参数化的HashMaps：编译时错误：类型不兼容

Question

public Class Test{


    HashMap<String, HashMap<String, String>> GlobalMap = new HashMap();
    private String X;



    public static void main(String[] args){
        Test t1 = new Test();
        t1.read(someTreeObject);
        }


@Override
    public Void method1(someCtx) {
        String A = "some value";
        String B = "some other value";

        Map Map1 = new HashMap();
        Map1.put(A,B);

        System.out.println("Local Map : "+Map1.entrySet);

        GlobalMap.put(X, (HashMap<String, String>)GlobalMap.get(X).putAll(Map1));  //<<<Compile time error details with the indicator pointing at Map1 within this line:  error: incompatible types: void cannot be converted to HashMap<String,String>

        // GlobalMap.put(X, (HashMap<String, String>)GlobalMap.get(X).put(new HashMap(A,B)));  //<<<<<<<<<This is another approach, when tried gives a compile time error with the indicator pointing at A within this line:  error: incompatible types: String cannot be converted to int

        System.out.println("Global Map Details : \n"+GlobalMap.entrySet()+"\n");       

    }
    return super.SomeMethod(someCtx);

}

Method1 is an overridden method originally available for use from an Abstract Interface. I know there is nothing that I expect to send to the main method.

why are the putAll and put methods yielding different error messages? What am I really missing here ?

I am new to programming and Java and really trying to learn building advanced HashMaps. I did not encounter anything similar to this error when using the generic HashMap constructions earlier in my experience.

Answer 1

Your error is related to the fact that putAll method from HashMap returns nothing (void), because it is just copy all the mappings from a map to another. See more here . You can extract it outside and then put the map with copied elements.

((HashMap<String, String>)GlobalMap.get(X)).putAll(Map1);
GlobalMap.put(X, (HashMap<String, String>) Map1);

Answer 2

The method HM2.putAll(HM1) copies all the mappings from HM1 to HM2. It does not returns anything. So, possibly do this before you want to cast.

However, not sure what are you trying to achieve here in your code - by putting a get(X) without putting anything into global map, which would result in nullpointer when you run putAll() on it.