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         notes : There are three examples of homework at the end  

   encapsulation

    1.1、 Three characteristics of object-oriented ： encapsulation 、 Inherit 、 polymorphic 、
         With encapsulation , To inherit , With inheritance , There is polymorphism .
    
    1.2、 The first feature of object orientation ： encapsulation . 
         What is encapsulation ？ What's the role ？
             Many real examples in real life are encapsulated , for example ：
                 mobile phone , The TV , Computers have a hard shell on the outside , encapsulated ,
                 Protect internal components , Ensure that the internal components are safe . In addition, after encapsulation ,
                 For our users , We can't see the complex structure inside , We
                 There is no need to care about how complicated it is inside , We only need to operate a few on the shell
                 Button to complete the operation .
            
         So what does encapsulation do ？
             The first function ： Ensure the safety of the internal structure
             The second function ： Shielding is complicated , Exposure is simple
        
         At the code level , What's the use of encapsulation ？
             Data in a class body , Suppose after encapsulation , For the caller of the code ,
             You don't need to care about the complex implementation of the code , Just go through a simple entrance
             Visited , in addition , The data with higher security level in the class body is encapsulated , Outsiders
             Members are not allowed to visit at will , To ensure data security .

What are the disadvantages of code without encapsulation ：

//  Access... In an external program Person Data in this type .
public class PersonTest01
{
	public static void main(String[] args){
		//  establish Person object 
		Person p1 =new Person();
		//  The age of the interviewer 
		//  Accessing the properties of an object , There are usually two operations , One is to read data , One is to change the data .
		//  Reading data 	
		System.out.println(p1.age);	// read (get Said to get )

		//  Modifying data 
		p1.age =50;
		//  Read again 
		System.out.println(p1.age);

		//  stay PersonTest In this external program, you can now set attributes at will age operational .
		//  Random modification leads to data insecurity 
		p1.age =-100;
		System.out.println(p1.age);
	}
}

Packaged Person:

//  Packaged Person
public class Person
{	
	// private  For private , After being modified by this keyword , This data can only be accessed in this class .
	private int age;
}

//  Access... In an external program Person Data in this type .
public class PersonTest01
{
	public static void main(String[] args){
		//  establish Person object 
		Person p1 =new Person();
		
		// Person Of age, Completely inaccessible from the outside , But it's a little too safe .
		// age Cannot access , The meaning of this program is not great .
		
		//  Reading data 	
		System.out.println(p1.age);	// read (get Said to get )  The result is wrong 

		//  Modifying data 
		p1.age =50;
		//  Read again 
		System.out.println(p1.age);	//  The result is wrong 

		//  Modify the data again 
		p1.age =-100;
		System.out.println(p1.age); //  The result is wrong 
	}
}

  Modified encapsulation code ：【 First understand the instance method Revise 】

java There are requirements in the development specification ,set Methods and get Method should satisfy the following format ：
     get Method requirements :
         public return type get+ Attribute names are capitalized ( No arguments ){
            return xxx;
        }
    
     set Method requirements :
         public void set+ Attribute name is capitalized ( There is a parameter ){
            xxx = Parameters ;
        }

  Focus on

The encapsulated code implements two steps ：
     First step ： Property privatization (private)
     The second step ： One attribute provides two external set and get Method , External programs can only pass through set Methods to modify , Only through get
             Method reading , Can be in set Set up checkpoints in the method to ensure the security of data .

     Just a little bit more ：    
         set and get Methods are instance methods , You can't take static
         No static The method of is called instance method , The instance method must be called first new object

        set and get There are strict specification requirements when writing methods

public class Person
{	
	// private  For private , After being modified by this keyword , This data can only be accessed in this class .
	private int age;

	//  Provide simple access to the outside world 
	//  Write a method dedicated to reading .（get）
	//  Write a method dedicated to writing .（set）
	// get and set The method should not take static, Defined as instance method 
	//[ What you read and revise is your age   No object, no age ]
	// Format ： [ List of modifiers ]  return type   Method name ( Formal parameter type ){} 
	
	// get Take the data 
	public int getAge(){
		return age;	// return The data type and " Return type of method " identical ,age by int So the method is int
	}

	// set Modifying data 
	public void setAge(int nianLing){
		age =nianLing;
	}
}

//  Access... In an external program Person Data in this type .
public class PersonTest01
{
	public static void main(String[] args){
			//  establish Person object 
		Person p1 =new Person();
	
			//  Take the data 
		int nianling =p1.getAge();	//  The return value must require a variable of the same type to receive 
		System.out.println(nianling);	// 0  The default value is 
		//  The above codes are combined  System.out.println(p1.getAge());	
			
			// Modifying data 
		p1.setAge(123);
			//  When reading data again 
		System.out.println(p1.getAge());	// 123
		
