There are many ways to write a calculator, such as:

package Recursion; /** * Created by lirui on 2018/12/6. */ public class JiSuanQi { int a; int b; public JiSuanQi(int a, int b) { this.a = a; this.b = b; } public int add(int a, int b) {return a + b; } public int sub(int a, int b) {return a-b; }}Copy the code

But this kind of writing is not very good, according to the principle of single responsibility to build, calculator not only has addition, subtraction, multiplication, division, if subsequent to add multiplication, division, that is about to change in the original class, add these two methods, but if, in accordance with the single responsibility principle, a class focus on doing one thing, take apart in a class, such as:

1. Add class AddJiSuanQi

public class AddJiSuanQi extends JiSuanQi { public AddJiSuanQi(int a, int b) { super(a, b); } public int add(int a, int b) {return a + b; }}Copy the code

2. Subtraction class SubJiSuanQi

public class SubJiSuanQi extends JiSuanQi { public SubJiSuanQi(int a, int b) { super(a, b); } public int add(int a, int b) {return a-b; }}Copy the code

By separating addition and subtraction into a single class, if you need to add multiplication and division later on, you can simply add these two classes without making any changes to the original class.

1. The class diagram
   
   
2. Code implementation

/** * Created by lirui on 2018/12/6. */ public IBook {// Book name public String getName(); Public int getPrice(); Public String getAuthor(); }Copy the code

package Book; /** * Created by lirui on 2018/12/6. */ public class implements IBook {// book name private String name; Private int price; Private String author; public NovelBook(String name, int price, String author) { this.name = name; this.price = price; this.author = author; } @Override public String getName() { return this.name; } @Override public int getPrice() { return this.price; } @Override public String getAuthor() { return this.author; }}Copy the code

package Book; import java.text.NumberFormat; import java.util.ArrayList; /** * Created by lirui on 2018/12/6. */ public class BookStore { private final static ArrayList<IBook> bookList=new ArrayList<>(); Booklist. add(new NovelBook(" Romance of The Three Kingdoms ",100," Lo Guanzhong ")); static {booklist. add(new NovelBook(" Romance of The Three Kingdoms ",100," Lo Guanzhong ")); Booklist.add (new NovelBook(" journey to the West ",200," Wuchen-en ")); Booklist.add (new NovelBook(" Dream of Red Mansions ",300," Cao Xuechen ")); } / / simulate the bookstore to buy books public static void main (String [] args) {NumberFormat format. = NumberFormat getCurrencyInstance (); format.setMaximumFractionDigits(2); System. The out. Println (" = - = - = - = - = - = - = - = bookstore sell books record - = - = - = - = - = - = - = - "); For (IBook book:bookList){system.out.println (" book name: "+book.getName()+"\t book author: "+ book. GetAuthor () +" \ t books price: "+ format. The format (book. GetPrice () / 100.0) +" RMB "); }}} running results - = - = - = - = - = - = - = - = bookstore sell books record - = - = - = - = - = - = - = - name of books: the romance of The Three Kingdoms: author luo guanzhong book price: RMB 1.00 books name: journey to the west book author: wu chengen books price: RMB 2.00 books name: A Dream of Red Mansions by Cao XueqinCopy the code

The project was put into operation and the bookstore made a profit. However, in order to expand the market, the bookstore decided to give a 20% discount for more than 40 yuan and a 10% discount for less than 40 yuan. How to solve this problem?

1. The first method:

Modify an interface. Add a new method on the IBook, getOffPrice(), for discounting, that all implementation classes implement. However, the consequence of such modification is that the implementation class NovelBook should be modified, and the main method of BookStore should also be modified. Meanwhile, Ibook as an interface should be stable and reliable, and should not change frequently, otherwise the interface as a contract will lose its effectiveness. Other books that don’t want to be discounted will also have to be discounted for implementing the book interface, so the scheme is rejected.

