CODEWITHSUNDEEP
java3dpointpoints
dtm247
dtm247

Reputation: 13

Cannot find z variable (3D Points - Java 24 hours)

I've just started learning Java so sorry if this is a noob question but I've been through each line of code and I don't understand what's going wrong. The code is from the book "Java in 24 hours".

The aim of the code is to take a starting 2D and 3D point, then move them and translate them. First it asks me to make a 3D point class:

package com.java24hours;

import java.awt.*;

public class Point3D extends Point
{
    public int z;

    public Point3D(int x, int y, int z)
    {
        super(x,y);
        this.z = z;
    }
    public void move(int x, int y, int z)
    {
        this.z = z;
        super.move(x,y);
    }
    public void translate(int x, int y, int z)
    {
        this.z += z;
        super.translate(x,y);
    }
}

It then calls for a tester class which uses the Points3D to move and translate the 3D points:

package com.java24hours;

import java.awt.*;

class PointTester
{
    public static void main(String[] arguments)
    {
        Point location1 = new Point(11,22);
        Point location2 = new Point3D(7,6,64);       

        System.out.println("The 2D point is at ("+location1.x + "," + location1.y +")");

        System.out.println("It's being moved to (4,11)");
        location1.move(4,11);
        System.out.println("The 2D point is now at (" + location1.x + "," + location1.y + ")");

        System.out.println("It's now being moved -10 in both the x and y axis");
        location1.translate(-10,-10);
        System.out.println("The 2D point is now at (" + location1.x +"," + location1.y + ")\n");

        System.out.println("The 3D point is at (" + location2.x + "," + location2.y + "," + location.z + ")");

        System.out.println("It's being moved to (10,22,71)");
        location2.move(10,22,71);
        System.out.println("The 3D point is now at (" + location2.x + "," + location2.y + "," + location2.z + ")");

        System.out.println("It's now going to be moved -20 units in the x y and z axis");
        location2.translate(-20,-20,-20);
        System.out.println("It's now at (" + location2.x + "," + location2.y + "," + location2.z + ")");
    }
}

This creates the following errors on the lines featuring location2.z:

cannot find symbol method move cannot be applied to given types method translate cannot be applied to given types

Source is here: https://www.informit.com/library/content.aspx?b=STY_Java2_24hours&seqNum=140

This is roughly what it should give me (I changed some of the wording):

The 2D point is located at (11, 22) 
    It's being moved to (4, 13) 
The 2D point is now at (4, 13) 
    It's being moved -10 units on both the x and y axes 
The 2D point ends up at (-6, 3) 

The 3D point is located at (7, 6, 64) 
    It's being moved to (10, 22, 71) 
The 3D point is now at (10, 22, 71) 
    It's being moved -20 units on the x, y and z axes 
The 3D point ends up at (-10, 2, 51)

I think my confusion is that I declared the z variable in the Point3D class, I created the newPoint3D which accepted three variables, and then when I try and use the third variable later on it can't find it.

Thanks

Upvotes: 1

Views: 450

Answers (5)

Stephen C
Stephen C

Reputation: 719239

The problem is this line:

    Point location2 = new Point3D(7,6,64);

You have created a Point3D instance and assigned it to a Point variable. The object that is currently in location2 at that point has a z field, but since you declared the variable as a Point ... the compiler has to assume that it is a Point instance, which doesn't have a z field.

Hence the compiler tells you that location2.z is a compilation error.

The fix is this:

    Point3D location2 = new Point3D(7,6,64);

OK, so why is Java designed to not allow that?

Consider an example like this:

    Point location2 = test ? new Point3D(7,6,64) : new Point(7,6);
    System.out.println(location2.z);

If the compiler accepted that, and test was false, what should happen? In a dynamically typed language, that would give you a runtime error because you attempted to access a field that does not exist. But Java is statically typed by design, and that is not acceptable.

In fact, Java takes the view that when you declare a variable has a type Point the following code can only treat its value as a Point.

Upvotes: 0

osama yaccoub
osama yaccoub

Reputation: 1866

Either define 'x' and 'move' in Point or refer to location1 and location2 by Point3D

Upvotes: 0

burglarhobbit
burglarhobbit

Reputation: 2291

You have a syntax problem in your code because you are using location.z instead of location2.z

Also, you are using Point class as a reference to create an object of Point3D. You cannot access variable z from a Point class due to Polymorphism concepts(I suggest you look into it) as the Point3D inherits the Point class and so, the variable z is defined within the Point3D class and not the Point.

Concluding, you should instantiate your 3D Point as below:

Point3D location2 = new Point3D(7,6,64);

Upvotes: 1

Mage Xy
Mage Xy

Reputation: 1803

I'm assuming that the base Point's move() method accepts two parameters. However, you're calling that method with three: location2.move(10,22,71);

Sure, you know that the Point contained in location2 is actually a Point3D, but the compiler thinks it's just a regular Point. Change the type of location2 to Point3D and you should be fine.

Upvotes: 0

William Morrison
William Morrison

Reputation: 11006

This is happening because you've declared your location2 variable as a Point. Its real type is Point3D, but you're referencing it as type Point and so Point3D variable z is not visible. (This is a fundamental principal of object oriented programming called polymorphism, but don't worry about that yet as you're just starting to learn!)

You can fix this by declaring your location2 variable as a Point3D, like so:

Point location1 = new Point(11,22);

//here's the fixed line of code.
Point3D location2 = new Point3D(7,6,64);

//[...] rest of your code

Upvotes: 0

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