Question

I am absolutely new to Java and having trouble referencing a class that I have declared in another .java file. I read a lot about it on the Oracle Website and also many stackoverflow questions, but couldn’t solve the problem. Here are the steps I have taken:

Set the class path.
I am running this on UNIX. So I typed in
% java -classpath /users/myUserName/algs4/assignment1. Instead of changing the class path it gives me a long list of other options.

Include the ‘package name’ in each of my .java files which have class definitions I want to refer to.

I also ensured that all my files that I want to refer are in the same directory.
Here is some of my code:

package assignment1;
public class Percolation
{
    private int[][] grid;
    ....
    //Other method definitions etc.
   public Percolation(int N)
   {
      uf = new WeightedQuickUnionUF(N);
      grid = new int[N][N];
      ...
      //Other code

   }

 }

My WeightedQuickUnionUF class is also defined as:

package assignment1;
public class WeightedQuickUnionUF {
    private int[] id;    // id[i] = parent of i
    private int[] sz;    // sz[i] = number of objects in subtree rooted at i
    private int count;   // number of components

    //Create an empty union find data structure with N isolated sets.
    public WeightedQuickUnionUF(int N) {
        count = N;
        id = new int[N];
        sz = new int[N];
        for (int i = 0; i < N; i++) {
            id[i] = i;
            sz[i] = 1;
        }
     }
}

But now when I compile using javac Percolation.java. It gives me the errors:

Percolation.java:7: cannot find symbol
symbol  : class WeightedQuickUnionUF
location: class assignment1.Percolation
WeightedQuickUnionUF uf;
^

I get it that it is because the compiler doesn’t know what is the class WeightedQuickUnionUF. It cannot refer it. But how do I do it then? I have already tried the popular solutions.

I am not using any IDE. Just a text editor and then compiling it on the terminal.

(algs4 folder has files like stdlib.jar and others)

EDIT: I missed the ‘new’ keyword. I have added that. Also I am compiling my WeightedQuickUnionUF class before Percolation.java

Answer 1

The java command is not setting the classpath but trying to run a java class. That’s why the long option list.

You can specify the classpath upon compilation: run javac -classpath . assignment1/Percolation.java from the algs4 directory. Pay attention to the dot it means current directory.

There is no need to compile the files in order.

To run your program: java -cp . assignment1.Percolation from the algs4 directory.