Gil Shoam

Thanks for your answer but it didn't really answer my question.
With try/catch I define what happens in case of an exception. (for example i can print "an exception occured"). In this case (main throws) what actually happens? what does the program do?

public class Value{ private int num=0; public Value() throws Exception{ if(something) throw new Exception(); } } public static void main(String[] args) throws Exception{ Value val= new Value(); }

in the above code, where is the Exception actually handled? It just keeps getting thrown...
isn't there supposed to be a try/catch somewhere to define what happens if the Exception is thrown?
I know that if I would delete the "throws" from the main, I would be forced to try/catch, but if main throws where is it caught?
Thanks for your time

Jeff Verdegan wrote:Put the code in a method.

isn't there a "redo case(a)"
or something similar?

Hi,

I'm trying to do an inner loop using the switch option.
I want to do something like this:switch(options){ case(a): //do something break; case(b): //do something break; case(c) //do something if(something) //redo case(a) if(somethingElse) //redo case(b) break; }

I know I can just copy paste the code.. but is there a more elegant way to do it?

Thanks

Stephan van Hulst wrote:

Why are you using such a method anyway? Why not provide a constructor that takes an array?

You are right.. that is what I meant to do.....

(it's 2:30 a.m here... )

but anyway can you elaborate on this:

Stephan van Hulst wrote:Because you are redeclaring the type parameter E in your method.

Hi,

I'm trying to implement a generic class used to define a set of different types. I'm trying to add a constructor that stores an array into the set. for some reason the add method does not work. (The compiler says: "Add (E) in arrayList can not be applied to (E)"

Here is my code: public class Set<E> { private ArrayList<E> elements; public Set() { elements = new ArrayList<E>(); } public <E> void setFromArray(E[] inputArray) { elements = new ArrayList<E>(inputArray.length); for (E e : inputArray) elements.add(e); } }

Any idea why It won't add the elements?

Thanks

I'll clarify: (the Object identifies and translates 'A' into new Character('A'), so that is not the problem)

this is the Node class:
public class Node { private Object _object; public Node(Object obj) { _object= obj; } public String toString() { return _object.toString(); } public boolean equals(Node node){ if(this._object.toString().equals(node.toString())) return true; return false; } public static void main(String[] args) { //Character character = new Character('A'); Node node = new Node('A'); Node node1= new Node('A'); System.out.println(node.toString()); System.out.println(node1.toString()); System.out.println(node.equals(node1));

it prints :
A
A
true

meaning it compares two different nodes and 'equals' checks if they have the same name.

this is the Edge Class:
public class Edge { private Node _source; private Node _dest; public Edge(Node source, Node dest) { _source = source; _dest = dest; } public String toString() { return "(" + _source.toString() + "," + _dest.toString() + ")"; } public boolean equals(Edge edge) { if (this.toString().equals(edge.toString())) return true; return false; } public static void main(String[] args) { Node nodeS = new Node('C'); Node nodeD = new Node('D'); Node nodeS1= new Node('C'); Node nodeD1= new Node('D'); Edge edge = new Edge(nodeS, nodeD); Edge edge1 = new Edge(nodeS1,nodeD1); System.out.println(edge.toString()); System.out.println(edge1.toString()); System.out.println(edge.equals(edge1));
prints
(C,D)
(C,D)
true

so here the equals also works.

in the graph class i did this:

