Graph Using An Array of Sets

Graph.java

import java.util.NoSuchElementException;

public class Graph
{
    private static final String NEWLINE = System.getProperty( "line.separator" );

    private final int V;
    private int E;
    private Bag<Integer>[] adj;
    
    public Graph( int V )
    {
        if ( V < 0 ) throw new IllegalArgumentException( "Number of vertices must be nonnegative" );
        this.V = V;
        this.E = 0;
        adj = ( Bag<Integer>[] ) new Bag[ V ];

        for ( int v = 0; v < V; v++ )
        {
            adj[ v ] = new Bag<Integer>();
        }
    }

    public Graph( In in )
    {
        try
        {
            this.V = in.readInt();

            if ( V < 0 ) throw new IllegalArgumentException( "number of vertices in a Graph must be nonnegative" );
            adj = ( Bag<Integer>[] ) new Bag[ V ];

            for ( int v = 0; v < V; v++ )
            {
                adj[ v ] = new Bag<Integer>();
            }
            int E = in.readInt();

            if ( E < 0 ) throw new IllegalArgumentException( "number of edges in a Graph must be nonnegative" );

            for ( int i = 0; i < E; i++ )
            {
                int v = in.readInt();
                int w = in.readInt();
                validateVertex( v );
                validateVertex( w );
                addEdge( v, w ); 
            }
        } catch ( NoSuchElementException e ) {
            throw new IllegalArgumentException( "invalid input format in Graph constructor", e );
        }
    }

    public Graph( Graph G )
    {
        this( G.V() );
        this.E = G.E();

        for ( int v = 0; v < G.V(); v++ )
        {
            Stack<Integer> reverse = new Stack<Integer>();

            for ( int w : G.adj[ v ] )
            {
                reverse.push( w );
            }

            for ( int w : reverse )
            {
                adj[ v ].add( w );
            }
        }
    }

    public int V() { return V; }

    public int E() { return E; }

    private void validateVertex( int v )
    {
        if ( v < 0 || v >= V )
            throw new IllegalArgumentException( "vertex " + v + " is not between 0 and " + ( V - 1 ) );
    }

    public void addEdge( int v, int w )
    {
        validateVertex( v );
        validateVertex( w );
        E++;
        adj[ v ].add( w );
        adj[ w ].add( v );
    }

    public Iterable<Integer> adj( int v )
    {
        validateVertex( v );
        return adj[ v ];
    }

    public int degree( int v )
    {
        validateVertex( v );
        return adj[ v ].size();
    }

    public String toString()
    {
        StringBuilder s = new StringBuilder();
        s.append( V + " vertices, " + E + " edges " + NEWLINE );

        for ( int v = 0; v < V; v++ )
        {
            s.append( v + ": " );
            
            for ( int w : adj[ v ] )
            {
                s.append( w + " " );
            }
            s.append( NEWLINE );
        }
        return s.toString();
    }

    public static void main( String[] args )
    {
        In in = new In( args[ 0 ] );
        Graph G = new Graph( in );
        StdOut.println( G );
    }
}