Create a map from two different lists based on common conditions using Java 8 Streams

Ravi :

I have two list instances like this:

List<NameAndAge> nameAndAgeList = new ArrayList<>();
nameAndAgeList.add(new NameAndAge("John", "28"));
nameAndAgeList.add(new NameAndAge("Paul", "30"));
nameAndAgeList.add(new NameAndAge("Adam", "31"));

List<NameAndSalary> nameAndSalaryList = new ArrayList<>();
nameAndSalaryList.add(new NameAndSalary("John", 1000));
nameAndSalaryList.add(new NameAndSalary("Paul", 1100));
nameAndSalaryList.add(new NameAndSalary("Adam", 1200));

where NameAndAge is

class NameAndAge {
    public String name;
    public String age;

    public NameAndAge(String name, String age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return name + ": " + age;
    }
}

and NameAndSalary is

private class NameAndSalary {
    private String name;
    private double salary;

    public NameAndSalary(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    @Override
    public String toString() {
        return name + ": " + salary;
    }
}

Now, I want to create a map with key as NameAndAge object from the first list and value as NameAndSalary from the second list where the name is equal in both the objects.

So, when I print the map, it should look like this:

{John: 28=John: 1000.0}
{Paul: 30=Paul: 1100.0}
{Adam: 31=Adam: 1200.0}

I have tried doing this, but the end return type is 'void' so I'm stuck clueless as I am new to Streams.

nameAndAgeList
    .forEach(n ->
        nameAndSalaryList
            .stream()
            .filter(ns -> ns.name.equals(n.name))
            .collect(Collectors.toList()));

Can someone please advise how can this be achieved with Java Streams API?

Matt :

This should do the trick, too:

Map<NameAndAge, NameAndSalary> map = new HashMap<>();
nameAndAgeList.forEach(age -> {
     NameAndSalary salary = nameAndSalaryList.stream().filter(
              s -> age.getName().equals(s.getName())).
              findFirst().
              orElseThrow(IllegalStateException::new);
      map.put(age, salary);
});

Mind that it would throw an IllegalStateException if a matching name can't be found.