Line.java

public class Line {
    private final Point start;
    private final Point end;

    public Line(Point start,Point end){
        this.start = start;
        this.end = end;
    }

    public double getLength(){
        return length();
    }
    private double length(){
        double length = Math.sqrt((Math.pow(end.getX()-start.getX(),2)+Math.pow(end.getY()-start.getY(),2)));
        return length;
    }

    @Override
    public String toString() {
        return "Line startpoint[" + start.toString()+"] endpoint [ "+ end.toString()+"] length is "+ length();
    }
}

Lines.java

public class Lines {


        private ArrayList<Line> lines= new ArrayList<Line>();

        public void add(Line line){
            lines.add(line);
        }

        private double sumLength(){
            double sum = 0;
            for(Line line:lines)
                sum += line.getLength();
            return sum;
        }
        private Line longestLine(){
            Line line = lines.get(0);
            double max = 0;
            for(Line l: lines){
               if(l.getLength()>max) {
                   max = l.getLength();
                   line = l;
               }
            }
            return line;
        }

        public void printLines(){
            for(Line line: lines){
                System.out.println(line.toString());
            }
        }

    public String longestLinetoString() {
        return "The longest line is "+ longestLine().toString();
    }

    public String lengthString(){
            return "Lines length is " + sumLength();
        }
}

LinesRunner.java

public class LinesRunner {
    public static void main(String[] args) {
        Point point1 = new Point(2,3);
        Point point2 = new Point(3,5);
        Point point3 = new Point(5,3);
        Point point4 = new Point(3,1);


        Line line1 = new Line(point1,point2);
        Line line2 = new Line(point3,point4);


        Lines lines = new Lines();
        lines.add(line1);
        lines.add(line2);
        lines.longestLinetoString();

        System.out.println(line1.toString() );              //print a line
        System.out.println(lines.lengthString());           //print the sum of lengths of lines
        System.out.println(lines.longestLinetoString());    //print the longest line
        lines.printLines();

    }
}

Point.java

public class Point {
    private final double x;
    private final double y;

    public Point(double x , double y){
        this.x = x;
        this.y = y;
    }

    public double getX() {
        return x;
    }

    public double getY() {
        return y;
    }

    @Override
    public String toString() {
        return "x = "+ x+ " y ="+y;
    }
}