在HashMap中使用自定义类作为键，但无法搜索键 [英] Using a customed class as keys to in HashMap but not able to search the keys

问题描述

我创建了一个使用自定义类Location作为键的HashMap. 使用put()将所有条目插入HashMap后，我无法搜索键.

我尝试使用get()或containsKey()进行搜索，但都没有给我带来积极的结果.但是，我确定键确实存在于代码中，因为我使用了HashMap迭代来打印键.

下面是代码:

public HashMap<Location, Integer>beenTo = new HashMap<>();
public int uniquePathsIII(int[][] grid) {
        for (int i=0; i<grid.length; i++){
             for (int j=0; j<grid[0].length; j++){
                 if (grid[i][j] == 0 || grid[i][j] == 2){
                     Location newSquare = new Location(i,j);
                     notBeen.put(newSquare, 1);                    
                 }           
             }
        }
        Location newSquare = new Location(0,1);
        if (notBeen.get(newSquare) != null){
             return 10;
         }
        if (notBeen.isEmpty()){
            return -1;
        }
}

下面是课程位置:

class Location{
        int i;  // row  
        int j;  // column
        public Location(int _i, int _j){
            i = _i;
            j = _j;
        }
        public int getI(){
            return i;
        }
        public int getJ(){
            return j;
        }
        public void setI(int _i){
            i = _i;
        }
        public void setJ(int _j){
            j = _j;
        }
}

在上面的代码中，我想搜索键Location(0,1).我已经确保Hashmap notBeen不为空，并尝试该键确实存在.但是我无法使用containsKey()或get()来找到它.

解决方案

如果希望自定义对象用作Java HashMap中的键，则需要实现/覆盖hashCode和equals方法.

仅供参考: _variableName违反Java命名约定 Oracle Java命名约定.这也是不必要的，因为您可以使用以下方法获得相同的结果:

public Location(int i, int j){
  this.i = i;
  this.j = j;
}

I created a HashMap that uses a customed class Location as keys. After inserting all the entries into the HashMap using put(), I am not able to search the keys.

I have tried to use get() or containsKey() to search, but neither give me positive results. However, I am sure that the keys do exists in the code because I have used HashMap iteration to print out the keys.

Below is the code:

public HashMap<Location, Integer>beenTo = new HashMap<>();
public int uniquePathsIII(int[][] grid) {
        for (int i=0; i<grid.length; i++){
             for (int j=0; j<grid[0].length; j++){
                 if (grid[i][j] == 0 || grid[i][j] == 2){
                     Location newSquare = new Location(i,j);
                     notBeen.put(newSquare, 1);                    
                 }           
             }
        }
        Location newSquare = new Location(0,1);
        if (notBeen.get(newSquare) != null){
             return 10;
         }
        if (notBeen.isEmpty()){
            return -1;
        }
}

Below is the class Location:

class Location{
        int i;  // row  
        int j;  // column
        public Location(int _i, int _j){
            i = _i;
            j = _j;
        }
        public int getI(){
            return i;
        }
        public int getJ(){
            return j;
        }
        public void setI(int _i){
            i = _i;
        }
        public void setJ(int _j){
            j = _j;
        }
}

In the above code, I wanted to search for the key Location(0,1). I have made sure that the Hashmap notBeen is not empty, and tried that the key does exist. But I am never able to find it using containsKey() nor get().

解决方案

You need to implement/override hashCode and equals methods if you want a custom Object to work as a key in a HashMap in Java.

FYI: _variableName goes against Java naming conventions Oracle Java Naming Convention. It is also not necessary as you can get the same result using:

public Location(int i, int j){
  this.i = i;
  this.j = j;
}

这篇关于在HashMap中使用自定义类作为键，但无法搜索键的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！