2. The second method:

Modify the implementation class. Modifying a method in an NovelBook class to implement discount processing directly in getPrice() is one of the most common ways you can do this in your project. You can make some business (or bug fix) changes by replacing class files. This is an excellent approach when the project has a clear charter (intra-team constraints) or a good architectural design. However, this method still has defects, for example, the procurement of books is also to see the price, because the method has realized the discounted price, so the procurement personnel see the discounted price, which produces the effect of information blinding, resulting in information asymmetry and decision-making errors. The scheme is also not an optimal one.

3. The third way:

Optimal solution, through extension to achieve change. Add a subclass OffNovelBook that overrides the getPrice method. High-level modules (i.e., static module areas) generate new objects through the OffNovelBook class to accomplish the task of developing business changes. Good idea, low risk

Code implementation

/** * Create by lirui on 2018/12/6. */ public class extends NovelBook {public OffNovelBook(String)  name, int price, String author) { super(name, price, author); } @override public int getPrice(){// getPrice = super.getprice (); int offPrice=0; if (prePrice>=200){ offPrice=prePrice*80/100; }else{ offPrice=prePrice*90/100; }return offPrice; }}Copy the code

The bookstore class

/** * Created by lirui on 2018/12/6. */ public class BookStore { private final static ArrayList<IBook> bookList=new ArrayList<>(); Static {booklist. add(new OffNovelBook(" Romance of The Three Kingdoms ",100," Novelbook ")); Booklist.add (new OffNovelBook(" journey to the West ",200," Wuc en ")); Booklist.add (new OffNovelBook(" Dream of Red Mansions ",300," Cao Xueqing ")); } / / simulate the bookstore to buy books public static void main (String [] args) {NumberFormat format. = NumberFormat getCurrencyInstance (); format.setMaximumFractionDigits(2); System. The out. Println (" = - = - = - = - = - = - = - = bookstore sell books record - = - = - = - = - = - = - = - "); For (IBook book:bookList){system.out.println (" book name: "+book.getName()+"\t book author: "+ book. GetAuthor () +" \ t books price: "+ format. The format (book. GetPrice () / 100.0) +" RMB "); }}} running results - = - = - = - = - = - = - = - = bookstore sell books record - = - = - = - = - = - = - = - name of books: the romance of The Three Kingdoms: author luo guanzhong book price: RMB 0.90 books name: journey to the west book author: wu chengen books price: RMB 1.60 books name: A Dream of Red Mansions by Cao XueqinCopy the code

Inductive change:

Logic changes. Change only one logic without involving other modules. For example, the original algorithm is AB + C, and now ab* C is required, which may be accomplished by modifying the methods in the original class, on the premise that all dependent or associated classes are processed according to the same logic. Submodule changes. A module change, affect other modules, especially a low-level module changes must cause the change of high-level modules, it changes, done in by extending the high-level module change is inevitable, just books discount price is similar to the processing module, and even cause the change of the interface that part of the change.

4. Extended interface and extended implementation: computer books have been added to the bookstore, which has a unique feature: for what field

Code implementation

/** * Created by lirui on 2018/12/6. */ public interface IComputerBook extends IBook{// Computer book scope public String getScope(); }Copy the code

** * Created by Lirui on 2018/12/6. */ Public class ComputerBook implements IComputerBook {private String name; private int price; private String author; private String scope; public ComputerBook(String name,int price,String author,String scope){ this.name=name; this.price=price; this.author=author; this.scope=scope; } @Override public String getName() { return this.name; } @Override public int getPrice() { return this.price; } @Override public String getAuthor() { return this.author; } @Override public String getScope() { return this.scope; }}Copy the code

