The code provided defines a `Rectangle` class that extends a...

April 3, 2025 at 03:17 PM

package rectangle;
/** 3/18/2025
This version uses inheritance from class Shape
* We get everything from our parent, but we can only call or access the public
* members. 
* private fields are inherited but are not accessible.
* 
* toString() should represent fields that are inherited as well.
* So we have "our own fields" and the "inherited fields"
* General rule: we deal with our own fields and we ask our parent
* to deal with the inherited fields.
* 
* Who's responsibility is it to INITIALIZE the stuff that I got from my parent
* (i.e. the inherited fields)?
* Parent's --- parent has access to all private fields and it knows itself
* and understands itself and knows how to initialize itself.
* 
* Next time: we will show 3 layers so we can see chaining of methods and 
* constructors.
*/
import java.util.Scanner;

public class Rectangle extends Shape {
// state (instance variables)
    private double length=0.0;
    private double width=0.0;
    private static final String DEFCOLOR="blue";
    private static int countObj = 0;
    
//  no-arg constructor
    public Rectangle() {
        super(DEFCOLOR, false); // invoke the parent's constructor
        length=width=1.0;
      //  setColor(DEFCOLOR); this is OK but not the best way to initialize
        countObj++;
    }
//  parameter constructor
    public Rectangle(double length, double width, String color) throws Exception {
        // first, ask your parent to initialize the inherited fields
        // called: constructor chaining 
        super(color, true);
        // then, initialize your own stuff
        setLength(length);
        setWidth(width);
        countObj++;
    }
// overloaded constructor
    public Rectangle(double dim) throws Exception {
        this(dim, dim,  DEFCOLOR);
        /*********
        setLength(dim);
        setWidth(dim);
        countObj++;
        **********/
    }
// behavior
    // getters (accessors)
    public double getLength() {
        return length;
    }
    public double getWidth() {
        return width;
    }
    public static int getCountObj() {
        return countObj;
    }
    // setters (mutator)
    public void setLength(double length) throws Exception {
        if (length<0) {
            throw new Exception("negative length not allowed!");
        } 
        this.length=length;
    }
    public void setWidth(double width) throws Exception {
        if (width<0) {
             throw new Exception("negative width not allowed!");
        }
        this.width=width; // set the this object's width to the parameter width
    }
    @Override
    public String getColor() {
        return super.getColor().toUpperCase(); // chaining
    }
    // toString method creates a textual representation of an object
    /* overriding means writing a method with the same signtature as an 
    a method inherited from the parent. This method HIDES the parent's method.
    In other words, you will not be able to call the Shape class's toString 
    method outside of the Rectangle class. */
    @Override
    public String toString() {
        return "length: " + this.length + " width: " + this.width + " "
              //  " color: " + getColor() + " date created: " + getDateCreated()
              //  + " filled: " + isFilled();
        // how about if we "rely on our parent" to convert all my inherited fields to a string
               + super.toString();
    }
    // static method meaning "belonging to the class and not to an instance"
    // no 'this' object in a static method
    public static Rectangle newRect() throws Exception {
        return new Rectangle(1,2, DEFCOLOR );
    }
    /**
     * This method reads in 2 double values from the Scanner sc parameter.
     * Then, it creates a new Rectangle object initialized to the values read in
     * and returns the new Rectangle.
     * @param sc
     * @return 
     */
    public static Rectangle read(Scanner sc) throws Exception {
        double len=sc.nextDouble();
        double w=sc.nextDouble();
        return new Rectangle(len, w, DEFCOLOR);
    }
    /**
     * this method doubles the length of the 'this' object
     */
    public void doubleLength() {
        length*=2;
    }
    public double perimeter() {
        return length*2+width*2;
    }
    @Override
    public double area() {
        return length*width;
    }
    public boolean equals(Rectangle other) {
        return this.length==other.length && this.width==other.width;
    }
    public int compareTo(Rectangle other) {
        if (area()<other.area()) return -1;
        if (area()>other.area()) return 1;
        return 0;
    }
}
set 限制解除

The code provided defines a Rectangle class that extends a parent class Shape. Here's a breakdown of what the code does:

Package and Imports

It belongs to the rectangle package.
It imports java.util.Scanner.

Purpose

The Rectangle class represents a rectangle while demonstrating object-oriented principles such as inheritance from the Shape class, encapsulation, and method overriding.

Key Components and Behavior

1. Instance Variables (State):

length and width: The dimensions of the rectangle. Both are private fields with default values of 0.0.
DEFCOLOR: A constant representing the default color for a rectangle (set to "blue").
countObj: A static variable keeping track of the number of Rectangle objects created.

2. Constructors:

No-argument constructor (public Rectangle()):
- Calls the parent class's constructor with a default color ("blue") and false for filling.
- Initializes both length and width to 1.0.
- Increments countObj.
Parameter constructor (public Rectangle(double length, double width, String color)):
- Calls the parent class's constructor (constructor chaining) with specific color and filled status.
- Initializes length and width using setters, ensuring no negative values.
- Increments countObj.
Overloaded constructor (public Rectangle(double dim)):
- Square-like constructor that initializes a rectangle with equal length and width, using the above parameter constructor.

3. Accessors and Mutators:

getLength() and setLength(double length): Access and modify length.
- The setter throws an exception if length is negative.
getWidth() and setWidth(double width): Access and modify width.
- Similarly, the setter prohibits negative values.
getColor() (Overridden): Access the color, converted to uppercase. It delegates to the parent class's getColor() method.
getCountObj(): Returns the total number of Rectangle objects created.

4. Area, Perimeter, and Behavior:

area() (Overridden): Calculates the area of the rectangle (length * width). Overrides the parent class's implementation.
perimeter(): Calculates the perimeter (2 * (length + width)).
doubleLength(): Doubles the rectangle's length.
equals(Rectangle other): Checks whether two rectangles are equal by comparing their length and width.
compareTo(Rectangle other): Compares rectangles based on their areas. Returns:
- -1 if the current rectangle's area is smaller.
- 1 if the current rectangle's area is larger.
- 0 if they are equal.

5. Overridden Methods - `toString()`:

Constructs a textual representation of the object, incorporating both the current class's fields and the parent class's fields via super.toString(). It hides the parent class's toString() outside the Rectangle class.

6. Static Utility Methods:

newRect(): A factory method that creates a new rectangle with dimensions 1x2 and a blue color.
read(Scanner sc): Reads two double values (representing length and width) from a Scanner instance and creates a rectangle.

Summary

This Rectangle class:

Demonstrates inheritance from a Shape parent class.
Handles initialization of both inherited (Shape) and unique (Rectangle) fields through constructor chaining.
Validates dimensions using setters to prevent invalid states (e.g., negative dimensions).
Implements overriding, such as redefining the toString() and area() methods from the parent class.
Tracks object creation with the static counter countObj.
Defines utility methods for creating (newRect) and reading (read) rectangles.

Chaining multiple constructors and methods provides flexibility and reusability, showcasing good object-oriented programming practices.

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