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定义一个抽象类GeometricObject 属性包括color colorFill
2023-06-15 13:31:14 深夜i     --     --
继承 抽象类 多态

我来分享一下我在学习《Java面向对象程序设计第三版》时整理的课后题答案。希望能对大家学习Java编程有所帮助。

以下是几个我认为比较重要的部分的答案：

## 1. 继承和多态

### 1.1

illed(),setFill()。定义类Circle和Rectangle分别继承GeometricObject并实现其方法。定义一个测试类TestGeometricObject，创建两个GeometricObject的对象circle和rectangle,然后遍历输出半径，颜色，填充状态、周长、面积。

### 1.2 答案



public abstract class GeometricObject {


  protected String color;


  protected boolean filled;


  public GeometricObject()


    color = "white";


    filled = false;


  


  public GeometricObject(String color, boolean filled)


    this.color = color;


    this.filled = filled;


  


  public boolean isFilled()


    return filled;


  


  public void setFilled(boolean filled)


    this.filled = filled;


  


  public String getColor()


    return color;


  


  public void setColor(String color)


    this.color = color;


  


  @Override


  public String toString() {


    return "GeometricObject{" +


        "color='" + color + '\'' +


        ", filled=" + filled +


        '}';


  }


  public abstract double getArea();


  public abstract double getPerimeter();


}


public class Circle extends GeometricObject {


  private double radius;


  public Circle()


    radius = 1.0;


  


  public Circle(double radius)


    this.radius = radius;


  


  public Circle(double radius, String color, boolean filled) {


    super(color, filled);


    this.radius = radius;


  }


  public double getRadius()


    return radius;


  


  public void setRadius(double radius)


    this.radius = radius;


  


  @Override


  public double getArea() {


    return Math.PI * Math.pow(radius, 2);


  }


  @Override


  public double getPerimeter() {


    return 2 * Math.PI * radius;


  }


}


public class Rectangle extends GeometricObject {


  private double width;


  private double height;


  public Rectangle()


    width = 1.0;


    height = 1.0;


  


  public Rectangle(double width, double height)


    this.width = width;


    this.height = height;


  


  public Rectangle(double width, double height, String color, boolean filled) {


    super(color, filled);


    this.width = width;


    this.height = height;


  }


  public double getWidth()


    return width;


  


  public void setWidth(double width)


    this.width = width;


  


  public double getHeight()


    return height;


  


  public void setHeight(double height)


    this.height = height;


  


  @Override


  public double getArea() {


    return width * height;


  }


  @Override


  public double getPerimeter() {


    return 2 * (width + height);


  }


}


public class TestGeometricObject {


  public static void main(String[] args) {


    GeometricObject circle = new Circle(5, "red", true);


    GeometricObject rectangle = new Rectangle(3, 4, "blue", false);


    System.out.println("Circle's radius is " + ((Circle)circle).getRadius());


    System.out.println("Circle's color is " + circle.getColor());


    System.out.println("Circle isFilled? " + circle.isFilled());


    System.out.println("Circle's perimeter is " + circle.getPerimeter());


    System.out.println("Circle's area is " + circle.getArea());


    System.out.println();


    System.out.println("Rectangle's width is " + ((Rectangle)rectangle).getWidth());


    System.out.println("Rectangle's height is " + ((Rectangle)rectangle).getHeight());


    System.out.println("Rectangle's color is " + rectangle.getColor());


    System.out.println("Rectangle isFilled? " + rectangle.isFilled());


    System.out.println("Rectangle's perimeter is " + rectangle.getPerimeter());


    System.out.println("Rectangle's area is " + rectangle.getArea());


  }


}

## 2. 接口和Lambda表达式

### 2.1 题目

定义接口Resizable，内含resize(double)方法，再用Resizable修改GeometricObject，然后编写一个程序ResiableCircle从Circle继承并实现Resizable接口,并且可以调整半径。

### 2.2 答案



public interface Resizable {


  void resize(double percent);


}


public abstract class GeometricObject {


  protected String color;


  protected boolean filled;


  public GeometricObject()


    color = "white";


    filled = false;


  


  public GeometricObject(String color, boolean filled)


    this.color = color;


    this.filled = filled;


  


  public boolean isFilled()


    return filled;


  


  public void setFilled(boolean filled)


    this.filled = filled;


  


  public String getColor()


    return color;


  


  public void setColor(String color)


    this.color = color;


  


  @Override


  public String toString() {


    return "GeometricObject{" +


        "color='" + color + '\'' +


        ", filled=" + filled +


        '}';


  }


  public abstract double getArea();


  public abstract double getPerimeter();


}


public class Circle extends GeometricObject {


  private double radius;


  public Circle()


    radius = 1.0;


  


  public Circle(double radius)


    this.radius = radius;


  


  public Circle(double radius, String color, boolean filled) {


    super(color, filled);


    this.radius = radius;


  }


  public double getRadius()


    return radius;


  


  public void setRadius(double radius)


    this.radius = radius;


  


  @Override


  public double getArea() {


    return Math.PI * Math.pow(radius, 2);


  }


  @Override


  public double getPerimeter() {


    return 2 * Math.PI * radius;


  }


}


public class ResizableCircle extends Circle implements Resizable {


  public ResizableCircle() {


    super();


  }


  public ResizableCircle(double radius) {


    super(radius);


  }


  public ResizableCircle(double radius, String color, boolean filled) {


    super(radius, color, filled);


  }


  @Override


  public void resize(double percent) {


    this.setRadius(this.getRadius() * percent / 100);


  }


}


public class TestResizableCircle {


  public static void main(String[] args) {


    ResizableCircle circle = new ResizableCircle(5, "red", true);


    System.out.println("Before resizing: radius=" + circle.getRadius() + ", area=" + circle.getArea());


    circle.resize(50);


    System.out.println("After resizing: radius=" + circle.getRadius() + ", area=" + circle.getArea());


  }


}

关键词：接口、Lambda表达式、多重继承

## 3. 异常处理

### 3.1 题目

编写一个简单的程序，从命令行传入一个文件名，读入该文件的内容。当文件不存在或读取失败时，抛出自定义异常FileReadFailException并处理。最后输出读取的文件内容。

### 3.2 答案



class FileReadFailException extends Exception {


  public FileReadFailException(String msg) {


    super(msg);


  }


}


public class TestFileReader {


  public static void main(String[] args) {


    try {


      FileReader file = new FileReader(args[0]);


      int ch;


      while ((ch = file.read()) != -1) {


        System.out.print((char) ch);


      }


      file.close();


    } catch (FileNotFoundException e) {


      System.out.println("File not found!");


    } catch (IOException e) {


      System.out.println("I/O Exception!");


    } catch (Exception e) {


      System.out.println("Other Exception!");


    } finally {


      System.out.println("\nFile reading finished.");


    }


  }


}

关键词：异常、自定义异常、异常处理

综上可知，学习Java编程需要不断练习做题，并深入理解Java面向对象的思想，才能够在编写程序时轻松自如。

  
  

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