这段代码有什么问题?(多态性)

Question

我试图修复代码，但我不知道如何修复它。

1. print方法中，编写一个 for 循环以遍历数组形状中的每个元素并调用该方法 draw 。

2. 在方法TestCase2中，调用上面的方法print并将数组形状传递给此方法。

   package Polymorphism.test;
   import Polymorphism.*;
   
   public class TestShape {
   
       public static void main(String[] args) {
       int testCase = 1;
       switch (testCase)
       {
           case 1:
               TestCase1();
               break;
           case 2:
               TestCase2();
               break;
           default:
               System.out.println("Invalid test case selection!");
       }
       }
       public static void print()
       {
           Shape[] shapes;
           System.out.println("Test an array of Shape:");
           for(int i = 0; i < shapes.length; i++)
           {
               shapes[i].draw();
           }
       }
       public static void TestCase1()
       {
           Shape shape;
           shape = new Shape();
           shape.draw();
           shape = new Polygon();
           shape.draw();
           shape = new Rectangle();
           shape.draw();
           shape = new Square();
           shape.draw();
           shape = new Circle();
           shape.draw();
       }
       public static void TestCase2()
       {
           Shape[] shapes = {new Shape(), new Polygon(), new Rectangle(), new  Square(), new Circle(), new Polygon(), new Rectangle()};
           shapes.print();
       }
   }
   

Answer 1

在 TestCase2 方法中，调用上面的方法打印并将数组形状传递给此方法

这意味着

public static void print()

应该接受形状数组：

public static void print(Shape[] shapes)

你应该把这个数组传递给它，所以改变：

shapes.print();

自

TestShape.print(shapes);

您的打印方法将变为：

public static void print(Shape[] shapes)
{
    System.out.println("Test an array of Shape:");
    for(int i = 0; i < shapes.length; i++)
    {
        shapes[i].draw();
    }
}

Answer 2

您的问题在于

public static void print()
{
    Shape[] shapes;
    System.out.println("Test an array of Shape:");
    for(int i = 0; i < shapes.length; i++)
    {
        shapes[i].draw();
    }
}

您需要初始化形状数组，否则将得到NullPointerException

Answer 3

更改print()如下：

public static void print(Shape [] shapes)   // HERE PASS shapes AS ARGUMENT
{
    System.out.println("Test an array of Shape:");
    for(int i = 0; i < shapes.length; i++)
        {
            shapes[i].draw();
        }
    }
}

TestCase2()如下：

public static void TestCase2()
{
    Shape[] shapes = {
        new Shape(), new Polygon(), new Rectangle(),
        new  Square(), new Circle(), new Polygon(),
        new Rectangle()
    };
    print(shapes); // HERE CALL STATIC METHOD
}

Answer 4

您在打印中定义形状，但不填充它：

public static void print()
{
    Shape[] shapes;
    System.out.println("Test an array of Shape:");
    // shapes is empty always!!!!
    for(int i = 0; i < shapes.length; i++)
    {
        shapes[i].draw();
    }
}

将 Shape[] 形状定义为类的属性，以便在任何地方使用它：

package Polymorphism.test;
import Polymorphism.*;

public class TestShape {
// declare field!!!
Shape[] shapes;

public static void main(String[] args) {
int testCase = 1;
switch (testCase)
{
    case 1:
        TestCase1();
        break;
    case 2:
        TestCase2();
        break;
    default:
        System.out.println("Invalid test case selection!");

    // execute the print function
    print();
}
}
public static void print()
{
    System.out.println("Test an array of Shape:");
    for(int i = 0; i < shapes.length; i++)
    {
        shapes[i].draw();
    }
}
public static void TestCase1()
{
    shapes = new Shape[5];
    shape = new Shape();
    shapes[0] = shape;
    shape = new Polygon();
    shapes[1] = shape;
    shape = new Rectangle();
    shapes[2] = shape;
    shape = new Square();
    shapes[3] = shape;
    shape = new Circle();
    shapes[4] = shape;
}
public static void TestCase2()
{
    Shape[] shapes = {
         new Shape(), new Polygon(), new Rectangle(), 
         new  Square(), new Circle(), new Polygon(),
         new Rectangle()
   };
}