Jgrapht: how to convert a directed cyclic graph into a DAG?

One thought is run a spanning tree algorithm (e.g., Kruskal) to get the spanning tree, and then construct a new graph from the spanning tree, which is quite messy in terms of code. Looks like there is no available implementation to obtain a DAG subgraph of a directed graph. Please instruct if there are better ways of doing this with jgrapht. Thanks!

As described in the previous section.

Your suggestion is not necessary bad. Not sure why this would be messy in code:

Graph<Integer,DefaultEdge> graphWithCycles = ...; //Your graph with cycles

//Compute spanning tree
SpanningTreeAlgorithm<DefaultEdge> sa = new KruskalMinimumSpanningTree<>(graphWithCycles);
SpanningTreeAlgorithm.SpanningTree<DefaultEdge> tree = sa.getSpanningTree();

//Create new graph without cycles induced by the spanning tree
Graph<Integer,DefaultEdge> graphWithoutCycles = ...; //Your graph without cycles, definition depends on graphWithCycles
for(DefaultEdge edge: tree.getEdges())
    Graphs.addEdgeWithVertices​(graphWithoutCycles, graphWithCycles, edge);

There's various other approaches, e.g. you could also simply run a BreadthFirstSearch on your graph with cycles and create a new graph induced by the BFS tree as you go:

Graph<Integer,DefaultEdge> graphWithCycles = ...; //Your graph with cycles
Graph<Integer,DefaultEdge> graphWithoutCycles = ...; //Your graph without cycles
BreadthFirstIterator​ bfs = new BreadthFirstIterator​(graphWithCycles);
while(bfs.hasNext()){
    Integer vertex = bfs.next(); 
    graphWithoutCycles.addVertex(vertex);
    DefaultEdge edge = bfs.getSpanningTreeEdge(vertex);
    if(edge != null) //Edge is null if the vertex is the root of the BFS tree
        graphWithoutCycles.addEdge(bfs.getParent(vertex),vertex,edge);
}