Abstract classes and interfaces (sorting out some knowledge points)

Abstract classes and interfaces

This is what I'm learning 《Java Language programming 》（Y.Daniel Liang） I took notes when I was studying .
1. The parent class defines the common behavior of related subclasses . Interfaces can be used to define the common behavior of classes （ Include unrelated classes ）.
2. Abstract classes cannot be used to create objects . An abstract class can contain abstract methods , These methods will be implemented in specific subclasses .
3. occasionally , A parent class is designed to be very abstract , So that it has no specific examples . Such a class is called abstract class .

public abstract class GeometricObject {
    
  private String color = "white";
  private boolean filled;
  private java.util.Date dateCreated;

  /** Construct a default geometric object */
  protected GeometricObject() {
    
    dateCreated = new java.util.Date();
  }

  /** Construct a geometric object with color and filled value */
  protected GeometricObject(String color, boolean filled) {
    
    dateCreated = new java.util.Date();
    this.color = color;
    this.filled = filled;
  }

  /** Return color */
  public String getColor() {
    
    return color;
  }

  /** Set a new color */
  public void setColor(String color) {
    
    this.color = color;
  }

  /** Return filled. Since filled is boolean, * the get method is named isFilled */
  public boolean isFilled() {
    
    return filled;
  }

  /** Set a new filled */
  public void setFilled(boolean filled) {
    
    this.filled = filled;
  }

  /** Get dateCreated */
  public java.util.Date getDateCreated() {
    
    return dateCreated;
  }

  /** Return a string representation of this object */
  public String toString() {
    
    return "created on " + dateCreated + "\ncolor: " + color +
      " and filled: " + filled;
  }

  /** Abstract method getArea */
  public abstract double getArea();

  /** Abstract method getPerimeter */
  public abstract double getPerimeter();
}

4. A class containing abstract methods must be declared as an abstract class .
5. The construction method of an abstract class is defined as protected, Because it is only used by subclasses . When creating an instance of a concrete subclass , The constructor of its parent class is called to initialize the data field defined in the parent class .
6. You can define an abstract class that does not contain abstract methods , This kind is used to define the base class of new subclasses .
7. Even if the parent of a subclass is concrete , This subclass can also be abstract .
8. Abstract classes can be used as a data type .
9. Interface Is a structure similar to a class , Contains only constants and abstract methods .
10. You can use interfaces as data types of reference variables or as the result of type conversions .
11. The relationship between classes and interfaces is called Interface inheritance .
12.Java Allow modifiers in interfaces to be ignored .