Referencing basic Java object in another class

Question

I'm a beginner developing in Java, and I have run into a bit of a problem referencing an object (from a different class) in a class.

This is the code I used to create the object, from the file "Neighborhoods.java".

public class Neighborhoods {

    // variables
    String name;
    int vertices;
    double[] latCoords;
    double[] longCoords;

    public Neighborhoods() {
        Neighborhoods fisherHill = new Neighborhoods();
        fisherHill.name = "Fisher Hill";
        fisherHill.vertices = 4;
        fisherHill.latCoords = new double[] {42.331672, 42.326342, 42.334464, 42.335733};
        fisherHill.longCoords = new double[] {-71.131277, -71.143036, -71.148615, -71.141062};
    }
}

I then tried to use the object I created, "fisherHill" (from the class Neighborhoods) in my main class when calling a function from another different class (called "inPolygon").

inPolygon.check(Neighborhoods.fisherHill.vertices);

But for some reason, I'm getting an error when I try to reference the fisherHill object, as it says it can't be found.

I know I'm making some dumb mistake here, but I'm not sure what it is. Sorry if I used the wrong terminology in describing the code. Any help or suggestions would be much appreciated.

Why are you making recursive call in your constructor? That will at some point of time result in `StackOverflowError`. — Rohit Jain, Jul 23 '13 at 18:54
`Neighborhoods fisherHill = new Neighborhoods();` Wrong. Use `this` instead (e.g. `this.name = "Fisher Hill";`). Please get a Java book. — m0skit0, Jul 23 '13 at 18:55
Ok thanks for the help. I'll rewrite this a bit more properly. — Rowan K., Jul 23 '13 at 18:56

score 1 · Answer 1 · edited May 23 '17 at 12:03

1

You have several things wrong in there:

Neighborhoods fisherHill = new Neighborhoods();

Why are you instantiating a new object of the same class inside the constructor? Your constructor is called because a new object of this class is already going to be created. This new object is referenced as this. This is the proper way to initialize your class fields for a new object:

this.name = "Fisher Hill";
this.vertices = 4;
this.latCoords = new double[] {42.331672, 42.326342, 42.334464, 42.335733};
this.longCoords = new double[] {-71.131277, -71.143036, -71.148615, -71.141062};

As you can see in the other answers, this can be ommited. I personally prefer to put it, it makes code more readable for me.

And

inPolygon.check(Neighborhoods.fisherHill.vertices);

There's no such static field Neighborhoods.fisherHill. Even if it was there, fisherHill.vertices cannot be accessed because it has default accessibility.

You should create a Neighborhoods object, keep reference to it and extract the vertices field through a getter:

final Neighborhoods n = new Neighborhoods();
final int numVertices = n.getVertices();
inPolygon.check(numVertices);

Add in Neighborhoods class a getter for vertices field:

public int getVertices() {
    return this.vertices;
}

I suggest you get a Java book because you're obviously lacking knowledge of basic Java and OOP.

edited May 23 '17 at 12:03

Community

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answered Jul 23 '13 at 18:59

m0skit0

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Thank you for the reply. I rewrote the code so that it now cleanly and properly creates the object inside the class. But that I'm still stuck with being unable to access a variable of an object from a different class. If I create a getter method as you suggested, I'd have to explicitly create "numVertices" variables for every object, which would be inefficient. – Rowan K. Jul 23 '13 at 20:26
1

If `numVertices` is the same among all `Neighborhoods` objects then you can declare a private static constant `private static final int NUM_VERTICES = 4`. You can reference this inside any object of this class as `Neighborhoods.NUM_VERTICES` or simply `NUM_VERTICES`. If `numVertices` varies between objects, then there's no way than replicate it over each object. – m0skit0 Jul 23 '13 at 21:16
Also in your case you don't even need this value assuming you mean the size of the arrays. You can simply do `public int getVertices() { return this.latCoords.length; }`, which is less prone to bugs, e.g. if you change the array size but forget to change the constant. – m0skit0 Jul 23 '13 at 21:21
This case fits the scenario you mentioned above. numVertices is different for every object, therefore there's no real efficient way to do implement a getter method. I kind of solved this myself by just instantiating the object in the main class, and not the Neighborhoods one. I could then access all the variables. – Rowan K. Jul 23 '13 at 21:27
1

Actually what you did is what I meant by `final Neighborhoods n = new Neighborhoods(); final int numVertices = n.getVertices(); inPolygon.check(numVertices);`. Glad to have helped. – m0skit0 Jul 23 '13 at 21:51

score 0 · Answer 2 · answered Jul 23 '13 at 18:57

0

You have a recursive call in your constructor.

Offending line:

Neighborhoods fisherHill = new Neighborhoods();

answered Jul 23 '13 at 18:57

mach

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score 0 · Answer 3 · answered Jul 23 '13 at 18:58

It should probably be something like

public class Neighborhoods {

  // variables
  private final String name;
  private final int vertices;
  private final double[] latCoords;
  private final double[] longCoords;

  public Neighborhoods() {
    name = "Fisher Hill";
    vertices = 4;
    latCoords = new double[] {42.331672, 42.326342, 42.334464, 42.335733};
    longCoords = new double[] {-71.131277, -71.143036, -71.148615, -71.141062};
  }

  public int getVertices() {
     return vertices;
  }

  //and some more getters
}

and then in your other class call

inPolygon.check(new Neighborhoods().getVertices());

score 0 · Answer 4 · answered Jul 23 '13 at 19:01

To answer your questions:

inPolygon.check(Neighborhoods.fisherHill.vertices);

But for some reason, I'm getting an error when I try to reference the fisherHill object, as it says it can't be found.

Here you are making a static reference, fisherHill in this way will only be accessible if it a static member.

Also, dont create instance inside constructor.

Referencing basic Java object in another class

4 Answers4