如何检查在Java中收到的通用my方法？

Question

I have a set method that need to accept 2 kind of generics :

• Map<Long,Map<String,Object>> , and
• Map<Long,Map<String,String>>

However I need to set it as Map<Long,Map<String,String>>

I know that String is an Object, so in the parameter I can use Map<Long,Map<String,Object>> , however I don't want to undergo a conversion if there no need to. Using instanceof seems not working, throwing me with error:

Cannot perform instanceof check against parameterized type Map<Long,Map<String,String>> . Use the form Map<?,?> instead since further generic type information will be erased at runtime

Below is current codes I have now.

public void setRates(Map<Long,Map<String,Object>> inputRates){

    if(inputRates!=null){
        Map<Long,Map<String,String>> result = new HashMap<>();
        // if <Map<Long,Map<String,String>> do below :
        if(inputRates instanceof Map<Long,Map<String,String>>){
        //instanceof not working....   
            this.rates= inputRates;
        }else{
            // if really Map<Long,Map<String,Object>> do below - converting Object to String
            for (Map.Entry<Long, Map<String,Object>> entry1 : inputRates.entrySet()) {
                Map<String,String> resultMap = new HashMap<>(); 

                Long key = entry1.getKey();
                Map<String,Object> inputMap = entry1.getValue();
                for(Map.Entry<String,Object> entry2 : inputMap.entrySet()) { 
                    String key2 = entry2.getKey();
                    Object value2 = entry2.getValue();
                    if(value2!=null){
                        resultMap.put(key2, value2.toString());  
                    }else{
                        resultMap.put(key2, null);  
                    }
                }
                result.put(key, resultMap);
            }
            this.rates = result;
        } 
    }
}

Any idea on how I implement this?

Answer 1

You can have a generic method .

public <T> void setRates(Map<Long,Map<String, T>> inputRates){ }

Then use generic type T in your method's body.

Answer 2

Generics are erased after compilation. So you could never use instanceof with a parameterized type class :

if(inputRates instanceof Map<Long,Map<String,String>>){

Using instanceof with Map<?,?> will be useless either as it is still broader. Map<Object, Object> being broader than Map<String,Object> .

If the clients of your method declare a broader generic variable : Map<String,Object>String,Object> , you is stuck to cast Object to String in the processing of the Map .

Other option : create a new method with Map<Long,Map<String,String>> as parameter.
The client would call so the first one or the second according to what one manipulates.

Answer 3

You can overload with named classes such:

class ObjectMap extends Map<Long,Map<String,Object>>{}
class StringMap extends Map<Long,Map<String,String>>{}
public void setRates(ObjectMap inputRates){ }
public void setRates(StringMap inputRates){ }

They will work as their parametrized parts but you will need to instantiate then instead.