The document shows an example of converting an external iteration over a list using a for loop into an internal iteration using Java streams. First, a for loop is used to iterate over a list of numbers and add each number multiplied by 2 to a new list. Then, this is rewritten using streams to map each number by multiplying it by 2 and collecting the results into a new list, eliminating the need for an anonymous class.

25. From External Iteration
To Internal Iteration
List<Long> nums = ...;
List<Long> nums2
= new ArrayList<>();
for (long n: nums) {
nums2.add(n*2L);
}
Before

26. From External Iteration
To Internal Iteration
List<Long> nums = ...;
List<Long> nums2 = nums.stream()
.map(new Function<Long,Long>(){
@Override
public Long aply(Long n) {
return n*2L;
}
})
.collect(Collectors.toList());

27. From External Iteration
To Internal Iteration
List<Long> nums = ...;
List<Long> nums2 = nums.stream()
.map(new Function<Long,Long>(){
@Override
public Long aply(Long n) {
return n*2L;
}
})
.collect(Collectors.toList());
Boilerplate!

28. From External Iteration
To Internal Iteration
List<Long> nums = ...;
List<Long> nums2 = nums.stream()
.map(
n -> n*2L
)
.collect(Collectors.toList());
After