05 - 深入理解封装、抽象、继承、多态

2021-08-06  本文已影响0人  舍是境界

在什么是面向对象中，简单介绍了面向对象的一些基本概念和知识点，比如，什么是面向对象编程，什么是面向对象编程语言等等。其中，我们还提到，理解面向对象编程及面向对象编程语言的关键就是理解其四大特性：封装、抽象、继承、多态。不过，对于这四大特性，光知道它们的定义是不够的，我们还要知道每个特性存在的意义和目的，以及它们能解决哪些编程问题

对于这四大特性，尽管大部分面向对象编程语言都提供了相应的语法机制来支持，但不同的编程语言实现这四大特性的语法机制可能会有所不同。所以，本文在讲解四大特性的时候，并不与具体某种编程语言的特定语法相挂钩，同时，也希望你不要局限在你自己熟悉的编程语言的语法思维框架里

封装（Encapsulation）

public class Wallet {
  private String id;
  private long createTime;
  private BigDecimal balance;
  private long balanceLastModifiedTime;
  // ...省略其他属性...
  public Wallet() {
     this.id = IdGenerator.getInstance().generate();
     this.createTime = System.currentTimeMillis();
     this.balance = BigDecimal.ZERO;
     this.balanceLastModifiedTime = System.currentTimeMillis();
  }
  // 注意：下面对get方法做了代码折叠，是为了减少代码所占文章的篇幅
  public String getId() { return this.id; }
  public long getCreateTime() { return this.createTime; }
  public BigDecimal getBalance() { return this.balance; }
  public long getBalanceLastModifiedTime() { return this.balanceLastModifiedTime;  }
  public void increaseBalance(BigDecimal increasedAmount) {
    if (increasedAmount.compareTo(BigDecimal.ZERO) < 0) {
      throw new InvalidAmountException("...");
    }
    this.balance.add(increasedAmount);
    this.balanceLastModifiedTime = System.currentTimeMillis();
  }
  public void decreaseBalance(BigDecimal decreasedAmount) {
    if (decreasedAmount.compareTo(BigDecimal.ZERO) < 0) {
      throw new InvalidAmountException("...");
    }
    if (decreasedAmount.compareTo(this.balance) > 0) {
      throw new InsufficientAmountException("...");
    }
    this.balance.subtract(decreasedAmount);
    this.balanceLastModifiedTime = System.currentTimeMillis();
  }
}

抽象（Abstraction）

public interface IPictureStorage {
  void savePicture(Picture picture);
  Image getPicture(String pictureId);
  void deletePicture(String pictureId);
  void modifyMetaInfo(String pictureId, PictureMetaInfo metaInfo);
}
public class PictureStorage implements IPictureStorage {
  // ...省略其他属性...
  @Override
  public void savePicture(Picture picture) { ... }
  @Override
  public Image getPicture(String pictureId) { ... }
  @Override
  public void deletePicture(String pictureId) { ... }
  @Override
  public void modifyMetaInfo(String pictureId, PictureMetaInfo metaInfo) { ... }
}

继承（Inheritance）

多态（Polymorphism）

public class DynamicArray {
  private static final int DEFAULT_CAPACITY = 10;
  protected int size = 0;
  protected int capacity = DEFAULT_CAPACITY;
  protected Integer[] elements = new Integer[DEFAULT_CAPACITY];
  
  public int size() { return this.size; }
  public Integer get(int index) { return elements[index];}
  //...省略n多方法...
  
  public void add(Integer e) {
    ensureCapacity();
    elements[size++] = e;
  }
  
  protected void ensureCapacity() {
    //...如果数组满了就扩容...代码省略...
  }
}
public class SortedDynamicArray extends DynamicArray {
  @Override
  public void add(Integer e) {
    ensureCapacity();
    int i;
    for (i = size-1; i>=0; --i) { //保证数组中的数据有序
      if (elements[i] > e) {
        elements[i+1] = elements[i];
      } else {
        break;
      }
    }
    elements[i+1] = e;
    ++size;
  }
}
public class Example {
  public static void test(DynamicArray dynamicArray) {
    dynamicArray.add(5);
    dynamicArray.add(1);
    dynamicArray.add(3);
    for (int i = 0; i < dynamicArray.size(); ++i) {
      System.out.println(dynamicArray.get(i));
    }
  }
  
  public static void main(String args[]) {
    DynamicArray dynamicArray = new SortedDynamicArray();
    test(dynamicArray); // 打印结果：1、3、5
  }
}

public interface Iterator {
  boolean hasNext();
  String next();
  String remove();
}
public class Array implements Iterator {
  private String[] data;
  
  public boolean hasNext() { ... }
  public String next() { ... }
  public String remove() { ... }
  //...省略其他方法...
}
public class LinkedList implements Iterator {
  private LinkedListNode head;
  
  public boolean hasNext() { ... }
  public String next() { ... }
  public String remove() { ... }
  //...省略其他方法... 
}
public class Demo {
  private static void print(Iterator iterator) {
    while (iterator.hasNext()) {
      System.out.println(iterator.next());
    }
  }
  
  public static void main(String[] args) {
    Iterator arrayIterator = new Array();
    print(arrayIterator);
    
    Iterator linkedListIterator = new LinkedList();
    print(linkedListIterator);
  }
}

class Logger:
    def record(self):
        print(“I write a log into file.”)
        
class DB:
    def record(self):
        print(“I insert data into db. ”)
        
def test(recorder):
    recorder.record()
def demo():
    logger = Logger()
    db = DB()
    test(logger)
    test(db)

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