问题
Hi awesome person!
I have a problem... I get a NullPointerException when my test case reaches allEdges.add(newEdge); in the connectNodes method.
I think it has something to do with Edge newEdge = new Edge( n1, n2, weight ); previously in that same method.
Is the problem my use of generics in the Edge class or something like that?.
I previously got an error directing me at the Edge newEdge = new Edge( n1, n2, weight ); line, saying something like "class not found". But now i seem to get the NullPointerException at allEdges.add(newEdge); instead without having changed anything.
Very thankful for every bit of help!
import java.util.*;
public class MyMiniGraph<T extends Comparable<? super T>> implements MiniGraph<T>
{
// The Graph containing all the nodes and their edges
private Map< T, HashSet<Edge> > theGraph = new HashMap< T, HashSet<Edge> >( );
// Keeps track of theGraphs current size
private int currentSize = 0;
// Keeps track of the current Edge quantity
private int numEdges;
// TreeSet containing all edges
private TreeSet<Edge> allEdges;
// edge representing class with its associated nodes and weight
private class Edge implements Comparable<Edge>
{
public int cost;
public T n1;
public T n2;
public Edge(T n1, T n2 , int cost)
{
this.n1 = n1;
this.n2 = n2;
this.cost = cost;
}
public int compareTo(Edge e)
{
// returns 0 if edges are equal
if(e.cost == cost)
return 0;
// returns 1 if edge is greater than other edge,
// -1 if edge is smaller than other edge
return e.cost < cost ? 1 : -1;
}
}
/**
* Method for adding a node to the graph.
* Silently ignores any duplicates
*
* @param n The node to add to the graph.
*/
public void addNode(T n)
{
if(n == null)
throw new IllegalStateException("Invalid Node");
if(!theGraph.containsKey(n))
{
theGraph.put(n,new HashSet<Edge>());
++currentSize;
}
}
/**
* Method for removing a node from the graph.
* Before the node is removed, all edges associated with the node
* must be removed.
* Silently ignores any nodes not already in the graph.
*/
public void removeNode(T n)
{
if(theGraph.containsKey(n))
{
// If node n has edges, remove all those edges.
// Firstly, remove the edges connecting to this
// node from other nodes, then, remove this node
// and its edges with it.
if( !theGraph.get(n).isEmpty() )
{
//iterator to iterate over the edges of node n
Iterator<Edge> edgeIt = theGraph.get(n).iterator();
// remove this node from all its connecting nodes edge lists
Edge localEdge;
/**Edge foreignEdge;*/
while(edgeIt.hasNext())
{
localEdge = edgeIt.next();
T foreignNode = localEdge.n2 == n ? localEdge.n1 : localEdge.n2;
// iterator to iterate over the edges of adjacent node of n (foreignNode)
/**Iterator<Edge> forEdgeIt =
theGraph.get(foreignNode).iterator();
while(forEdgeIt.hasNext())
{
foreignEdge = forEdgeIt.next();
if( foreignEdge.equals( localEdge ) )
forEdgeIt.remove();
}*/
// removes all edges occurring in n from all foreign nodes
theGraph.get(foreignNode).remove(localEdge);
allEdges.remove(localEdge);
--numEdges;
}
}
//remove the node itself thereby also removing its local edge list
theGraph.remove(n);
--currentSize;
}
}
/**
* Method for creating an unidirectional edge between two nodes.
*
* @param n1 The first node to create an edge between
* @param n2 The second node to create an edge between
* @param weight The cost for traversing the edge
*/
public void connectNodes(T n1, T n2, int weight)
{
if(!contains(n1) || !contains(n2))
throw new IllegalStateException("node not in graph");
if(!edgeExistsBetween(n1,n2))
{
Edge newEdge = new Edge( n1, n2, weight );
theGraph.get(n1).add( newEdge );
theGraph.get(n2).add( newEdge );
allEdges.add(newEdge);
++numEdges;
}
}
/**
* Method for removing an edge between two nodes.
*
* @param n1 The first node that identifies the edge.
* @param n2 The second node that identifies the edge.
