To calculate the volume of a tetrahedron in Java, you can use the following formula:

V = (a^3 * √2) / 12

Where a is the length of one of the edges of the tetrahedron.

To implement this formula in Java, you can write a method that takes in the length of an edge and returns the volume of the tetrahedron. Here's an example implementation: 

public class TetrahedronVolume {
    public static void main(String[] args) {
        double edgeLength = 5.0;
        double volume = calculateTetrahedronVolume(edgeLength);
        System.out.println("The volume of the tetrahedron is: " + volume);
    }

    public static double calculateTetrahedronVolume(double edgeLength) {
        double volume = (Math.pow(edgeLength, 3) * Math.sqrt(2)) / 12;
        return volume;
    }
}

In this example, the program calculates the volume of a tetrahedron with an edge length of 5.0. The calculateTetrahedronVolume method takes in the edge length as a parameter, uses the formula to calculate the volume, and then returns the volume.

Note that the Math.pow and Math.sqrt methods are used to perform the necessary mathematical operations.