我可以在每个对象上使用contains方法吗？

Question

Can the contains method be used on instances of classes I've written myself?

I've created a class named Ana and in another class, I've created two instance of it called cookie and maccaroni . This is one of them:

Ana cookie = new Ana(cc.c, 20, 450);

At some point I am trying to invoke

if (maccaroni.contains(cookie.image(), cookie.xi(), cookie.yi())
{
    return true;
}

but Netbeans underlined it to indicate a mistake. When I hovered over it, it said:

 no suitable method found for contains(Image,int,int) method Component.contains(int,int) is not applicable (actual and formal argument lists differ in length) method Component.contains(Point) is not applicable (actual and formal argument lists differ in length) method JComponent.contains(int,int) is not applicable (actual and formal argument lists differ in length) 

Why the following code is regarded as a mistake by Netbeans?

This is the class I'm referring to.

import java.awt.Graphics;
import java.awt.Image;
import javax.swing.JComponent;

public class Ana extends JComponent
{
    public static Image p;
    public static int xx;
    public static int yy;

    public Ana(Image io, int x, int y)
    {
        p = io;
        xx = x;
        yy = y;
        repaint();
    }

    @Override
    public void paint(Graphics g)
    {
       g.drawImage(p, xx, yy, null);
    }


    public Image image()
    {
        return p;
    }

    public int xi()
    {
        return xx;
    }

    public int yi()
    {
        return yy;
    }
}

Answer 1

There is no contains() method defined in Ana . There is a contains() method defined in JComponent . It takes two integers. You're trying to invoke it with an image and two integers. You can probably get what you need by writing it like this:

if (maccaroni.contains(cookie.xi(), cookie.yi())
{
    return true;
}

So the kind of error in this code is: it's a compile error. NetBeans isn't just underlining it, there is a message somewhere on your screen telling you what is wrong. You may have to mouse over the underline or look in a different section of the screen--I don't use NetBeans on a regular basis so I don't recall exactly--but I assure you the message is there.

And no, you can't invoke contains() on any object, only on objects which classes define a contains() method (with proper parameters), or that inherit such a method from one of their supertypes (from an ancestor class or interface they implement).

Methods which all objects could use are those inherited from java.lang.Object class, since all classes explicitly or implicitly extend it (parent of parent of parent... finally will need to extend the Object class).

But there is no contains(Image, int, int) method defined on java.lang.Object . The only contains() method available to instances of the Ana class is the one inherited from JComponent which is contains(int, int) . As you see it accepts two int arguments, which prevents you from using it with the additional Image argument.

By the way, as @PShemo has noted, your fields p , xx and yy should likely be private and most definitely not static .