如何在不覆盖以前的数据的情况下使用 hashmap.put？ （爪哇）

Question

I have something along the lines of this:

public HashMap<Boolean, String> map = new HashMap();
map.put(this.inverted, "Inverted");
map.put(this.active, "Loading");
System.out.println(map.size());

after seeing that the size was always 1, I realised that using map.put was overriding the previous data. I am currently trying to iterate over the hashmap. Is there a way to add mappings to it without overriding previous ones?

Answer 1

You have declared your HashMap as: -

public HashMap<Boolean, String> map = new HashMap();

Now, just think how many maximum mapping can you have in your map ? The answer you can get by thinking of, what all values can your Boolean type take. This is because, you cannot have duplicate keys in a HashMap .

So, probably you got it now, that you can at max have only 2 mappings in your map, one for true and other for false (In fact you can have a 3rd one too, as you can have a mapping for a null key too in your HashMap ).

So, in your case, if both this.inverted and this.active are either true or false . Then only one of them can be there, and that would be the later value inserted.

Is there a way to add mappings to it without overriding previous ones?

Probably you have build your HashMap wrongly. You should declare your map as: -

private Map<String, Boolean> map = new HashMap();

And now you can put two mappings as: -

map.put("Inverted", this.inverted);
map.put("Loading", this.active);

Answer 2

It's because this.inverted.equals(this.active) and this.inverted.hashcode()==this.active.hashcode()

Maybe you need redefine the equals method for the key.

Answer 3

In MAP

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value. ---> fromMap Api

from your implementation, may be this.inverted and this.active both have same value.

Check the input once. print the keySet, then check.

or change the input to Map<String, Boolean>

Answer 4

As @Frank suggest you should invert your Map.

public final Map<String, Boolean> map = new HashMap<>();

map.put("Inverted", this.inverted);
map.put("Loading", this.active);
System.out.println(map);

Answer 5

If the keys are the same than the previous value is overwritten in a standard Java Map. If you don't want this, you can have a look at a multimap which is implemented for example in commons-collections. It can hold different values for one key.

Answer 6

Hashmap is based on key/value pairs. If your keys are equal (they have the same hashcode), it will behave as you described.

For your use case, reversing your key/value pairs will help you.

public HashMap<String, Boolean> map = new HashMap();
map.put("Inverted", this.inverted); 
map.put("Loading", this.active);
System.out.println(map.size());

Answer 7

Get object of innermap ,by passing outer map key .. Then check if key of innermap exists then update values with previous data. else create new object of inner map.