(102,10000,0.023));
        bankobj.getCustomer(0).getAccount().withdraw(2000);
        System.out.println(bankobj.getCustomer(0).getAccount().getBalance());
    }
}