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