Sensing when the mouse is inside a rectangle with Processing

int buttonSize = 100;

void setup()
{
  size(300, 300);
}

void draw()
{
  button(0, 0, buttonSize, buttonSize);  // top left
  button(width - buttonSize, 0, buttonSize, buttonSize);  // top right  
  button(0, height - buttonSize, buttonSize, buttonSize);  // top right      
  button(width - buttonSize, height - buttonSize, buttonSize, buttonSize);  // top right
}

void button( int x, int y, int w, int h )
{
  // if mouse is inside rectangle
  if ( mouseX >= x  &&  mouseX <= x+w &&
       mouseY >= y  &&  mouseY <= y+h) { 
         
    if (mousePressed) {              
      fill(255, 0, 0);   // the color if the mouse is pressed and over the button
    }
    else {
      fill(255, 255, 0); // the color if the mouse is over the button
    }
    
  } 
  else {
    fill(0, 0, 255);     // the color if the mouse is not over the button
  }
  
  rect(x, y, w, h);
}