public class Graph { private HashSet<Node> _nodes; private HashSet<Edge> _edges; public Graph() { _nodes = new HashSet<Node>(); _edges = new HashSet<Edge>(); } public Graph(ArrayList<Node> nodeList, ArrayList<Edge> edgeList) { _nodes = new HashSet<Node>(nodeList); _edges = new HashSet<Edge>(edgeList); } public void addNode(Node node) { _nodes.add(node); } public void removeNode(Node node) { _nodes.remove(node); } public void addEdge(Edge edge) { _edges.add(edge); } public void removeEdge(Edge edge) { _edges.remove(edge); } public boolean isNode(Node node) { return _nodes.contains(node); } public boolean isEdge(Edge edge) { return _edges.contains(edge); } public boolean equals(Graph graph) { if (this._nodes.equals(graph._nodes)) //&& _edges.equals(graph._edges)) return true; return false; } public String toString() { String nodes = "Nodes:" + _nodes.toString(); String edges = "Edges" + _edges.toString(); return nodes + "\n" + edges; /* String nodes = ""; Iterator iterator = _nodes.iterator(); while (iterator.hasNext()) nodes = iterator.next() + ", "; return nodes; */ } public static void main(String[] args) { //new list of nodes ArrayList<Node> nodes = new ArrayList<Node>(); //creates new nodes Node nodeA = new Node('A'); Node nodeB = new Node('B'); Node nodeC = new Node('C'); Node nodeD = new Node('D'); Node nodeA2= new Node('A'); //add nodes to the list nodes.add(nodeA); nodes.add(nodeB); nodes.add(nodeC); nodes.add(nodeD); nodes.add(nodeA2); //new list of edges ArrayList<Edge> edges = new ArrayList<Edge>(); //creates new edges Edge edge1 = new Edge(nodeA, nodeB); Edge edge2 = new Edge(nodeA, nodeC); Edge edge3 = new Edge(nodeB, nodeC); //add edges to the list edges.add(edge1); edges.add(edge2); edges.add(edge3); Graph graph = new Graph(nodes, edges); //create another similar graph ArrayList<Node> nodes1 = new ArrayList<Node>(); Node nodeE = new Node('A'); Node nodeF = new Node('B'); Node nodeG = new Node('C'); Node nodeH = new Node('D'); nodes1.add(nodeE); nodes1.add(nodeF); nodes1.add(nodeG); nodes1.add(nodeH); ArrayList<Edge> edges1 = new ArrayList<Edge>(); Edge edge4 = new Edge(nodeA, nodeB); Edge edge5 = new Edge(nodeA, nodeC); Edge edge6 = new Edge(nodeB, nodeC); edges1.add(edge4); edges1.add(edge5); edges1.add(edge6); Graph graph1 = new Graph(nodes1, edges1); System.out.println(graph.toString()); System.out.println(graph1.toString()); System.out.println(graph.equals(graph1)); System.out.println(graph._nodes.contains(nodeA)); } }
it prints:
Nodes: [A,D,B,A,C]
Edges: [(A,B),(B,C)(A,C)]
nodes1: [A,C,B,D]
Edges 1:[(A,B),(B,C)(A,C)]
false
true

so there are two problems here.
1) How can I add another 'A' to Nodes? a hashset isn't supposed to allow it. i.e it has two similar elements. I know it sees them as different because it is looking at the 'Node' wrapping.
2) If I erase the extra 'A' from Nodes, and to the equals method again (which currently only checks the _nodes)
it still returns false even though they have the same elements in the set.
the last line gives 'true' because the set has wrapped the Characters with the nodes, so I guess the equals method is comparing that. (So these 2 problems are actually the same I think..?)
That is where I am stuck..
Hope this helps...

Hi,
I'm writing a program that represents a graph.
I have 3 classes.
1. a Node class (which takes an object)
2. a Edge class (which build an edge from a source Node to destination Node)
3. A graph class. Here it is:
public class Graph { private HashSet<Node> _nodes; private HashSet<Edge> _edges; public Graph() { _nodes = new HashSet<Node>(); _edges = new HashSet<Edge>(); } public Graph(ArrayList<Node> nodeList, ArrayList<Edge> edgeList) { _nodes = new HashSet<Node>(nodeList); _edges = new HashSet<Edge>(edgeList); }

I'm trying to override the equals and hashcode methods to check if two graphs are similar.
meantime I wrote it just to check the nodes but with no luck:
public boolean equals(Graph graph) { if (this._nodes.equals(graph._nodes)) //&& _edges.equals(graph._edges)) return true; return false; }
when I tried System.out.println(graph.equals(graph1)); System.out.println(graph._nodes.contains(nodeA));

I got:
false
true

in other words I wasn't able to implement the method for the actual Node which contains a char, but only for it's wrapping. for the same reason I am able to add two similar char to the _nodes hashset (which should in theory not be possible). I did so and the did toString and got
Nodes: [A,B,D,A,C]
So it sees is as two different object although I want it to see the two A's as the same (and then it won't add another A right?)
I understand what the problem is, I just don't know how to solve it.

BTW, I wrote equal methods in the Node and Edge classes, both work fine. (in both cases i based the methods on the toString result)
Here:
public boolean equals(Node node){ if(this._object.toString().equals(node.toString())) return true; return false; } public static void main(String[] args) { //Character character = new Character('A'); Node node = new Node('A'); Node node1= new Node('A'); System.out.println(node.toString()); System.out.println(node1.toString()); System.out.println(node.equals(node1));
prints:
A
A
true

and in Edge:
public boolean equals(Edge edge) { if (this.toString().equals(edge.toString())) return true; return false; } public static void main(String[] args) { Node nodeS = new Node('C'); Node nodeD = new Node('D'); Node nodeS1= new Node('C'); Node nodeD1= new Node('D'); Edge edge = new Edge(nodeS, nodeD); Edge edge1 = new Edge(nodeS1,nodeD1); System.out.println(edge.toString()); System.out.println(edge1.toString()); System.out.println(edge.equals(edge1));
returns:
(C,D)
(C,D)
true


Please Help, and sorry for the long post

Thanks

Gil

I got it!

