public class JumpingThing {

   // You might want to play around with this:
   public static final float METER_TO_PIXELS = 32f;
   public static final float SECONDS_TO_FRAMES = 1f / 60f;
   // 9.81 [m/s²] is the average ground factor on earth. This is the value to play around, 
   // if you want the game to play on the moon, for example
   public static final float GRAVITY = 9.81f * METER_TO_PIXELS * SECONDS_TO_FRAMES;

   // Play around with this value as well:
   public static final float JUMP_POWER = 10f; // The higher this value, the higher (stronger) it jumps.

   // Usually you'd also have X position and so on, but I leave it out, since it's only going up or down
   private float velocityY;
   private float positionY;

   public void update() {
      // Make the object accelerate towards the bottom
      velocityY += GRAVITY;
      // If you use this method names I'll kill you :P
      if (thisThingWantsToJump() && thisThingIsAbleToJump()) {
         // It's minus, because it's going down, not up.
         // Remove the minus, when you have your y values 
         // increasing to the top.
         velocityY = -JUMP_POWER;
      }
      positionY += velocityY;
   }
} // That's it!