State in Java
Why read if you can watch?
Watch State's video tutorialBefore and after
BEFORE - 3-speed ceiling fan state machine
// Not good: unwieldy "case" statement
class CeilingFanPullChain
{
private int m_current_state;
public CeilingFanPullChain()
{
m_current_state = 0;
}
public void pull()
{
if (m_current_state == 0)
{
m_current_state = 1;
System.out.println(" low speed");
}
else if (m_current_state == 1)
{
m_current_state = 2;
System.out.println(" medium speed");
}
else if (m_current_state == 2)
{
m_current_state = 3;
System.out.println(" high speed");
}
else
{
m_current_state = 0;
System.out.println(" turning off");
}
}
}
public class StateDemo
{
public static void main(String[] args)
{
CeilingFanPullChain chain = new CeilingFanPullChain();
while (true)
{
System.out.print("Press ");
get_line();
chain.pull();
}
}
static String get_line()
{
BufferedReader in = new BufferedReader(new InputStreamReader(System.in))
;
String line = null;
try
{
line = in.readLine();
}
catch (IOException ex)
{
ex.printStackTrace();
}
return line;
}
}
Press
low speed
Press
medium speed
Press
high speed
Press
turning off
Press
low speed
Press
medium speed
Press
high speed
Press
turning off
AFTER - The CeilingFanPullChain class is now a wrapper that delegates to its m_current_state reference. Each clause from the "before" case statement is now captured in a State derived class.
For this simple domain, the State pattern is probably over-kill.
class CeilingFanPullChain
{
private State m_current_state;
public CeilingFanPullChain()
{
m_current_state = new Off();
}
public void set_state(State s)
{
m_current_state = s;
}
public void pull()
{
m_current_state.pull(this);
}
}
interface State
{
void pull(CeilingFanPullChain wrapper);
}
class Off implements State
{
public void pull(CeilingFanPullChain wrapper)
{
wrapper.set_state(new Low());
System.out.println(" low speed");
}
}
class Low implements State
{
public void pull(CeilingFanPullChain wrapper)
{
wrapper.set_state(new Medium());
System.out.println(" medium speed");
}
}
class Medium implements State
{
public void pull(CeilingFanPullChain wrapper)
{
wrapper.set_state(new High());
System.out.println(" high speed");
}
}
class High implements State
{
public void pull(CeilingFanPullChain wrapper)
{
wrapper.set_state(new Off());
System.out.println(" turning off");
}
}
public class StateDemo
{
public static void main(String[] args)
{
CeilingFanPullChain chain = new CeilingFanPullChain();
while (true)
{
System.out.print("Press ");
get_line();
chain.pull();
}
}
static String get_line()
{
BufferedReader in = new BufferedReader(new InputStreamReader(System.in))
;
String line = null;
try
{
line = in.readLine();
}
catch (IOException ex)
{
ex.printStackTrace();
}
return line;
}
}
