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Q&A Map<?, Optional<T>> to Map<?, T>

Based on the code snippet in the question (which uses collect etc), I'm assuming you want to use streams. I'm also infering that "empty values" means those values for which Optional.isEmpty() retur...

posted 1mo ago by hkotsubo‭ · edited 1mo ago by hkotsubo‭

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#2: Post edited by

hkotsubo‭ · 2024-09-11T18:16:54Z (about 1 month ago)

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Based on the code snippet in the question (which uses `collect` etc), I'm assuming you want to use streams. I'm also infering that "empty values" means those values for which `Optional.isEmpty()` returns `false` (and consequently `Optional.isPresent()` returns `true`).
In that case, just create a stream for the map entries, filter the non-empty ones and collect to a new map, by getting the `Optional`'s values:
```java
Map<K, T> result = input
// get stream of map entries
.entrySet().stream()
// check if value is present (which means it's not empty)
.filter(e -> e.getValue().isPresent()) // alternative: .filter(e -> ! e.getValue().isEmpty())
// collect to a new map, getting the values from the Optional
.collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().get()));
```
But of course you can also do it without streams, using the traditional loop approach:
```java
Map<K, T> result = new HashMap<>();
for (Map.Entry<K, Optional<T>> entry : input.entrySet()) {
// value is present (AKA: not empty)
if (entry.getValue().isPresent()) { // alternative: if (! entry.getValue().isEmpty())
// add the Optional's value to result
result.put(entry.getKey(), entry.getValue().get());
}
}
```

Based on the code snippet in the question (which uses `collect` etc), I'm assuming you want to use streams. I'm also infering that "empty values" means those values for which `Optional.isEmpty()` returns `false` (and consequently `Optional.isPresent()` returns `true`).
In that case, just create a stream for the map entries, filter the non-empty ones and collect to a new map, by getting the `Optional`'s values:
```java
Map<K, T> result = input
// get stream of map entries
.entrySet().stream()
// check if value is present (which means it's not empty)
.filter(e -> e.getValue().isPresent()) // alternative: .filter(e -> ! e.getValue().isEmpty())
// collect to a new map, getting the values from the Optional
.collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().get()));
```
But of course you can also do it without streams, using the traditional loop approach:
```java
Map<K, T> result = new HashMap<>();
for (Map.Entry<K, Optional<T>> entry : input.entrySet()) {
// value is present (AKA: not empty)
if (entry.getValue().isPresent()) { // alternative: if (! entry.getValue().isEmpty())
// add the Optional's value to result
result.put(entry.getKey(), entry.getValue().get());
}
}
```
---
IMO, the second approach makes it easier to change the `Map` implementation - if you want a `TreeMap` instead of a `HashMap`, for example, just need to change the line `Map<K, T> result = new HashMap<>();` to use whatever implementation type you want.
With streams, it's also possible, but not so straighforward. You'd have to change the collector to:
```
Map<K, T> result = input
....
.collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().get(), (a, b) -> a, TreeMap::new));
```
The fourth parameter tells what implementation type we'd like to return. But to use this, we must provide the third parameter, which is used to resolve collisions between values associated with the same key. In this case I'm assuming there will be no collisions (as the question has no indication that such situation can happen), so I'm just returning the first occurrence.

#1: Initial revision by

hkotsubo‭ · 2024-09-11T18:01:48Z (about 1 month ago)

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Based on the code snippet in the question (which uses `collect` etc), I'm assuming you want to use streams. I'm also infering that "empty values" means those values for which `Optional.isEmpty()` returns `false` (and consequently `Optional.isPresent()` returns `true`).

In that case, just create a stream for the map entries, filter the non-empty ones and collect to a new map, by getting the `Optional`'s values:

```java
Map<K, T> result = input
    // get stream of map entries
    .entrySet().stream()
    // check if value is present (which means it's not empty)
    .filter(e -> e.getValue().isPresent()) // alternative: .filter(e -> ! e.getValue().isEmpty())
    // collect to a new map, getting the values from the Optional
    .collect(Collectors.toMap(Map.Entry::getKey, e -> e.getValue().get()));
```

But of course you can also do it without streams, using the traditional loop approach:

```java
Map<K, T> result = new HashMap<>();
for (Map.Entry<K, Optional<T>> entry : input.entrySet()) {
    // value is present (AKA: not empty)
    if (entry.getValue().isPresent()) { // alternative: if (! entry.getValue().isEmpty())
        // add the Optional's value to result
        result.put(entry.getKey(), entry.getValue().get());
    }
}
```

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