Calculating the angle between two lines without having to calculate the slope? (Java)

Question

I have two Lines: L1 and L2. I want to calculate the angle between the two lines. L1 has points: {(x1, y1), (x2, y2)} and L2 has points: {(x3, y3), (x4, y

Answer 1

dx1=x2-x1 ; dy1=y2-y1 ; dx2=x4-x3 ;dy2=y4-y3.

Angle(L1,L2)=pi()/2*((1+sign(dx1))* (1-sign(dy1^2))-(1+sign(dx2))*(1-sign(dy2^2)))
           +pi()/4*((2+sign(dx1))*sign(dy1)-(2+sign(dx2))*sign(dy2))
           +sign(dx1*dy1)*atan((abs(dx1)-abs(dy1))/(abs(dx1)+abs(dy1)))
           -sign(dx2*dy2)*atan((abs(dx2)-abs(dy2))/(abs(dx2)+abs(dy2)))

Answer 2

Check this Python code:

import math
def angle(x1,y1,x2,y2,x3,y3):

  if (x1==x2==x3 or y1==y2==y3):
    return 180
  else:
    dx1 = x2-x1
    dy1 = y2-y1
    dx2 = x3-x2
    dy2 = y3-y2
    if x1==x2:
      a1=90
    else:
      m1=dy1/dx1
      a1=math.degrees(math.atan(m1))
    if x2==x3:
      a2=90
    else:
      m2=dy2/dx2
      a2=math.degrees(math.atan(m2))
    angle = abs(a2-a1)
    return angle

print angle(0,4,0,0,9,-6)