Java creating a custom Map

Question

I have two questions regarding my following code: 1) Is there any way I can `implement Map` in this case, rather than extend just one direct implementation of the interface? I do not wish to write a whole Map implementation though. But it could be nice seeing that my implementation does not care what underlying Map implementation is being used. 2) Are there any bad practices? ExtremesMap.java ``` package ocr.util; import java.util.Arrays; import java.util.HashMap; import java.util.HashSet; import java.util.List; import java.util.Set; /** * * @author student */ public class ExtremesMap extends HashMap { private Set smallest; private int smallestValue; private Set biggest; private int biggestValue; public ExtremesMap() { super(); smallest = new HashSet<>(); smallestValue = Integer.MAX_VALUE; biggest = new HashSet<>(); biggestValue = Integer.MIN_VALUE; } @Override public void put(String key, Integer value) { if (value == null) { throw new IllegalArgumentE…

Accepted Answer

As the error says, the `put` signature is:

```
 V put(K key, V value)
```

That means that your method `put` should return `Integer` and not `void`:

```
public Integer put(String key, Integer value) { ... }
```

Concerning your first two questions, I would say that it is not a recommended practice to extend `HashMap`, because you are changing the way a Map is working when you override `put`. You should better use composition instead (a normal class which contains a private field `Map` for internal use).

For a whole code review of your code, you could ask at Code Review StackExchange.

Java creating a custom Map

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