HashMap（Java）给出错误的结果

Question

I have created a hash map in which each entry corresponds to 3 values Key object values ( which are two in number)

I have created a class ,whose object i create and store the results in a hash map This is my code below in which i compare my incoming data with the previous values in the hash map.If the same data comes then i just increment the counter of that data. I have taken the print statements in the the for loop . though the two strings match but still my code never comes in the if loop for increment the counter.Why?

 for(i=1;i<=hMap.size();i++)
 {
      String skey = Integer.toString(i);
      if(hMap.get(skey).olddata==comingdata)
      {
           hMap.get(skey).counter=  hMap.get(skey).counter+1;
      }
 }

Answer 1

You haven't given nearly enough information about the types involved, but, but I strongly suspect that this is the problem:

if(hMap.get(skey).olddata==comingdata)

That will be comparing references , rather than for equality, if olddata and comingdata are references of some kind. (EDIT: By the sounds of it, they're string references.)

My guess is that you want:

String skey = Integer.toString(i);
if(hMap.get(skey).olddata.equals(comingdata))
{
   hMap.get(skey).counter=  hMap.get(skey).counter+1;
}

Or rather more efficiently, avoiding pointless lookups:

WhateverType value = hMap.get(Integer.toString(i));
if (value.olddata.equals(comingdata))
{
    value.counter++;
}

I'd also suggest that if you're always going to look up by an integer, why not use an Integer key instead of always converting the integer into a string?

Additionally, it's worth following Java naming conventions, and you should make your fields private if they're not already.

If none of this helps, please post more code. The chances of the problem being in HashMap rather than in your code are incredibly small.

Answer 2

You don't compare objects with == in Java unless you're trying to see if they have the same reference value .

if (hMap.get(skey).olddata.equals(comingdata)) {
...

You also shouldn't be exposing olddata like that; it should be available via a getter; eg getOldData()

Answer 3

It's not clear the type of olddata , but maybe you should compare the values using equals() :

if (hMap.get(skey).olddata.equals(comingdata))

In Java, == is used for either comparing primitive data types for equality or comparing object types for identity . If you need to compare two object types for equality , then you must use the equals() method, which is defined for all objects since it's inherited from the Object class, being aware that you also must override equals() and hashCode() in your class, providing implementations meaningful for that class.

Answer 4

它应该是

if(hMap.get(skey).olddata.equals(comingdata))

Answer 5

Do you actually mean comingdata.equals(hMap.get(skey).olddata) ? Furthermore be aware that equals(Object) and hashCode() must be correclty implemented.