		// Be careful : I found that I could change my age to -100  It's not very realistic [ unsafe ]
		//  Then we need to set Method set up a checkpoint at the entrance 
		p1.setAge(-100);
		System.out.println(p1.getAge());	// -100
		
	}
}

set Method set up a checkpoint at the entrance ：

public class Person
{	
	// private  For private , After being modified by this keyword , This data can only be accessed in this class .
	private int age;

	//  Provide simple access to the outside world 
	//  Write a method dedicated to reading .（get）
	//  Write a method dedicated to writing .（set）
	// get and set The method should not take static, Defined as instance method 
	//[ What you read and revise is your age   No object, no age ]
	// Format ： [ List of modifiers ]  return type   Method name ( Formal parameter type ){} 
	
	// get Take the data 
	public int getAge(){
		return age;	// return The data type and " Return type of method " identical ,age by int  So the method is int
	}

	// set Modifying data 
	public void setAge(int nianLing){
		//  Can you in this set Method set up a level on the location ！！！
		//  Yes, it was int nianLing  Make conditional restrictions 
		if (nianLing<0||nianLing>150)
		{
			System.out.println(" Sorry, the age is illegal , Please re-enter ！");
			return;
		}
		age =nianLing;
	}
}

Example method ( No static Methods )

Objects are called instances
Strength is related to ： Instance variables 、 Example method
Instance variables are object variables . Instance methods are object methods
step ： All instances related need to be new object , adopt " quote ." The way to visit

  The code is shown as follows ：

public class OOTest02
{
	public static void main(String[] args){
		
		//  with static Method call ( Common method )
		//  Class name .  It can be omitted ( In the same class )
		dosome();

		//  Without static Method call ( Example method or Object methods )  You need to create the object first   Re pass " quote ." Method call 
		
		OOTest02 o =new OOTest02();
		o.doOther();
	}

	//  with static
	public static void dosome(){
		System.out.println("do some");
	}

	//  This method doesn't have static, This method is called :  Example method ( Object methods )
	public void doOther(){
		System.out.println("do other.....");
	}
}

Null pointer problems caused by instance methods

public class OOTest03
{
	public static void main(String[] args){
		
		User u =new User();
		u.eat();
		
		//  The reference becomes null No longer point to the heap User object  
		
		u=null;
		//  The invocation of an instance method must also have the existence of an object   Therefore, a null pointer exception occurs in the above case 
		u.eat();	
	}
	public static void doSome(){
		System.out.println("do some");
	}	
}
class User
{
	//  Instance variables 
	int id;

	// Example method ( Object related methods   There must be an object )
	//( Such as ： having dinner   So the object is people or animal    To play basketball ---> object   people ... So you need to call first new An object )
	//【 The common method only needs " Class name ." To make the call 】
	public void eat(){
		System.out.println(" Eat something ");
	}
}

Homework 1：

Please encapsulate... Through code , Realize the following requirements ：
     Write a class Book, Representative textbook :
    1. With attributes : name (title) 、 the number of pages (pageNum)
    2. The number of pages cannot be less than 200 page , Otherwise, an error message will be output , And give default values 200
    3. Provide assignment and value taking methods for each attribute
    4. There is a way :detail, It is used to output the name and pages of each textbook on the console
    5. Writing test classes BookTest To test : by Book The properties of the object are given initial values , And call Book Object's detail

public class HomeWork
{
	public static void main(String[] args){
		Book b =new Book("java Introductory textbook ",160);// [ There are constructors   Parameters are passed directly to attributes through the constructor ]
		b.detail();

		//  Take the name  or Take pages  
		String i=b.getTitle();
		System.out.println(i);
		//  Modify name  or Change the number of pages 
		b.setTitle("python.");
		System.out.println(b.getTitle());
		
	}
}

class Book
{	
	private String title;
	private int pageNum;
	
	//  Construction method   No parameter 
	public Book(){
		
	}
	
	//  There are parameter constructors 
	public Book(String t,int p){
		title =t;
		pageNum =p;
	}

	// set  and  get Method 
	public String getTitle(){
		return title;
	}
	public void setTitle(String t){
		title =t;
	
	}
	public int getpageNum(){
		return pageNum;
	}
	public void setTitle(int p){
		if (p<200||p<0)
		{
			System.out.println(" The number of pages in this book cannot be less than 200 page   Default assignment 200 page .");
			pageNum =200;
			return;
		}
		pageNum =p;
	}
	
	public void detail(){
		System.out.println(" Book title :"+this.title+" Book pages :"+this.pageNum);
	}
}

 

In the parameterless constructor and There are parameter constructors that can also modify data or define conditions

public class HomeWork
{
	public static void main(String[] args){
		
		Book b1 =new Book();
		//  Don't pass it on   Modifying attribute data in a parameterless constructor 
		String i =b1.getTitle();
		System.out.println(i);
		//  Then read the data 
		int b =b1.getpageNum();
		System.out.println(b);
		
