float velx = 0, vely = 0, MaxSpeed = 100f;

    //место массива directions поставь проверку на нажатие кнопок
    boolean keyUp = directions[0];
    boolean keyDown = directions[1];
    boolean keyLeft = directions[2];
    boolean keyRight = directions[3];

        if (keyUp) {
            velx += MaxSpeed / 10f;
            if (velx > MaxSpeed) velx = MaxSpeed;
        }
        if (keyDown) {
            velx -= MaxSpeed / 10f;
            if (velx < -MaxSpeed) velx = -MaxSpeed;
        }
        if (!keyUp && !keyDown) {
            if (velx > (MaxSpeed / 20.0f)) {
                velx -= (MaxSpeed / 20.0f);
            } else if (velx < -(MaxSpeed / 20.0f)) {
                velx += (MaxSpeed / 20.0f);
            } else {
                velx = 0.0f;
            }
        }
        if (keyRight) {
            vely += MaxSpeed / 10f;
            if (vely > MaxSpeed) vely = MaxSpeed;
        }
        if (keyLeft) {
            vely -= MaxSpeed / 10f;
            if (vely < -MaxSpeed) vely = -MaxSpeed;
        }
        if (!keyRight && !keyLeft) {
            if (vely > (MaxSpeed / 20.0f)) {
                vely -= (MaxSpeed / 20.0f);
            } else if (vely < -(MaxSpeed / 20.0f)) {
                vely += (MaxSpeed / 20.0f);
            } else {
                vely = 0.0f;
            }
         }