使用HashMap在我自己的图类中使用NPE

Question

This is for a school project. We're supposed to write a program handling graphs with the help of a HashMap. Each node (or corner?) has a key (which is another node to which we have a connection) and a value (the cost of this connection). I'm almost done and all my test cases works except for the toString() method. It is supposed to write something like this:

{(0,1), (2,1)}

This means that 0 and 1 are connected and also 2 and 1 are connected.

/**
 * Returns a string representation of this graph.
 * 
 * @return a String representation of this graph
 */
@Override
public String toString() {
    // TODO
    StringBuilder sb = new StringBuilder();
    sb.append("{");
    for (int i = 0; i < edges.length; i++) {
        for (int j = 0; j < edges[i].size(); j++) {

            if (edges[i].containsKey(j)) {
                if (j < edges[i].size() - 1) {
                    sb.append("(" + i + "," + j + "),");
                } else if (j == edges[i].size() - 1) {
                    sb.append("(" + i + "," + j + ")");
                }

            }
        }
        if (i != edges.length - 1) {
            sb.append(", ");
        }
    }
    sb.append("}");
    System.out.println(sb.toString());
    return sb.toString();
}

Since our code structure (the code we started with) has a static size for the HashMap I get a NPE if I make a graph for 5 nodes and only add 3 of them. When I try to print these I will get a NPE due to edges[i].containsKey(j)) .

Answer 1

It's possible that you aren't initializing every element of the edges array.

You can test that the edges index is not null before you even attempt to check if the key is contained.

Change:

if (edges[i].containsKey(j))

To:

if (edges[i] != null && edges[i].containsKey(j))

This is possible because of short-circuit evaluation:

http://en.wikipedia.org/wiki/Short-circuit_evaluation

You also might not even want to enter the nested for-loop if the value is null. In this situation, wrap the nested for-loop around a condition to ensure edges[i] != null.