一.面向对象编程的引入

写一个简单的程序输出张三，李四的名字

class Person {String name;String getName() {return "guangdong "+name;}
};public class Oop {public static void main(String args[]) {Person p1 = new Person();p1.name = "zhangsan";Person p2 = new Person();p2.name = "lisi";System.out.println(p1.getName());System.out.println(p2.getName());}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Oop.java
root@ubuntu:/home/topeet/guyilian# java Oop
guangdong zhangsan
guangdong lisi

在以上代码的基础上我们，添加构造函数，在定义对象的时候就把名字传递过去，构造函数的名字跟类名一样，在构造这个对象的时候就会自动执行这个构造方法。

class Person {String name;String getName() {return "guangdong "+name;}/* construct function */	public Person (String n) {name = n;}};public class Oop2 {public static void main(String args[]) {Person p1 = new Person("zhangsan");Person p2 = new Person("lisi");System.out.println(p1.getName());System.out.println(p2.getName());}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Oop2.java 
root@ubuntu:/home/topeet/guyilian# java Oop2
guangdong zhangsan
guangdong lisi

在5代码的基础上，进行构造函数的重载，同时引入this，如果在一个类里面，类的属性与函数的参数名同名，我们加上this修饰表示是当前对象的属性

class Person {String name;int age;String getName() {return "guangdong "+name;}/* construct method */	public Person () {name = "null";age  = 0;}public Person (String name) {this.name = name;}public Person (String name, int age) {this.name = name;this.age  = age; }};public class Oop3 {public static void main(String args[]) {Person p1 = new Person("zhangsan");Person p2 = new Person("lisi");Person p3 = new Person();Person p4 = new Person("wangwu", 6);System.out.println(p1.getName());System.out.println(p2.getName());System.out.println(p3.getName());System.out.println(p4.getName());}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Oop3.java 
root@ubuntu:/home/topeet/guyilian# java Oop3
guangdong zhangsan
guangdong lisi
guangdong null
guangdong wangwu

在以上代码的基础上，我们添加类方法printPerson，那样我们不需要定义具体的类对象我们就可以直接用这个方法，同时我们添加一个类属性count，用来统计人数的多少。

class Person {static int count;String name;int age;String getName() {return "guangdong "+name;}/* construct method */	public Person () {count++;name = "null";age  = 0;}public Person (String name) {count++;this.name = name;}public Person (String name, int age) {count++;this.name = name;this.age  = age; }static void printPerson () {System.out.println("This is a class of Person");}};public class Oop4 {public static void main(String args[]) {Person p1 = new Person("zhangsan");Person p2 = new Person("lisi");Person p3 = new Person();Person p4 = new Person("wangwu", 6);System.out.println(p1.getName());System.out.println(p2.getName());System.out.println(p3.getName());System.out.println(p4.getName());Person.printPerson();System.out.println(Person.count);}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Oop4.java 
root@ubuntu:/home/topeet/guyilian# java Oop4
guangdong zhangsan
guangdong lisi
guangdong null
guangdong wangwu
This is a class of Person
4

在以上代码的基础上，我们引入构造代码块， 每实例化一个对象前，都执行；先于构造方法执行。以上例子中我们要在每个构造函数进行count ++,有了构造代码块我们只需要在构造代码块写一次就够了。同时引入静态构造代码块，实例化第一个对象前，执行；只执行一次。

class Person {static int count;String name;int age;String getName() {return "guangdong "+name;}static {System.out.println("static block");		}{System.out.println("construct block");count ++;}/* construct method */	public Person () {System.out.println("construct method: Person 1");name = "null";age  = 0;}public Person (String name) {System.out.println("construct method: Person 2");this.name = name;}public Person (String name, int age) {System.out.println("construct method: Person 3");this.name = name;this.age  = age; }static void printPerson () {System.out.println("This is a class of Person");}};public class Oop5 {public static void main(String args[]) {Person p1 = new Person("zhangsan");Person p2 = new Person("lisi");Person p3 = new Person();Person p4 = new Person("wangwu", 6);System.out.println(p1.getName());System.out.println(p2.getName());System.out.println(p3.getName());System.out.println(p4.getName());Person.printPerson();System.out.println(Person.count);}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Oop5.java 
root@ubuntu:/home/topeet/guyilian# java Oop5
static block
construct block
construct method: Person 2
construct block
construct method: Person 2
construct block
construct method: Person 1
construct block
construct method: Person 3
guangdong zhangsan
guangdong lisi
guangdong null
guangdong wangwu
This is a class of Person
4