This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.

Constrain.

Move the mouse across the screen to move the circle. The program constrains the circle to its box.

 
float mx;
float my;
float easing = 0.05;
int radius = 24;
int edge = 100;
int inner = edge + radius;

void setup() {
  size(640, 360);
  noStroke(); 
  ellipseMode(RADIUS);
  rectMode(CORNERS);
}

void draw() { 
  background(51);
  
  if (abs(mouseX - mx) > 0.1) {
    mx = mx + (mouseX - mx) * easing;
  }
  if (abs(mouseY - my) > 0.1) {
    my = my + (mouseY- my) * easing;
  }
  
  mx = constrain(mx, inner, width - inner);
  my = constrain(my, inner, height - inner);
  fill(76);
  rect(edge, edge, width-edge, height-edge);
  fill(255);  
  ellipse(mx, my, radius, radius);
}