Converting Collection to Map with JDK 8 Collectors

Main.java

package com.example;

import static java.lang.System.out;

import java.util.*;
import java.util.function.BinaryOperator;
import java.util.stream.Collectors;

public class Main {

    private static final User[] users;

    static {
        users = new User[]{
                new User("u-1", "rich", 30),
                new User("u-2", "tom", 20),
                new User("u-1", "rich lee", 30),
                new User("u-3", "Jackson", 45)
        };
    }

    private static Map<String, User> convertArrayToMap(final User[] users) {
        return Arrays.stream(users).collect(
                Collectors.toMap(User::getId, user -> user));
    }

    private static Map<String, User> convertListToMap(final List<User> users) {
        return users.stream().collect(
                Collectors.toMap(User::getId, user -> user));
    }

    /**
     * Use initial value when the mapped keys contains duplicates.
     *
     * @param users
     * @return
     */
    private static Map<String, User> convertListToMapAndMerge(final List<User> users) {
        return users.stream().collect(
                Collectors.toMap(User::getId, user -> user, usingInitialValue()));
    }

    private static Map<String, User> convertSetToMap(final Set<User> userSet) {
        return userSet.stream().collect(
                Collectors.toMap(User::getId, user -> user));
    }

    private static <T> BinaryOperator<T> usingInitialValue() {
        return (a,b) -> a;
    }

    private static <T> BinaryOperator<T> usingLastValue() {
        return (a,b) -> b;
    }

    public static void main(String[] args) {

        final List<User> usersList = Arrays.asList(users);
        /*
         If the mapped keys contains duplicates (according to  Object.equals(Object)), an IllegalStateException is 
         thrown when the collection operation is performed. 
         
         If the mapped keys may have duplicates, use toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction) instead.
         */
        
//        out.println("LIST -> MAP:\n" + convertListToMap(usersList));
        out.println("LIST -> MAP:\n" + convertListToMapAndMerge(usersList));

    }
}

User.java

package com.example;

public class User {

    final String id;

    final String name;

    final int age;

    public User(String id, String name, int age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "User{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}