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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     15     0
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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