Another common problem that we encounter in applications entails replacing the elements of a List that matches certain conditions.

In the following example, let's consider the Melon class:

public class Melon {

  private final String type;
  private final int weight;

  // constructor, getters, equals(), hashCode(),
  // toString() omitted for brevity
}

And then, let's consider a List of Melon:

List<Melon> melons = new ArrayList<>();

melons.add(new Melon("Apollo", 3000));
melons.add(new Melon("Jade Dew", 3500));
melons.add(new Melon("Cantaloupe", 1500));
melons.add(new Melon("Gac", 1600));
melons.add(new Melon("Hami", 1400));

Let's assume that we want to replace all melons weighing less than 3,000 grams with other melons of the same types and that weigh 3,000 grams.

A solution to this problem will entail iterating the List and then using List.set(int index, E element) to replace the melons accordingly.

This is a snippet of spaghetti code as follows:

for (int i = 0; i < melons.size(); i++) {

  if (melons.get(i).getWeight() < 3000) {

    melons.set(i, new Melon(melons.get(i).getType(), 3000));
  }
}

Another solution relies on Java 8 functional style or, more precisely, on the UnaryOperator functional interface.

Based on this functional interface, we can write the following operator:

UnaryOperator<Melon> operator = t 
  -> (t.getWeight() < 3000) ? new Melon(t.getType(), 3000) : t;

Now, we can use the JDK 8, List.replaceAll(UnaryOperator<E> operator), as follows:

melons.replaceAll(operator);

Both approaches should perform almost the same.