Java Stream Examples

### Problem #1: Given an integer array sorted in increasing order, return an array of the squares of each number sorted in increasing order.
### Solution:
```
    public static int[] sortedSquaresArray(int[] numbers) {
        return Arrays.stream(numbers).boxed()
                .map(val -> val * val)
                .sorted()
                .mapToInt(value -> value.intValue())
                .toArray();
    }

```

If you want to return the squares of each number sorted in descending order then you can use *.sorted(Collections.reverseOrder())*


### Problem #2: Given ArrayList of Employee (name, age) object. Print the names of employees having age greater than 20 saperated with comma. 
### Solution:
```
	String str = employeeList.stream()
                        .filter(e -> e.age >20)
                        .map(e -> e.name)
                        .collect(Collectors.joining(","));

    System.out.println(str);
```

### Problem#3: Create a reverse map in Java	, which means change the key as value and value as key.
### Solution:
```
 	sourceMap.forEach((key,value) -> inverseMap.put(value,key))
```
This is equivalent to:

```
 		for (Map.Entry entry : sourceMap.entrySet()){
            inverseMap.put(entry.getValue(), entry.getKey());
        }
```

### Problem#4: Given a List of Integer. Find out sum of top 3 elements in the given list.
### Solution:

```
	 List intList = new ArrayList<>();
      
      intList.add(34);
      intList.add(45);
      intList.add(50);
      intList.add(300);
      
     int topElementSum = intList.stream().sorted(Comparator.reverseOrder())
              .limit(3)
              .mapToInt(Integer::intValue)
              .sum();
     System.out.println(topElementSum);
```
We create a stream of Integer objects via *Collection.stream()* and then perform *Stream.sorted()* to sort the stream in descending order followed by *Stream.limit()* to get first 3 element.
Then convert stream to IntStream using *Stream.mapToInt()* to get sum() of elements.