为什么多态性不能像我期望的那样起作用？

Question

I'm quite new to Java and have come accross to a strange behaviour that I can not explain why this happens or where is the mistake in my code.

Here's the code:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;

abstract class Shape {
    public abstract void printMe(String no);
}

final class Circle extends Shape {
    @Override
    public void printMe(String no){
        System.out.println("This is Circle no: " + no);
    }
}

final class Square extends Shape {
    @Override
    public void printMe(String no) {
        System.out.println("This is Square no: " + no);
    }
}

final class Triangle extends Shape {
    @Override
    public void printMe(String no) {
        System.out.println("This is Triangle no: " + no);
    }
}

public class Foo {
    private ArrayList<Shape> shapes;

    public Foo(){
        this.shapes   = new ArrayList<Shape>();

        this.shapes.add(new Circle());
        this.shapes.add(new Square());
        this.shapes.add(new Triangle());
    }

    public void printShapes(ArrayList<String> numbers){
        for(String s:numbers){
            Iterator<Shape> iter = this.shapes.iterator();
            Shape shape = iter.next();
            shape.printMe(s);
        }
    }

    public static void main(String[] args) {
        ArrayList<String> numbers = new ArrayList<String>(Arrays.asList("1", "2", "3"));
        Foo foo = new Foo();
        foo.printShapes(numbers);
    }
}

The output I'd expect would be:

This is Circle no: 1
This is Square no: 2
This is Triangle no: 3

However, the output I get is:

This is Circle no: 1
This is Circle no: 2
This is Circle no: 3

What am I doing wrong?

Answer 1

将此行拉出循环：

 Iterator<Shape> iter = this.shapes.iterator();

Answer 2

You're always getting a new iterator--instead of using the same one.

It's not clear to me why you're doing it like this anyway; either pass an integer and loop until it runs out, or iterate over the shapes and keep a counter. Passing a string array strikes me as clumsy.

public void printShapes() {
    int i = 1;
    for (Shape shape : shapes) {
        shape.printMe(i++); // And modify the method to take an int.
    }
}

I'm uncomfortable with a shape needing to be aware it can have a position. If this is a requirement, create a "PositionalShape" or something (but ew), or have shapes output a string representation that can be composited with additional info like a list position, or create a shape decorator, etc.

---

// (If you're really trying to print the first n shapes)
public void printShapes(int n) {
    Iterator<Shape> iter = shapes.iterator();
    for (int i = 0; i < n; i++) {
        Shape shape = iter.next();
        shape.printMe("" + i+1);
    }
}

Answer 3

Look at Iterator<Shape> iter inside your loop.

  public void printShapes(ArrayList<String> numbers){
        for(String s:numbers){
            Iterator<Shape> iter = this.shapes.iterator();
            Shape shape = iter.next();
            shape.printMe(s);
        }
    }

You are always grabbing the first shape (initialize iterator, grab next)

Answer 4

I suspect you need to see this in your debugger however with

Iterator<Shape> iter = this.shapes.iterator();
Shape shape = iter.next();
shape.printMe(s);

You are using the first share every time (which is a Circle)

You can move the iter declaration outside the loop to fix it.

Answer 5

The following 2 lines need to be in a loop: Shape shape = iter.next(); shape.printMe(s); Shape shape = iter.next(); shape.printMe(s);

Answer 6

您总是重置迭代器：

Iterator<Shape> iter = this.shapes.iterator();