I'm trying to make a triangle rotate left or right depending on the arrow keys pressed. I am having trouble making the triangle stay in that position ones it rotates. When I hit the arrow key again the triangle has to start rotating from its new position.

This is what I have so far:

 boolean up, down, left, right;
 Triangle t1;
 float rotAmt;
 
 void setup()
 {
   t1 = new Triangle();
   size(800,600);

 }
 
 void draw()
 {

   stroke(255);
   fill(173,255,47);
   
   t1.turn();
   t1.drawTriangle();
   
 
   
   

   
 }
   
   class Triangle{
    int x1, y1, x2, y2, x3, y3;
   
   Triangle(){
    x1 = 355;
    y1 = 300;
    x2 = 383;
    y2 = 245;
    x3 = 411;
    y3 = 300;
   }
 
     void drawTriangle()
     {
       triangle(x1,y1,x2,y2,x3,y3);

 
     }
   
   
   void turn(){
      if (left)
     {
     translate (383,272);
     rotate(-rotAmt);
     translate (-383,-272);      
     }
   
    if(right)
   {
     translate (388,272);
     rotate(rotAmt);
     translate (-388,-272);
   }
   }
 
     }
   void keyPressed(){
      switch(keyCode){
      case(LEFT):
      left = true;
      rotAmt += .02;
      break;
      case(RIGHT):
      right = true;
      rotAmt += .02;
      break;
  }
 }

    void keyReleased(){
      switch(keyCode){
      case(LEFT):
      left = false;
      rotAmt=0;
      break;
      case(RIGHT):
      right = false;
      rotAmt = 0;
      break;
}
}
 

The best approach to dealing with keypresses is to use a boolean and switch it on keypress/keyrelease.  This avoids any issues with repeating keys, is more responsive and makes it easy to deal with multiple key presses.  For an example see this post.