How to implement an ADT in Java

Problem:

Implement the ADT below:
ADT: Line
contains(Point): Boolean
equals(Line): Boolean
isHorizontal(): Boolean
isVertical(): Boolean
slope(): Real
toString(): String
xIntercept(): Real
yIntercept(): Real

Output:

Not applicable.

Solution:

public class MyLine implements Line {
 private double m, b; // slope, intercept
 public static Line X_AXIS = new MyLine();
 private MyLine() {
 }
 public MyLine (double m, double b) {
 this.m = m;
 this.b = b;
 }
 public boolean contains(Point point) {
 double x = point.xCoordinate();
 double y = point.yCoordinate();
 return y == m*x + b;
 }
 public boolean equals(Object object) {
 if (object==this) {
 return true;
 } else if (!(object instanceof MyLine)) {
 return false;
 }
 MyLine that = (MyLine)object;
 return (that.m == this.m && that.b == this.b);
 }
 public boolean isHorizontal() {
 return m == 0.0;
 }
 public boolean isVertical() {
 return m == Double.POSITIVE_INFINITY || m==Double.NEGATIVE_INFINITY;
 }
 public double slope() {
 return m;
 }
 public String toString() {
 return String.format("y = %.2fx + %.2f", m, b);
 }
public double xIntercept() {
 if (isHorizontal()) {
 throw new RuntimeException("this line is horizontal");
 }
 return -b/m;
 }
 public double yIntercept() {
 if (isVertical()) {
 throw new RuntimeException("this line is vertical");
 }
 return b;
 }
}