*/
public void disconnectNodes(T n1, T n2)
{
if(!contains(n1) || !contains(n2))
throw new IllegalStateException("node not in graph");
boolean n1n2EdgeExists = true;
// iterates over n1, removing all edges containing n2 from n2
Iterator<Edge> edgeIt = theGraph.get(n1).iterator();
Edge deadEdge = null;
while(edgeIt.hasNext())
{
deadEdge = edgeIt.next();
if( deadEdge.n1.equals(n1) )
theGraph.get(n2).remove(deadEdge);
else if( deadEdge.n2.equals(n1) )
theGraph.get(n1).remove(deadEdge);
else
n1n2EdgeExists = false;
}
if(n1n2EdgeExists){
// removes the n1-n2 edge from n1
theGraph.get(n1).remove(deadEdge);
allEdges.remove(deadEdge);
--numEdges;
}
}
/**
* Method for searching the graph for a certain node.
* If the node is present in the graph, the method returns
* true, otherwise, it returns false.
*
* @return boolean true if the graph contains n, otherwise false.
*/
public boolean contains(T n)
{
return theGraph.containsKey(n);
}
/**
* Method for finding the number of nodes in the graph.
*
* @returns int The number of nodes in the graph.
*/
public int size()
{
return currentSize;
}
/**
* Checks if there exists and edge between nodes n1 and n2.
* Used for testing purposes.
*
* @param n1 The first node that identifies the edge.
* @param n2 The second node that identifies the edge.
* @return true if and edge exists between n1 and n2, otherwise false.
*/
public boolean edgeExistsBetween(T n1, T n2)
{
if(contains(n1))
{
boolean n1ContainsN2 = false;
Iterator<Edge> edgeIt = theGraph.get(n1).iterator();
Edge adjToN1;
while(edgeIt.hasNext())
{
adjToN1 = edgeIt.next();
if( adjToN1.n1.equals(n2) )
n1ContainsN2 = true;
else if( adjToN1.n2.equals(n2) )
n1ContainsN2 = true;
else
;
}// while n1 has next edge
return n1ContainsN2;
}// if n1 in graph
return false;
}
/**
* Gets the number of edges in the graph.
* Used for testing purposes.
*
* @return the number of edges in the graph.
*/
public int getNumberOfEdges()
{
return numEdges;
}
/**
* Method for calculating a minimum spanning tree for the graph.
* The method is supposed to returning a String representing the
* minimum spanning tree. The method is not allowed to modify the
* graph during the calculation, ie. the original graph must be
* identical to how the graph looked before the invocation of
* the method.
*
* The minimum spanning tree is calculated using Kruskal's algorithm.
*
* @return Graph A new instance of the Graph class, representing a
* minimal spanning tree.
*/
public MyMiniGraph<T> generateMinimumSpanningTree()
{
int edgesAccepted = 0;
//give all nodes to a class representing disjoint sets
DisjSet<T> ds = new DisjSet<T>( theGraph.keySet() );
//set up a new graph to represent the minimum spanning tree
MyMiniGraph<T> minSpanTree = new MyMiniGraph<T>();
//initialize minSpanTree with all theGraphs nodes
Iterator<T> nodeIter = theGraph.keySet().iterator();
while(nodeIter.hasNext())
minSpanTree.addNode(nodeIter.next());
//order all edges in theGraph in a priority queue
PriorityQueue<Edge> pq = new PriorityQueue<Edge>(allEdges);
Edge e;
// Kruskals algorithm. Accepts the smallest edges in order
// if they are not part of the same set which would cause a cycle.
while(edgesAccepted < currentSize -1)
{
e = pq.poll( );
T uset = ds.find( e.n1 );
T vset = ds.find( e.n2 );
if(uset != vset)
{
// Accept the edge
edgesAccepted++;
ds.union(uset, vset);
//if the edge is accepted, add it to minSpanTree
minSpanTree.connectNodes(e.n1, e.n2, e.cost);
}
}
return minSpanTree;
}
}
回答1:
I don't find any place where you initialize the member allEdges to a valid TreeSet object. Try to initialize (proper place seems like just where you define it)
来源：https://stackoverflow.com/questions/5134727/instantiated-private-class-null-pointer-exception