Map containsKey返回false

Question

My got a HashMap data:

Map<CharSequence, MyObject> dataMap = GET_FROM_SOME_WHERE

There is a key in the dataMap which is a CharSequence type with value "company.name"

But the following code returns me false :

String field = "company.name";
dataMap.containsKey(field); //This return me false

I somehow feel that it is because my field variable is a String object while the key in HashMap is CharSequence . That's why it returns me false.

If my guess is correct, then how to get rid of it? I need the above code returns me true. I am sure the key "company.name" is in that map data as key.

Answer 1

The Java API Spec has this to say on the subject:

'Each object may be implemented by a different class, and there is no guarantee that each class will be capable of testing its instances for equality with those of the other. It is therefore inappropriate to use arbitrary CharSequence instances as elements in a set or as keys in a map.'

Answer 2

为什么不放

CharSequence field = "company.name";

Answer 3

Are you putting StringBuffers into a HashMap? Because that is not going to work, since StringBuffer does not define a hashCode method. It inherits hashCode from java.lang.Object which uses the object's identity as hash code.

String on the other hand calculates hash code from the actual string data.

Edit: StringBuffer doesn't implement equals() method either, so it won't work at all like one would expect. (new StringBuffer("1")).equals(new StringBuffer("1")) -> false.

Answer 4

In order for this to work, the implementation of CharSequence inside your map must recognize String s in checking for equality, produce identical hash codes with the String , and return true when compared for equality to a string with the same value (ie equals should work from both sides). This is not possible unless the implementation of CharSequence is actually a String .

One way to address this is converting the Map<CharSequence,MyObject> to a Map<String,MyObject> . Iterate through the entry set of the original map, and put the data into a copy that uses the String as the key, like this:

Map<String,MyObject> copy = new HashMap<String,MyObject>();
for (Map.Entry<CharSequence,MyObject> e : dataMap.entrySet()) {
    copy.put(e.getKey().toString(), e.getValue());
}

Answer 5

Using Collection can be quite tricky sometimes.

The way that a Map.containsKey() works is by examining object equality by calling the .equals() method of your 'key' objects. So, if you put things in the map via a StringBuffer - which also implements CharSequence - but try to ask whether the key exists in the map but providing a String as the key, you are really challenging the Collections framework. This can be shown by below example:

Map<CharSequence, Integer> dataMap = new HashMap<CharSequence, Integer>();
StringBuffer sb = new StringBuffer();
sb.append("company.name");
CharSequence cs = sb;
dataMap.put(cs, 123);

String k = "company.name";
// Below prints a 'false'
System.out.println(dataMap.containsKey(k));

Accordingly, my advice is to always use the same type of objects as key entries in a collection (not just Map). In this case, perhaps you can also define your Map as

Map<String, MyObject>

Answer 6

Possible reason is that equals() method is used in your map. Another is hashCode() is used. If key in map isn't a String, it can give you another values by these methods.

If you are getting map somewhere outside and cannot control it, iteration over key set seems to be the only approach here. Otherwise I would suggest to change map to Map<String, MyObject>

public static void main(String[] args) {
    Map<CharSequence, String> dataMap = new HashMap<>();
    dataMap.put(new StringBuilder("company.name"), "AAA");
    System.out.println(dataMap.containsKey("company.name"));
    System.out.println(mapContainsKeyNamed("company.name", dataMap));
}

static boolean mapContainsKeyNamed(String key, Map<CharSequence, ?> map) {
    for (CharSequence cs : map.keySet()) {
        if (key.equals(cs.toString())) {
            return true;
        }
    }
    return false;
}

output:

false
true