[英]Java: How to check hashmap's keys contain object?
Basically, I am trying to read in a text file using the following code in order to build a graph using the adjacency lists representation. 基本上,我试图使用以下代码读取文本文件,以便使用邻接列表表示形式构建图形。 However, I ran into 2 problems.
但是,我遇到了两个问题。
The first and the major problem is that: I do not understand when I check graph.contains(v_l)
, it always return false
. 第一个也是主要的问题是:当我检查
graph.contains(v_l)
时,我不明白,它总是返回false
。 I have been stuck in this problem for ages now. 我已经被这个问题困扰了很久了。 Really need some help here.
真的需要一些帮助。
The second problem: I do not understand why is that within the if
statement, I cannot do the following: 第二个问题:我不明白为什么在
if
语句中无法执行以下操作:
if(graph.containsKey(v_l) == false){
// this always fail
graph.put(v_l, edges_of_this_vertex.add(v_r));
// the following works though
ArrayList<Vertex> edges_of_this_vertex = new ArrayList<Vertex>();
edges_of_this_vertex.add(v_r);
graph.put(v_l, edges_of_this_vertex);
}
I have no idea why this is happening ? 我不知道为什么会这样?
class Vertex{
int node;
....
public Vertex(int node){
this.node = node;
}
public String toString(){
return Integer.toString(node);
}
}
class dgraph{
// constructor ...
// instance method
public HashMap<Vertex, ArrayList<Vertex>> read_file_and_populate(String file_loc, boolean reverse) throws IOException{
HashMap<Vertex, ArrayList<Vertex>> graph = new HashMap<Vertex, ArrayList<Vertex>>();
int l = 0;
int r = 0;
if(reverse == false){
r = 1;
} else{
l = 1;
}
FileInputStream fil = new FileInputStream(file_loc);
BufferedReader br = new BufferedReader( new InputStreamReader(fil));
String element = null;
while( (element = br.readLine()) != null){
String[] line = element.split("\\s");
Vertex v_l = new Vertex( Integer.parseInt(line[l]) );
Vertex v_r = new Vertex( Integer.parseInt(line[r]) );
System.out.println("l = " + l + " r = " + r );
if(graph.containsKey(v_l) == false){
ArrayList<Vertex> edges_of_this_vertex = new ArrayList<Vertex>();
edges_of_this_vertex.add(v_r);
graph.put(v_l, edges_of_this_vertex);
//graph.put(v_l, edges_of_this_vertex.add(v_r));
} else{
graph.get(v_l).add(v_r);
}
}
return graph;
}
}
Below is some example data : 以下是一些示例数据:
1 1
1 2
1 5
1 6
1 7
1 3
1 8
1 4
2 47646
2 47647
2 13019
2 47648
2 47649
2 47650
2 7700
2 47651
2 47652
3 511596
5 1
5 9
Your value class (Vertex) needs to implement hashCode & equals(), otherwise all the operations will be done by checking to see if its the same instance (which it'll never be in this case). 您的值类(Vertex)需要实现hashCode&equals(),否则所有操作都将通过检查其是否为同一实例(在这种情况下永远不会实现)来完成。 assuming the int is the only state in Vertex, the hashCode & equals functions should be based on those eg
假设int是顶点中的唯一状态,则hashCode&equals函数应基于例如
public int hashCode() {
return node *31;
}
public boolean equals(Object o) {
if (o == this) return true;
if (o == null || getClass() != o.getClass()) return false;
Vertex r = (Vertex)o;
return node == r.node;
}
In 在
// this always fail
graph.put(v_l, edges_of_this_vertex.add(v_r));
you are trying to add the result of edges_of_this_vertex.add(v_r) to the graph. 您正在尝试将edge_of_this_vertex.add(v_r)的结果添加到图形中。 The add() function to a arraylist returns a boolean (always true).
数组列表的add()函数返回一个布尔值(始终为true)。 So, when you do a graph.get(v_l), you will always get a boolean true.
因此,当您执行graph.get(v_l)时,将始终获得布尔值true。
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