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问题描述
真棒人！
我遇到问题...当我的测试用例达到 connectNodes方法中的allEdges.add（newEdge）; 。
我认为它与 Edge newEdge = new Edge（n1，n2，weight）; 之前有关用同样的方法。
问题是我在Edge类中使用泛型还是类似的东西？
我以前在 Edge newEdge = new Edge（n1，n2，weight）; 这一行发表了一个错误消息，指出类似class not found。但是现在我好像在 allEdges.add（newEdge）; 中得到了NullPointerException，而没有改变任何东西。
非常感谢您的帮助！
import java.util。*;
公共类MyMiniGraph< T扩展了Comparable< super T>>实现MiniGraph< T>
{
//包含所有节点及其边的图形
private Map< T，HashSet< Edge> > theGraph =新的HashMap< T，HashSet< Edge> >（）;
//跟踪图的当前大小
private int currentSize = 0;
//跟踪当前边数量
private int numEdges;
//包含所有边的TreeSet
私有TreeSet< Edge> allEdges;
//边表示具有关联节点和权重的类
私有类Edge实现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 =费用;
}
public int compareTo（Edge e）
{
//如果边相等则返回0
if（e.cost == cost）
返回0;
//如果边缘大于其他边缘则返回1，
// -1如果边缘小于其他边缘
返回e.cost<成本？ 1：-1;
}
}
/ **
*向图中添加节点的方法。
*默默地忽略任何重复项
*
* @param n要添加到图中的节点。
* /
public void addNode（T n）
{
if（n == null）
throw new IllegalStateException（Invalid Node）;
if（！theGraph.containsKey（n））
{
theGraph.put（n，new HashSet< Edge>（））;
++ currentSize;
}
}
/ **
*从图中删除一个节点的方法。
*在删除节点之前，必须删除与节点
*相关联的所有边。
*默默地忽略不在图中的任何节点。
* /
public void removeNode（T n）
{
if（theGraph.containsKey（n））
{
//如果节点n有边缘，删除所有这些边缘。
//首先，从其他节点删除连接到此
//节点的边，然后删除此节点
//及其边。
if（！theGraph.get（n）.isEmpty（））
{
//迭代器遍历节点n的边界
Iterator< Edge> edgeIt = theGraph.get（n）.iterator（）;
//将此节点从其所有连接节点边缘列表中删除
Edge localEdge;
/ ** Edge foreignEdge; * /
while（edgeIt.hasNext（））
{
localEdge = edgeIt.next（）;
T foreignNode = localEdge.n2 == n？ localEdge.n1：localEdge.n2;
//迭代器遍历n（foreignNode）
/ ** Iterator< Edge>相邻节点的边缘。 forEdgeIt =
theGraph.get（foreignNode）.iterator（）; （forEdgeIt.hasNext（））
{
foreignEdge = forEdgeIt.next（）;
if（foreignEdge.equals（localEdge））
forEdgeIt.remove（）;
} * /
//删除所有外部节点中出现的所有边。
theGraph.get（foreignNode）.remove（localEdge）;
allEdges.remove（localEdge）;
--numEdges;
//删除节点本身，从而删除它的本地边界列表
theGraph.remove（n）;
--currentSize;
}
}
/ **
*在两个节点之间创建单向边的方法。
*
* @param n1在
* @param n2之间创建边的第一个节点在
* @param之间创建边的第二个节点重量遍历如果（！包含（n1）||！包含（n2））
$ /
public void connectNodes（T n1，T n2，int weight）
{
$ b抛出新的IllegalStateException（节点不在图中）;
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;
}
}
/ **
*删除两个节点之间的边的方法。
*
* @param n1标识边的第一个节点。
* @param n2标识边的第二个节点。
* /
public void disconnectNodes（T n1，T n2）
{
if（！contains（n1）||！contains（n2））
throw new IllegalStateException（节点不在图中）;
布尔型n1n2EdgeExists = true;
//在n1上迭代，从n2中移除包含n2的所有边
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）{
//从n1中删除n1-n2边缘
theGraph.get（n1）.remove（deadEdge）;
allEdges.remove（deadEdge）;
--numEdges;
}
}
/ **
*用于搜索某个节点的图形的方法。
*如果节点存在于图中，则该方法返回
* true，否则返回false。
*
* @return boolean如果图形包含n，则返回true，否则返回false。
* /
public boolean contains（T n）
{
return theGraph.containsKey（n）;
}
/ **
*查找图中节点数量的方法。
*
* @returns int图中节点的数量。
* /
public int size（）
{
return currentSize;
}
/ **
*检查节点n1和n2之间是否存在边。
*用于测试目的。
*
* @param n1标识边的第一个节点。
* @param n2标识边的第二个节点。
* @return如果和n1和n2之间存在边缘，则返回true，否则返回false。
* /
public boolean edgeExistsBetween（T n1，T n2）
{
if（contains（n1））
{
boolean n1ContainsN2 = false;
迭代器< Edge> edgeIt = theGraph.get（n1）.iterator（）;
边缘adjToN1;
while（edgeIt.hasNext（））
{
adjToN1 = edgeIt.next（）;
if（adjToN1.n1.equals（n2））
n1ContainsN2 = true;
else if（adjToN1.n2.equals（n2））
n1ContainsN2 = true;
else
;
} // // n1有下一条边
return n1ContainsN2;
} //如果
中的n1返回false;
}
/ **
*获取图形中的边数。
*用于测试目的。
*
* @返回图中边的数量。
* /
public int getNumberOfEdges（）
{
return numEdges;
}
/ **
*计算图的最小生成树的方法。
*该方法应该返回一个表示
*最小生成树的字符串。该方法不允许在计算过程中修改
*图形，即。在调用
*方法之前，原始图必须与
*相同。
*
*最小生成树使用Kruskal算法计算。
*
* @return Graph Graph类的新实例，表示
*最小生成树。
* /
public MyMiniGraph< T> generateMinimumSpanningTree（）
{
int edgesAccepted = 0;
//给所有节点提供表示不相交集合的类
DisjSet< T> ds = new DisjSet< T>（theGraph.keySet（））;
//设置一个新图来表示最小生成树
MyMiniGraph< T> minSpanTree = new MyMiniGraph< T>（）;
//用所有的图形节点初始化minSpanTree
Iterator< T> nodeIter = theGraph.keySet（）。iterator（）;
while（nodeIter.hasNext（））
minSpanTree.addNode（nodeIter.next（））;
//在优先级队列中排列图中的所有边
PriorityQueue< Edge> pq =新的PriorityQueue< Edge>（allEdges）;
Edge e;
// Kruskals算法。如果它们不是引起循环的同一个集合的一部分，则接受
//的最小边。
while（edgesAccepted< currentSize -1）
{
e = pq.poll（）;
T uset = ds.find（e.n1）;
T vset = ds.find（e.n2）;
if（uset！= vset）
{
//接受边缘
edgesAccepted ++;
ds.union（uset，vset）;
//如果边缘被接受，将其添加到minSpanTree
minSpanTree.connectNodes（e.n1，e.n2，e.cost）;
}
}
返回minSpanTree;
}
}
解决方案
我没有找到任何将成员allEdges初始化为有效的TreeSet对象的地方。尝试初始化（适当的地方看起来就像你在哪里定义它）
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;
}
}
解决方案
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)
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