In the constructor I didn't use the obj I got. This solved it

public Node(Object obj) { _object= new Object(); _object= obj; }

Thanks for your help.

If I have more Questions related to this project should I keep posting on this thread or open a new one?

Thanks again,

Gil

So you could have Graph's toString() which calls Node's toString() which calls it's data's toString(), with each layer adding whatever structural stuff it knows about/is responsible for (commas, brackets, etc.).

I tired this in the node class and the output I got is the object address not the actual char:

public class Node { private Object _object; public Node(Object obj) { _object = new Object(); } public String toString() { String res = _object.toString(); return res; } public static void main(String[] args) { //Character character = new Character('A'); Node node = new Node('A'); System.out.println(node.toString()); } }
I also tried wrapping it with the Character class but still the same problem...

Any suggestions?
Thanks

Jeff Verdegan wrote:

Gil Shoam wrote:Fast reply

Not exactly, that was just a test to see if I can access the char from graph._nodes.

What I want is to write a String representation of the graph inc the nodes and edges.

Then your Graph class needs to override toString() to pring out the nodes and edges how you want. And if you have classes for Node and/or Edge, they need to override toString to display themselves appropriately.

That I know..

How do I do it? Since I don't know In advance what types the graph has.
i tried overriding the toString() in the Node Class using the StringBuffer but it does not make sense...
if I don't yet know what type it is how can I write a string method for it?

Fast reply

Not exactly, that was just a test to see if I can access the char from graph._nodes.

What I want is to write a String representation of the graph inc the nodes and edges.
i.e the output for the main (above) should be:
Nodes: {A,,B,C,D}
Edges: {(A,B),(A,C),(B,C)

Hi,
sorry if this should be in Java for beginners. I wasn't sure..

Anyway, I'm writing a program that deals with graphs of any primitive types.
I have 3 classes.
One represents the Nodes:
public class Node { private Object _object; public Node(Object obj) { _object = new Object(); } }
Another the Edges
public class Edge { private Node _source; private Node _dest; public Edge(Node source, Node dest) { _source = source; _dest = dest; } }

And finally the Graph:
public class Graph { private HashSet<Node> _nodes; private HashSet<Edge> _edges; public Graph() { _nodes = new HashSet<Node>(); _edges = new HashSet<Edge>(); } public Graph(ArrayList<Node> nodeList, ArrayList<Edge> edgeList) { _nodes = new HashSet<Node>(nodeList); _edges = new HashSet<Edge>(edgeList); } public void addNode(Node node) { _nodes.add(node); } public void removeNode(Node node) { _nodes.remove(node); } public void addEdge(Edge edge) { _edges.add(edge); } public void removeEdge(Edge edge) { _edges.remove(edge); } public boolean isNode(Node node) { return _nodes.contains(node); } public boolean isEdge(Edge edge) { return _edges.contains(edge); } public boolean equals(Graph graph) { if (_nodes.equals(graph._nodes) && _edges.equals(graph._edges)) return true; return false; }

Just to test it I ran this Main:
public static void main(String[] args) { //new list of nodes ArrayList<Node> nodes = new ArrayList<Node>(); //creates new nodes Node nodeA = new Node('A'); Node nodeB = new Node('B'); Node nodeC = new Node('C'); Node nodeD = new Node('D'); //add nodes to the list nodes.add(nodeA); nodes.add(nodeB); nodes.add(nodeC); nodes.add(nodeD); ArrayList<Edge> edges = new ArrayList<Edge>(); Edge edge1 = new Edge(nodeA, nodeB); Edge edge2 = new Edge(nodeA, nodeC); Edge edge3 = new Edge(nodeB, nodeC); Graph graph = new Graph(nodes, edges); Iterator iterator = graph._nodes.iterator(); while (iterator.hasNext()) System.out.print(iterator.next() + ", "); System.out.println(); }


Problem is as you can see, it prints the object address.
I actually need to write a String representation of the graph inc the nodes and edges.
i.e the output should be:
Nodes: {A,,B,C,D}
Edges: {(A,B),(A,C),(B,C)


(here I tried printing only the nodes to see if it works, but no luck)
Since the actual node is an object how can I downcast it to the type I initialized it with?
Or in general- How do I override object toString() method for the graph class?

Thanks in advance

Gil