		
	}
}

class Book
{	
	private String title;
	private int pageNum;
	
	//  Construction method   No parameter 
	public Book(){
		title =" Unknown course ";
		pageNum =360;
	}
	
	//  There are parameter constructors 
	public Book(String t,int p){
		title =t;
		pageNum =p;
	}

	// set  and  get Method 
	public String getTitle(){
		return title;
	}
	public void setTitle(String t){
		title =t;
	
	}
	public int getpageNum(){
		return pageNum;
	}
	public void setpageNum(int p){
		if (p<200||p<0)
		{
			System.out.println(" The number of pages in this book cannot be less than 200 page   Default assignment 200 page .");
			pageNum =200;
			return;
		}
		pageNum =p;
	}
	
	public void detail(){
		System.out.println(" Book title :"+this.title+" Book pages :"+this.pageNum);
	}
}

Homework 2：

Write a name for Account Class simulation account .
The properties and methods of this class are as follows .
This class includes properties : Account id, balance balance, Annual interest rate annualInterestRate:
Included methods : Each attribute of set and get Method . Method of withdrawal withdraw(), Method of deposit deposit()
Write a test program
(1) Create a Customer, Name is Jane Smith, He has an account number for 1000, The balance is 2000, The annual interest rate is 1
(2) Yes Jane Smith operation :
Deposit in 100 element , Take it out again 960 element , Take it out again 2000.
Print Jane Smith Basic information of
The information is shown below :
Successfully deposited : 100
Successfully removed : 960
Lack of balance , Failed to withdraw money

public class HomeWork01
{
	public static void main(String[] args){
		Account a =new Account("Jane Smith",2000,1);
		System.out.println(a.getId());
		a.deposit(100);
		a.withdraw(960);
		a.withdraw(2000);
		
		
		
		
	}	
}

class Account
{
	private String id;
	private int balance;
	private int annualInterestRate;

	public Account(){
		
	}
	public Account(String id,int balance,int annualInterestRate){
		this.id =id;
		this.balance =balance;
		this.annualInterestRate =annualInterestRate;
	}

	public String getId(){
		return id;
	}
	public void setId(String id){
		this.id =id;
	}

	public int getBalance(){
		return balance;
	}
	public void setId(int balance){
		this.balance =balance;
	}
	
	public int getAnnualInterestRate(){
		return annualInterestRate;
	}
	public void setAnnualInterestRate(int annualInterestRate){
		this.annualInterestRate =annualInterestRate;
	}
	
	//  Method of deposit 
	public void deposit(int money){
		int money1 =money+this.balance;
		this.balance =money1;
		System.out.println(" Successfully deposited ："+money);
	}
	
	// Method of withdrawal 
	public void withdraw(int money){
		int money2 =this.balance-money;
		if (money2<0)
		{
			System.out.println(" Lack of balance , Failed to withdraw money ");
			return;
		}
		this.balance =money2;
		System.out.println(" Successfully removed :"+money);		
	}

}

Homework 3:

( encapsulation ) A class is known Student The code is as follows :
class Student{
String name;
int age;
String address ;
String zipCode ;
String mobile;
requirement :
1、 hold student All properties are private , And provide get/set Methods and appropriate construction methods .
2、 by student Class to add a - individual getPostAddress Method , Ask to return student Object's ” Address " and ” Zip code ”
Be careful : The address here is : address, It's not a memory address .

public class HomeWork
{
	public static void main(String[] args){
		//  No arguments 
		Student s1 =new Student();
		System.out.println(s1.getPostAddress());
		//  Ginseng 
		Student s2 =new Student(" The small white ",0," China "," Stick to the road ",null);
		System.out.println(s2.getPostAddress());

		//  Call to read the name getName
		System.out.println(s2.getName());
		}
		
}


class Student
{
	private String name;
	private int age;
	private String address ;
	private String zipCode ;
	private String mobile;
	
	//  Parameterless construction method 
	public Student(){
		
	}
	//  There are parameter construction methods ( The passed value is assigned to the private variable first )
	public Student(String name,int age,String address,String zipCode,String mobile){
		this.name =name;
		this.age =age;
		this.address =address;
		this.zipCode =zipCode;
		this.mobile =mobile;
	}
	
	public String getName(){
		return name;
	}
	public void setName(String name){
		this.name =name;
	}

	public String getAddress(){
		return address;
	}
	public void setAddress(String address){
		this.address =address;
	}

	public String getZipCode(){
		return zipCode;
	}
	public void setZipCode(String zipCode){
		this.zipCode =zipCode;
	}
	
	public String getPostAddress(){
		//return " Address :"+this.address+", Zip code :"+this.zipCode; // this Omission  this In fact, it is the same as quoting s1 equally 
		return " Address ："+this.getAddress()+", Zip code :"+this.getZipCode(); //  ditto 
	}
}