将两个HashMaps组合成第三个

Question

Let's say I have an object of each class below, and I put each object in a hashmap where IDnumber is the key in both maps.

class1 {
 int IDNumber = 123;  //same person as class2
 String name = John;
 String company = Intel;

 class2 { 
 int IDNumber = 123;  //same person as class1
 int income = 500;
 int workYears = 3;
 } 

HashMap<Integer, class1> one = new Hashmap<Integer, class1>();
HashMap<Integer, class2> two = new HashMap<Integer, class2>();

Now, how can I mash these two HashMaps into a third HashMap so that I can have the key IDnumber, and the values name, company, income, and workyears?

Answer 1

You can not do that. You have two different classes, and java is not going to auto-magically make them one.

You could create a new third class to merge the info:

public Class3 {

   public Class3(Class1 class1, Class2 class2){
       //pull the info you want from each into variables in this class
   }
}

Then loop through your map to get the entries, creating new Class3 instance for each and place them in a new HashMap<Integer, Class3> .

//gets the keys from the hashmap
Set<Integer> keys = one.keySet();
//merge the keys from the second hashmap
keys.addAll(two.keySet());
//new hashmap
Map<Integer, Class3> newMap = new HashMap<Integer, Class3>();
for (Integer key : keys){
     //create new instance and place it in the map
     newMap.put(key, new Class3(one.get(key), two.get(key));
}

Answer 2

Create a 3rd class called Combined in one of these 2 ways:

Combined {
    class1 info1;
    class2 info2;
}

Or, better:

Combined {
    int IDNumber = 123;
    String name = John;
    String company = Intel;
    int income = 500;
    int workYears = 3;
}

• Create a 3rd (empty) HashMap
• Now iterate over all elements in the first HashMap you had before
• For each entry, look up the same key in the 2nd HashMap:
  • If it is found, combine the information from these 2 entries and add it as a single entry of instance Combined to the 3rd HashMap. Then remove both of these entries from both HashMaps one and two.
  • If it is not found, then create a Combined instance anyway based on the entry in HashMap one and just set the unavailable information that would have come from a corresponding entry from HashMap two to null. Remove the entry from HashMap one.
• Now the 1st HashMap should be empty. Iterate HashMap two to find any entries that did not have a corresponding ID in HashMap one. Add them to the 3rd HashMap as above.

Answer 3

You can't store multiple values (ie Class1 and Class2 in your case) with the same key in a java.util.Map. What you want is a Multimap . Guava has an implementation for this. The one you are looking for is ArrayListMultimap .

Answer 4

You need to make a new class which is a mashup of class1 and class2 along with a new Map for it. Each time you put something in one and 'two' you make a new mashup containing both objects and put it into three .

Answer 5

You can use an ArrayList to create an HashMap with multiple of values for one key

ArrayList<> list = new ArrayList();

/* populate your list here with two class having the same ID */

HashMap<Integer, List> three = new Hashmap();

/* Put list on the Hashmap */

/* Redo the operation with another ID */

But it's not very optimized, if you aren't obliged to have an HashMap at final, use directly a multiHashMap: http://commons.apache.org/collections/apidocs/org/apache/commons/collections/MultiHashMap.html

Answer 6

As your data structure is not solit you should use some adapter/wraper

public class UserClassWrapper {

  private final UserClass1 class1;
  private final UserClass2 class2;

  UserWithDetail(UserClass1 class1, UserClass2 class2) {
    //Check preconditions as class1 and class2 must not be null and have equal id
    this.class1 = class1;
    this.class2 = class2;
  }

}

Then in your code you can declare a map:

private Map<Integer,UserClassWrapper> userClassWrappeMap = new HashMap<>();

Answer 7

Are you expecting this kind of arrangment

class class1 {
 int IDNumber = 123;  //same person as class2
 String name = "John";
 String company = "Intel";
 }

 class class2 { 

 int IDNumber = 123;  //same person as class1
 int income = 500;
 int workYears = 3;
 } 


 public class MyData{
public static void main(String arg[]){
    HashMap<Integer, class1> one = new HashMap<Integer, class1>();
    HashMap<Integer, class2> two = new HashMap<Integer, class2>();
    one.put(1, new class1());
    two.put(2, new class2());

    HashMap<Integer, Object> three = new HashMap<Integer, Object>();
    three.putAll(one);
    three.putAll(two);

    System.out.println(three);
}
 }