OOP Tutorial: 2-dimensional Vector as Java Object

When starting to learn object orientated programming, a good first example is the following: Modelling a 2-dimensional vector over the real numbers could result in this

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package de.bigdev;   public class Vector {       private double x;     private double y;       public Vector(double x, double y) {         super();         this.x = x;         this.y = y;     }           /**      * Addition of two vectors      *      * @param v      * @return the sum with v      */     public Vector add(Vector v) {         return new Vector(x + v.x, y + v.y);     }           /**      * Scalar Mulitplication with a constant      *      * @param c      * @return the scalar multiplication with c      */     public Vector scalarMult(double c) {         return new Vector(c * x, c * y);     }       /**      * Length of two vectors      *      * @return length      */     public double length() {         return Math.sqrt(x * x + y * y);     }       /**      * Scalar Product of two vectors      *      * @param v      * @return the scalar product with v      */     public double scalarProd(Vector v) {         return this.x * v.getX() + this.y * v.getY();     }       /**      * Angle between two vectors      *      * @param v      * @return the angle with v in degrees      */     public double angle(Vector v) {         return Math.acos(this.scalarProd(v) / (this.length() * v.length()))                 * 180.0 / Math.PI;     }       public double getX() { return x; }     public double getY() { return y; }       @Override     public String toString() {         return "[" + x + ", " + y + "]";     }       /**      * for testing only. should be factored out into another class... or use      * JUnit!      *      * @param args      */     public static void main(String[] args) {         Vector v = new Vector(1, 1);         Vector w = new Vector(-1, -1);           System.out.println("Vector algebra");         System.out.println("==============");         System.out.println("addition      : " + v + " + " + w + " = " + v.add(w));         System.out.println("scalar multip.: 2 * " + v + " = " + v.scalarMult(2));         System.out.println("length        : ||" +v + "|| = " + v.length());         System.out.println("scalar product: " + v + " * " + w + " = " + v.scalarProd(w));         System.out.println("angle         : angle(" + v + ", " + w           + ") = " + Math.round(v.angle(w)) + "°");     } }

The output on the console is


Vector algebra
==============
addition      : [1.0, 1.0] + [-1.0, -1.0] = [0.0, 0.0]
scalar multip.: 2 * [1.0, 1.0] = [2.0, 2.0]
length        : ||[1.0, 1.0]|| = 1.4142135623730951
scalar product: [1.0, 1.0] * [-1.0, -1.0] = -2.0
angle         : angle([1.0, 1.0], [-1.0, -1.0]) = 180°