Java Math.atan() returning 0 for some inputs?

Question

Is the java Math.atan() function known to have any issues? I'm using it in my code and for some bizarre reason that I can't work out, the function will return 0 for 45 degrees of motion, jumps to 45 and stays at 45 until it reaches 63.4 degrees where it starts mapping properly until it reaches -63.4 degrees and the problem repeats. I'm so confused! I've triple checked that my input is always correct but yet the problem recurs!

Here's my code

int adjacent = p2.x - p1.x;
if (adjacent == 0) adjacent = 1; //Prevent division by 0
int opposite = p2.y - p1.y;
double pheta = Math.atan(opposite/adjacent);
System.out.println(opposite + "/" + adjacent + ", pheta=" + Math.toDegrees(pheta));

Here's some printouts on test data

73/102, angle=0.0
73/101, angle=0.0
74/100, angle=0.0
74/99, angle=0.0
75/99, angle=0.0
76/97, angle=0.0
76/96, angle=0.0
77/95, angle=0.0
78/95, angle=0.0
78/94, angle=0.0
79/92, angle=0.0
80/91, angle=0.0
81/90, angle=0.0
81/89, angle=0.0
82/87, angle=0.0
83/86, angle=0.0
84/85, angle=0.0
84/84, angle=45.0
84/83, angle=45.0
85/83, angle=45.0
85/82, angle=45.0
85/81, angle=45.0
85/80, angle=45.0
86/79, angle=45.0
87/78, angle=45.0
87/77, angle=45.0
87/76, angle=45.0
88/75, angle=45.0
88/74, angle=45.0
88/73, angle=45.0
89/72, angle=45.0
90/70, angle=45.0
90/69, angle=45.0
91/68, angle=45.0
92/67, angle=45.0
92/66, angle=45.0
93/65, angle=45.0
93/64, angle=45.0
93/62, angle=45.0
94/62, angle=45.0
94/61, angle=45.0
95/61, angle=45.0
95/60, angle=45.0
95/59, angle=45.0
95/57, angle=45.0
95/56, angle=45.0
96/55, angle=45.0
96/54, angle=45.0
96/53, angle=45.0
97/53, angle=45.0
97/52, angle=45.0
97/51, angle=45.0
98/51, angle=45.0
98/50, angle=45.0
98/49, angle=63.43494882292201
98/47, angle=63.43494882292201
99/45, angle=63.43494882292201
99/42, angle=63.43494882292201
100/40, angle=63.43494882292201
100/38, angle=63.43494882292201
100/36, angle=63.43494882292201
100/34, angle=63.43494882292201
100/33, angle=71.56505117707799
101/33, angle=71.56505117707799
101/32, angle=71.56505117707799
101/31, angle=71.56505117707799
101/30, angle=71.56505117707799
102/29, angle=71.56505117707799
102/28, angle=71.56505117707799
102/27, angle=71.56505117707799
103/27, angle=71.56505117707799
103/25, angle=75.96375653207353
103/24, angle=75.96375653207353
103/23, angle=75.96375653207353
103/22, angle=75.96375653207353
104/21, angle=75.96375653207353
104/19, angle=78.69006752597979
105/18, angle=78.69006752597979
105/16, angle=80.53767779197439
106/15, angle=81.86989764584403
106/14, angle=81.86989764584403
106/13, angle=82.87498365109819
106/12, angle=82.87498365109819
106/11, angle=83.6598082540901
106/10, angle=84.28940686250037
106/9, angle=84.8055710922652
107/8, angle=85.60129464500447
108/6, angle=86.82016988013577

Any ideas?

Answer 1

You perform integer division, which results in round numbers, try:

Math.atan((double)opposite/adjacent);

Answer 2

You almost certainly want to be using Math.atan2(y, x), which is much smarter about pointing in the right direction, as opposed to Math.atan, which has to assume that you're in one of the first two quadrants.