谁能解释下面 for 循环中发生的事情？

Question

My expected output is

4 vertices, 4 edges 

0: 0 1 0 1
1: 1 0 1 0 
2: 0 1 0 1 
3: 1 0 1 0 

And it works fine without any problem but I don't know what's happening inside the second for loop in the following toString() method statement.

can anyone say what is going on inside this method especially in for loop?? And also why this program uses StringBuilder instead of String data type.

---

 public String toString() {
            StringBuilder sb = new StringBuilder();
            sb.append(V + " vertices, " + E + " edges " + "\n");
            for(int v = 0; v < V; v++) {
                sb.append(v + ": ");
                for(int w : adjMatrix[v]) {
                    sb.append(w + " ");
                }
                sb.append("\n");
            }
            return sb.toString();
        }

---

Here is my full graph program.

    public class Graph {

    private int V; // number of vertices in Graph
    private int E; // number of edges in Graph
    private int[][] adjMatrix;

    public Graph(int nodes) {
        this.V = nodes;
        this.E = 0;
        this.adjMatrix = new int[nodes][nodes];
    }

    public void addEdge(int u, int v) {
        adjMatrix[u][v] = 1;
        adjMatrix[v][u] = 1;  // because it is an undirected graph
        E++;
    }

    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append(V + " vertices, " + E + " edges " + "\n");
        for(int v = 0; v < V; v++) {
            sb.append(v + ": ");
            for(int w : adjMatrix[v]) {
                sb.append(w + " ");
            }
            sb.append("\n");
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        Graph g = new Graph(4);
        g.addEdge(0, 1);
        g.addEdge(1, 2);
        g.addEdge(2, 3);
        g.addEdge(3, 0);
        System.out.println(g.toString());
    }
}

Answer 1

对于二维数组 adjMatrix 中的每个 Integer，您都将其值附加到 StringBuilder 中（实际上只是连接字符串）。