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## org.jgrapht.traverse
Class TopologicalOrderIterator<V,E>

    java.lang.Object
      org.jgrapht.traverse.AbstractGraphIterator<V,E>
          org.jgrapht.traverse.CrossComponentIterator<V,E,java.lang.Object>
              org.jgrapht.traverse.TopologicalOrderIterator<V,E>

All Implemented Interfaces:
  java.util.Iterator<V>, GraphIterator<V,E>

    public class TopologicalOrderIterator<V,E>

extends CrossComponentIterator<V,E,java.lang.Object>

Implements topological order traversal for a directed acyclic graph. A topological sort is a permutation `p` of the vertices of a graph such that an edge `(i,j)` implies that `i` appears before `j` in `p` (Skiena 1990, p. 208). See also http://mathworld.wolfram.com/TopologicalSort.html.

See "Algorithms in Java, Third Edition, Part 5: Graph Algorithms" by Robert Sedgewick and "Data Structures and Algorithms with Object-Oriented Design Patterns in Java" by Bruno R. Preiss for implementation alternatives. The latter can be found online at http://www.brpreiss.com/books/opus5/

For this iterator to work correctly the graph must be acyclic, and must not be modified during iteration. Currently there are no means to ensure that, nor to fail-fast; the results with cyclic input (including self-loops) or concurrent modifications are undefined. To precheck a graph for cycles, consider using `CycleDetector` or `StrongConnectivityInspector`.

Since:
  Dec 18, 2004
Author:
  Marden Neubert

Nested Class Summary

Nested classes/interfaces inherited from class org.jgrapht.traverse.CrossComponentIterator
`CrossComponentIterator.VisitColor`

Field Summary

Fields inherited from class org.jgrapht.traverse.AbstractGraphIterator
`nListeners`

Constructor Summary
`TopologicalOrderIterator(DirectedGraph<V,E> dg)` Creates a new topological order iterator over the directed graph specified, with arbitrary tie-breaking in case of partial order.
`TopologicalOrderIterator(DirectedGraph<V,E> dg, java.util.Queue<V> queue)` Creates a new topological order iterator over the directed graph specified, with a user-supplied queue implementation to allow customized control over tie-breaking in case of partial order.

Method Summary
`protected void` | `encounterVertex(V vertex, E edge)` Update data structures the first time we see a vertex.
`protected void` | `encounterVertexAgain(V vertex, E edge)` Called whenever we re-encounter a vertex.
`protected boolean` | `isConnectedComponentExhausted()` Returns `true` if there are no more uniterated vertices in the currently iterated connected component; `false` otherwise.
`protected V` | `provideNextVertex()` Returns the vertex to be returned in the following call to the iterator `next` method.

Methods inherited from class org.jgrapht.traverse.CrossComponentIterator
`finishVertex, getGraph, getSeenData, hasNext, isSeenVertex, next, putSeenData`

Methods inherited from class org.jgrapht.traverse.AbstractGraphIterator
`addTraversalListener, fireConnectedComponentFinished, fireConnectedComponentStarted, fireEdgeTraversed, fireVertexFinished, fireVertexTraversed, isCrossComponentTraversal, isReuseEvents, remove, removeTraversalListener, setCrossComponentTraversal, setReuseEvents`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### TopologicalOrderIterator

    public TopologicalOrderIterator(DirectedGraph<V,E> dg)

  Creates a new topological order iterator over the directed graph specified, with arbitrary tie-breaking in case of partial order. Traversal will start at one of the graph's sources. See the definition of source at http://mathworld.wolfram.com/Source.html.

  Parameters:
    `dg` - the directed graph to be iterated.

### TopologicalOrderIterator

    public TopologicalOrderIterator(DirectedGraph<V,E> dg,
                                    java.util.Queue<V> queue)

  Creates a new topological order iterator over the directed graph specified, with a user-supplied queue implementation to allow customized control over tie-breaking in case of partial order. Traversal will start at one of the graph's sources. See the definition of source at http://mathworld.wolfram.com/Source.html.

  Parameters:
    `dg` - the directed graph to be iterated.
    `queue` - queue to use for tie-break in case of partial order (e.g. a PriorityQueue can be used to break ties according to vertex priority); must be initially empty

Method Detail

### isConnectedComponentExhausted

    protected boolean isConnectedComponentExhausted()

  Description copied from class: `CrossComponentIterator`
  Returns `true` if there are no more uniterated vertices in the currently iterated connected component; `false` otherwise.

  Specified by:
    `isConnectedComponentExhausted` in class `CrossComponentIterator<V,E,java.lang.Object>`
  Returns:
    `true` if there are no more uniterated vertices in the currently iterated connected component; `false` otherwise.
  See Also:
    `CrossComponentIterator.isConnectedComponentExhausted()`

### encounterVertex

    protected void encounterVertex(V vertex,
                                   E edge)

  Description copied from class: `CrossComponentIterator`
  Update data structures the first time we see a vertex.

  Specified by:
    `encounterVertex` in class `CrossComponentIterator<V,E,java.lang.Object>`
  Parameters:
    `vertex` - the vertex encountered
    `edge` - the edge via which the vertex was encountered, or null if the vertex is a starting point
  See Also:
    `CrossComponentIterator.encounterVertex(Object, Object)`

### encounterVertexAgain

    protected void encounterVertexAgain(V vertex,
                                        E edge)

  Description copied from class: `CrossComponentIterator`
  Called whenever we re-encounter a vertex. The default implementation does nothing.

  Specified by:
    `encounterVertexAgain` in class `CrossComponentIterator<V,E,java.lang.Object>`
  Parameters:
    `vertex` - the vertex re-encountered
    `edge` - the edge via which the vertex was re-encountered
  See Also:
    `CrossComponentIterator.encounterVertexAgain(Object, Object)`

### provideNextVertex

    protected V provideNextVertex()

  Description copied from class: `CrossComponentIterator`
  Returns the vertex to be returned in the following call to the iterator `next` method.

  Specified by:
    `provideNextVertex` in class `CrossComponentIterator<V,E,java.lang.Object>`
  Returns:
    the next vertex to be returned by this iterator.
  See Also:
    `CrossComponentIterator.provideNextVertex()`

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