processing4/java/examples/Basics/Input/Constrain/applet/Constrain.java

import processing.core.*; 

import java.applet.*; 
import java.awt.*; 
import java.awt.image.*; 
import java.awt.event.*; 
import java.io.*; 
import java.net.*; 
import java.text.*; 
import java.util.*; 
import java.util.zip.*; 
import java.util.regex.*; 

public class Constrain extends PApplet {

/**
 * 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 = 0.05f;
float esize = 25.0f;
int box = 30;

public void setup() 
{
  size(200, 200);
  noStroke(); 
  smooth();
  ellipseMode(CENTER_RADIUS);  
}

public void draw() 
{ 
  background(51);
  
  if(abs(mouseX - mx) > 0.1f) {
    mx = mx + (mouseX - mx) * easing;
  }
  if(abs(mouseY - my) > 0.1f) {
    my = my + (mouseY- my) * easing;
  }
  
  float distance = esize * 2;
  mx = constrain(mx, box+distance, width-box-distance);
  my = constrain(my, box+distance, height-box-distance);
  fill(76);
  rect(box+esize, box+esize, box*3, box*3);
  fill(255);  
  ellipse(mx, my, esize, esize);
}

  static public void main(String args[]) {
    PApplet.main(new String[] { "Constrain" });
  }
}