java整理（面向对象基础知识--类与对象）

1. 定义类

class Book{    String title;    double price;    public  void getInfo() {        System.out.print("图书名称："+title+",价格:"+price);    }}

1. 使用类–在mian类中使用Book类

package head_first;class Book{    String title;    double price;    public  void getInfo() {        System.out.print("图书名称："+title+",价格:"+price);    }}public class Exp31 {    public static void main(String[] args) {        // TODO Auto-generated method stub         Book bk = new Book();         bk.title = "java开发";         bk.price = 32.4;         bk.getInfo();    }}

运行结果：图书名称：java开发,价格:32.4

1. 以分布的方式实例化对象

public class Exp31 {    public static void main(String[] args) {        // TODO Auto-generated method stub         Book bk = null;//声明对象         bk = new Book();//实例化对象（开辟内存）         bk.title = "java开发";         bk.price = 32.4;         bk.getInfo();    }}

运行结果：图书名称：java开发,价格:32.4

1. 使用未实例化的对象

public static void main(String[] args) {        // TODO Auto-generated method stub         Book bk = null;         bk.title = "java开发";         bk.price = 32.4;         bk.getInfo();    }

报错信息：Exception in thread "main" java.lang.NullPointerException    at head_first.Exp31.main(Exp31.java:17)

1. 声明两个对象

Book bk1 = new Book();Book bk2 = new Book();bk1.title = "java开发";bk1.price = 32.4;bk2.title = "python开发";bk2.price = 11.2;bk1.getInfo();   bk2.getInfo();

运行结果：   图书名称：java开发,价格:32.4        图书名称：python开发,价格:11.2

1. 对象引用传递

Book bk1 = new Book();Book bk2 = null;bk1.title = "java开发";bk1.price = 32.4;bk2 = bk1;bk2.price = 11.2;bk1.getInfo();bk2.getInfo();

运行结果：   图书名称：java开发,价格:11.2        图书名称：java开发,价格:11.2

1. 深入观察引用传递

public static void main(String[] args) {        // TODO Auto-generated method stub         Book bk1 = new Book();         Book bk2 = new Book();         bk1.title = "java开发";         bk1.price = 32.4;         bk1.title = "python开发";         bk1.price = 11.4;         bk2 = bk1;         bk2.price = 11.2;         bk1.getInfo();         bk2.getInfo();    }

运行结果：   图书名称：java开发,价格:11.2        图书名称：java开发,价格:11.2