/** * Created by lirui on 2018/12/6. */ public class BookStore {private final static ArrayList<IBook> bookList=new  ArrayList<>(); Booklist. add(new NovelBook(" Romance of The Three Kingdoms ",100," Lo Guanzhong ")); static {booklist. add(new NovelBook(" Romance of The Three Kingdoms ",100," Lo Guanzhong ")); Booklist.add (new NovelBook(" journey to the West ",200," Wuchen-en ")); Booklist.add (new NovelBook(" Dream of Red Mansions ",300," Cao Xuechen ")); Booklist.add (New ComputerBook("Think in Java",400,"Bruce Eckel"," Programming Languages "); } / / simulate the bookstore to buy books public static void main (String [] args) {NumberFormat format. = NumberFormat getCurrencyInstance (); format.setMaximumFractionDigits(2); System. The out. Println (" = - = - = - = - = - = - = - = bookstore sell books record - = - = - = - = - = - = - = - "); For (IBook book:bookList){system.out.println (" book name: "+book.getName()+"\t book author: "+ book. GetAuthor () +" \ t books price: "+ format. The format (book. GetPrice () / 100.0) +" RMB "); }} running results - = - = - = - = - = - = - = - = bookstore sell books record - = - = - = - = - = - = - = - name of books: the romance of The Three Kingdoms: author luo guanzhong book price: RMB 1.00 books name: journey to the west: the author wu chengen books price: RMB 2.00 books name: Think in Java by Bruce Eckel. Think in Java by Bruce Eckel. Think in JavaCopy the code

An interface should not rely on interfaces it does not need, and a class’s dependence on another class should be based on the smallest interface.

1. When a class that provides an interface is not the smallest interface for its subclass, its subclass must implement some unnecessary functions when implementing the class. As a result, the system becomes bloated and difficult to maintain.
2. When class A depends on B through interface I, and class C depends on D through interface I, then for classes A and C, if interface I is not the minimum interface, then classes B and D must implement methods they don’t need.
3. To solve the problem, comply with the principle of interface isolation. “Fat” interface I is split into several independent interfaces, and class A and class C establish A dependency relationship with their required interface classes respectively. So that the classes they depend on don’t need to implement methods they don’t need.
4. Example scenario: a senior three student needs to take a simulation test. Due to the difference in the teaching content of arts and science, the test content is also different. If the content that takes an exam now has language, mathematics, geography, physics these subjects. As liberal arts students, they only test Chinese, math and geography. As science students, they have to test Chinese, math and physics. Use Java to do this

There is no code to implement interface isolation

Test subjects interface class

public interface IExam {

    public void chinese();
    public void math();
    public void physics();
    public void geograp();
}
Copy the code

Arts examination

Public class implements IExam {@override public void Chinese () {system.out.println (" implements "); } @override public void math() {system.out.println (" math "); } @override public void physics() {} @override public void geograp() {system.out.println (" geography "); }}Copy the code

Science Examination

Public class implements IExam {@override public void Chinese () {system.out.println (" implements "); } @override public void math() {system.out.println (" math "); } @override public void physics() {system.out.println (" physics "); } @Override public void geograp() { } }Copy the code

There are obvious problems with this implementation, as to why an empty method would be so embarrassing in a class. If the subjects of arts and sciences are not only these four, but also biology, history and so on, there will be more empty methods. This is where the principle of interface isolation is used, so that the dependencies between classes are based on the principle of minimal interface.Copy the code

5. Implement the test interface base class with interface isolation principle

public interface IExam {

    public void chinese();
    public void math();
}
Copy the code

Liberal Arts Interface Class

*/ public interface IArtExam extends IExam {public void geograp(); */ public interface IArtExam extends IExam {public void geograp(); }Copy the code

Science Interface

*/ public interface extends IExam {public void physics(); */ public void physics(); }Copy the code

Arts examination

Public class ArtsExam implements IArtExam {@override public void Chinese () {system.out.println (" implements "); } @override public void math() {system.out.println (" math "); } @override public void geograp() {system.out.println (" geography "); }}Copy the code

Science Examination

Public implements IPhyExam {@override public void Chinese () {system.out.println (" implements "); } @override public void math() {system.out.println (" math "); } @override public void physics() {system.out.println (" physics "); }}Copy the code

However, when using the interface isolation principle, you need to control the interface granularity according to the situation. Too small interfaces will cause the interface overflow in the system, which is not conducive to maintenance. If the interface size is too large, the interface isolation rule is violated and the interface size is too large. So the general interface is just to provide custom methods for dependent classes, and don’t let customers implement methods they don’t need.