为什么我不能有私有抽象方法？

Question

假设我有 classic:

public abstract class Mammal {
    private int numLegs;
    private String voice;
    private Coat coat;

    public abstract void eat();

    public abstract void speak();
    public abstract void sleep();  


    private abstract void ingestFood(Food f);  //The abstract method ingestFood in type Mammal can only set a visibility modifier, one of public or protected
}

有了这些具体的实现：

public class Dog extends Mammal {
     private int numLegs = 4;
     private String voice = "Woof!";
     private Coat coat = new Coat(COATTYPE.FUR, COATCOLOR.BROWN);

     @Override
     public void eat()
     { 
          Rabbit r = chaseRabbits(); 
          if (r != null) ingest(r); 
          else {
                   Garbage g = raidBin(); 
                   if (g!= null) ingest(g); 
               }


     }

     @Override
     private void ingest(Food f)
     {
         gobbleItAllUpInFiveSeconds(f); 
         stillFeelHungry(); 
     }
}

public class Human extends Mammal {
     private int numLegs = 2;
     private String voice = "Howdy!!";
     private Coat coat = new Coat(COATTYPE.SKIN, COATCOLOR.PINK);

     @Override
     public void eat()
     { 
          Food f = gotoSuperMarket();
          ingest(f); 


     }

     @Override
     private void ingest(Food f)
     {
         burp();  
     }
}

现在，我想要 Mammal class 中的一个方法，该方法可由哺乳动物的所有实例调用，例如

public String describe() {
     return "This mammal has " + this.numLegs + "legs, a " + this.coat.getColor() + " " this.coat.getCoatType() + " and says " + this.voice;
}

我的问题是，通过使 Mammal class 不抽象，是否可以单独创造哺乳动物？例如

 Mammal me = new Mammal();

你不应该这样做。

但是，我确实想要一些由父 class 实现的 public 方法，所有子 class 都调用它们，但每个方法都调用自己的私有方法。

Answer 1

你完全可以在抽象中实现方法class:

"Abstract classes are similar to interfaces. You cannot instantiate them, and they may contain a mix of methods declared with or without an implementation."

https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html

Answer 2

如果您希望未实现的方法在子类上有不同的行为，请将它们声明为抽象方法，并且要继承的方法像普通方法一样保留它们。还使用 protected 而不是 private 以便继承类:

public abstract class Mammal
{
  protected int numLegs;
  protected String voice;
  protected Coat coat;

  abstract void eat();
  abstract void speak();
  abstract void sleep();

  public String describe()
  {
     return "This mammal has " + this.numLegs + "legs, a " 
     + this.coat.getColor() + " " this.coat.getCoatType() + " and says " + this.voice;
  }
}

并使用构造函数初始化变量和实现抽象方法：

public class Dog extends Mammal
{
  public Dog(){
     this.numLegs = 4;
     this.voice = "Woof!";
     this.coat = new Coat(COATTYPE.FUR, COATCOLOR.BROWN);
  }
  void eat(){
    System.out.println("eating like a dog");
  }
  void speak(){
    System.out.println("speaking like a dog");
  }
  void sleep(){
    System.out.println("sleeping like a dog");
  }
}

public class Human extends Mammal
{
  public Human(){
     this.numLegs = 2;
     this.voice = "Howdy!!";
     this.coat = new Coat(COATTYPE.SKIN, COATCOLOR.PINK);
  }
  void eat(){
    System.out.println("eating like a human");
  }
  void speak(){
    System.out.println("speaking like a human");
  }
  void sleep(){
    System.out.println("sleeping like a human");
  }
}

Answer 3

回答标题中的问题（“为什么我不能有私有抽象方法？”）：

不能有private abstract方法，因为abstract方法需要在子类中实现。但是 private 方法在子类中不可见。

(如果你想要一个只在子类中可见的方法，而不是公开的，那么你需要使方法protected)

所以你永远无法实现 private abstract 方法。这就是 Java 不允许它们的原因 - 它们没有意义。

为什么我不能有私有抽象方法？

Why can't I have an private abstract method?

java

oop

inheritance

abstract