Cannot Compile/Run the Default Package File in Java

I'm doing the Algorithm homework of Princeton University, which asked us to finish 2 functions.

The question is, when I want to compile "PercolationStats.java" which needs the class "Percolation" in the same directory, It shows the error message:

PercolationStats.java:14: error: cannot find symbol

I tried to find the solution of "how to import default package" on Stackoverflow and turned out the answer that there's no need to import. I'm wondering the reason and I don't wanna use import package method since the homework spec said that both files should be in default package.

I will put both file I'd like to compile & run down below. I hope there's someone can tell me the reason/solution why this happened.

public class Percolation {
    private boolean[] id;
    private int number;
    public WeightedQuickUnionUF a;
    private boolean flag;

    public Percolation(int var1) {
        if (var1 < 1) {
            throw new IllegalArgumentException("Please input parameter larger than 0!");
        }
        else {
            this.number = var1;
            this.id = new boolean[this.number * this.number + 2];
            this.a = new WeightedQuickUnionUF(this.number * this.number + 2);

            for (int var2 = 0; var2 < this.number * this.number + 2; ++var2) {
                this.id[var2] = false;
            }

        }
    }
}


public class PercolationStats {

    private int trial_num;
    private double[] log;

    public PercolationStats(int var1, int var2) {
        if (var1 >= 1 && var2 >= 1) {
            Percolation pr = new Percolation(var1);
            trial_num = var2;
        }
        else {
            throw new IllegalArgumentException("Please make sure both input greater equal than 1!");
        }
    }
}

For readability, I only put the constructor of both class. And I used following command to compile and run:

$  javac PercolationStats.java
$  java PercolationStats.java

The compile error happened in:

Percolation pr = new Percolation(var1);