History variables: Difference between revisions

=={{header|AspectJ}}==

AspectJ implementation for Java 7.

Type of the history variable (Java class):

<lang java>public class HistoryVariable

{

public HistoryVariable(final Object v)

{

super();

value = v;

}

public void update(final Object v)

{

value = v;

}

public Object undo()

{

return value;

}

@Override

public String toString()

{

return value.toString();

}

public void dispose()

{

}

private Object value;

}</lang>

Aspect (HistoryHandling.aj):

<lang java>import java.util.Deque;

import java.util.HashMap;

import java.util.LinkedList;

import java.util.Map;

public privileged aspect HistoryHandling

{

before() : execution(HistoryVariable.new(..))

{

history.put((HistoryVariable) thisJoinPoint.getTarget(), new LinkedList<>());

}

after() : execution(void HistoryVariable.dispose())

{

history.remove(thisJoinPoint.getTarget());

}

before(Object v) : execution(void HistoryVariable.update(Object)) && args(v)

{

final HistoryVariable hv = (HistoryVariable) thisJoinPoint.getThis();

history.get(hv).add(hv.value);

}

after() : execution(Object HistoryVariable.undo())

{

final HistoryVariable hv = (HistoryVariable) thisJoinPoint.getThis();

final Deque<Object> q = history.get(hv);

if (!q.isEmpty())

hv.value = q.pollLast();

}

String around() : this(HistoryVariable) && execution(String toString())

{

final HistoryVariable hv = (HistoryVariable) thisJoinPoint.getThis();

final Deque<Object> q = history.get(hv);

if (q == null)

return "<disposed>";

else

return "current: "+ hv.value + ", previous: " + q.toString();

}

private Map<HistoryVariable, Deque<Object>> history = new HashMap<>();

}</lang>

Usage:

<lang java>public final class Main

{

public static void main(final String[] args)

{

HistoryVariable hv = new HistoryVariable("a");

hv.update(90);

hv.update(12.1D);

System.out.println(hv.toString());

System.out.println(hv.undo());

System.out.println(hv.undo());

System.out.println(hv.undo());

System.out.println(hv.undo());

System.out.println(hv.toString());

hv.dispose();

System.out.println(hv.toString());

}

}</lang>

{{out}}

<pre>current: 12.1, previous: [a, 90]

12.1

90

a

a

current: a, previous: []

<disposed></pre>