Question

我尝试修改代码，但我不知道如何修复它。

在print方法中，编写for循环以循环遍历数组形状中的每个元素，并调用方法draw。

在TestCase2方法中，调用上面的方法print并将此方法传递给此方法。

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print并将数组形状传递给此方法

这意味着

public static void print()

应该接受Shapes数组：

public static void print(Shape[] shapes)

你应该将那个数组传递给它，所以改变：

shapes.print();

到

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

您的问题是

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，否则您将获得NullPointerException

Answer 3

您可以在打印中定义形状，但不要填写它：

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()
   };

}

Answer 4

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();
        }
    }
}

和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
}

这段代码有什么问题？（多态性）

4 个答案: