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In the following example I would like to print out the items Ids of the one that get discarded. Is there a way not to get only the values, but the objects themselves?

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

public class Main {
    public static void main(String[] args) {
        // Example list with potential duplicates
        List<Item> items = Arrays.asList(
                new Item(1, "Item A"),
                new Item(2, "Item B"),
                new Item(1, "Item C"), // Duplicate key
                new Item(3, "Item D")
        );

        // Building the Map
        Map<Integer, String> itemMap = items.stream()
            .collect(Collectors.toMap(
                Item::getId,                 // Key mapper
                Item::getName,               // Value mapper
                (existing, incoming) -> {    // Merge function
                    // Print info about the duplicate
                    System.out.println("Duplicate key found. Existing value: " + existing + ", Incoming value: " + incoming);
                    // HOW CAN I PRINT THE INCOMING ELEMENTS ID HERE? <<<<<<<<<<<<
                    return incoming; // Keep the incoming value
                }
            ));

        // Print the resulting map
        System.out.println("Resulting Map: " + itemMap);
    }
}

class Item {
    private final int id;
    private final String name;

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

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

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

In the following example I would like to print out the items Ids of the one that get discarded. Is there a way not to get only the values, but the objects themselves?

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

public class Main {
    public static void main(String[] args) {
        // Example list with potential duplicates
        List<Item> items = Arrays.asList(
                new Item(1, "Item A"),
                new Item(2, "Item B"),
                new Item(1, "Item C"), // Duplicate key
                new Item(3, "Item D")
        );

        // Building the Map
        Map<Integer, String> itemMap = items.stream()
            .collect(Collectors.toMap(
                Item::getId,                 // Key mapper
                Item::getName,               // Value mapper
                (existing, incoming) -> {    // Merge function
                    // Print info about the duplicate
                    System.out.println("Duplicate key found. Existing value: " + existing + ", Incoming value: " + incoming);
                    // HOW CAN I PRINT THE INCOMING ELEMENTS ID HERE? <<<<<<<<<<<<
                    return incoming; // Keep the incoming value
                }
            ));

        // Print the resulting map
        System.out.println("Resulting Map: " + itemMap);
    }
}

class Item {
    private final int id;
    private final String name;

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

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

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

• java
• java-stream

Share Improve this question Follow asked Nov 19, 2024 at 10:04 BeginnerBeginner 5,49766 gold badges3838 silver badges7474 bronze badges 4

• 2 This is not a particularly good candidate problem for a stream as streams do not prioritize or consider state. btw, your Item object (particularly because of your declared definition of 'duplicate') should override equals and hashCode – g00se Commented Nov 19, 2024 at 10:33
• 2 @g00se it's the id (the map key as an Integer) that is duplicate, not the Item type itself, so no need for equals/hashCode (although it's generally a good idea to have them). – k314159 Commented Nov 19, 2024 at 10:37
• 2 why not use Map<Integer, Item> instead. You are using itemId as key right? You will have the required info – Anubhav Sharma Commented Nov 19, 2024 at 10:38
• 2 @k314159 An object that has an id attribute that doesn't uniquely identify that object, should probably name its attribute differently ;) – g00se Commented Nov 19, 2024 at 10:39

Add a comment  | 

1 Answer 1

Sorted by: Reset to default 7

You may use

Map<Integer, String> itemMap = items.stream()
    .collect(Collectors.groupingBy(
        Item::getId,                                         // Key mapper
        Collectors.collectingAndThen(
            Collectors.reducing((existing, incoming) -> {    // Merge function
            // Print info about the duplicate
            System.out.println("Duplicate key " + existing.getId()
                + " found. Existing value: " + existing.getName()
                + ", Incoming value: " + incoming.getName());
            return incoming; // Keep the incoming value
        }), o -> o.get().getName()                           // Value mapper
    )));

Duplicate key 1 found. Existing value: Item A, Incoming value: Item C
Resulting Map: {1=Item C, 2=Item B, 3=Item D}

But you may also consider a tradional loop:

Map<Integer, String> itemMap = new HashMap<>();
for(Item i: items) {
    String old = itemMap.put(i.getId(), i.getName());
    if(old != null) {
        System.out.println("Duplicate key " + i.getId()
          + " found. Existing value: " + old + ", Incoming value: " + i.getName());
    }
}

In the following example I would like to print out the items Ids of the one that get discarded. Is there a way not to get only the values, but the objects themselves?

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

public class Main {
    public static void main(String[] args) {
        // Example list with potential duplicates
        List<Item> items = Arrays.asList(
                new Item(1, "Item A"),
                new Item(2, "Item B"),
                new Item(1, "Item C"), // Duplicate key
                new Item(3, "Item D")
        );

        // Building the Map
        Map<Integer, String> itemMap = items.stream()
            .collect(Collectors.toMap(
                Item::getId,                 // Key mapper
                Item::getName,               // Value mapper
                (existing, incoming) -> {    // Merge function
                    // Print info about the duplicate
                    System.out.println("Duplicate key found. Existing value: " + existing + ", Incoming value: " + incoming);
                    // HOW CAN I PRINT THE INCOMING ELEMENTS ID HERE? <<<<<<<<<<<<
                    return incoming; // Keep the incoming value
                }
            ));

        // Print the resulting map
        System.out.println("Resulting Map: " + itemMap);
    }
}

class Item {
    private final int id;
    private final String name;

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

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

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

In the following example I would like to print out the items Ids of the one that get discarded. Is there a way not to get only the values, but the objects themselves?

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

public class Main {
    public static void main(String[] args) {
        // Example list with potential duplicates
        List<Item> items = Arrays.asList(
                new Item(1, "Item A"),
                new Item(2, "Item B"),
                new Item(1, "Item C"), // Duplicate key
                new Item(3, "Item D")
        );

        // Building the Map
        Map<Integer, String> itemMap = items.stream()
            .collect(Collectors.toMap(
                Item::getId,                 // Key mapper
                Item::getName,               // Value mapper
                (existing, incoming) -> {    // Merge function
                    // Print info about the duplicate
                    System.out.println("Duplicate key found. Existing value: " + existing + ", Incoming value: " + incoming);
                    // HOW CAN I PRINT THE INCOMING ELEMENTS ID HERE? <<<<<<<<<<<<
                    return incoming; // Keep the incoming value
                }
            ));

        // Print the resulting map
        System.out.println("Resulting Map: " + itemMap);
    }
}

class Item {
    private final int id;
    private final String name;

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

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

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

• java
• java-stream

Share Improve this question Follow asked Nov 19, 2024 at 10:04 BeginnerBeginner 5,49766 gold badges3838 silver badges7474 bronze badges 4

• 2 This is not a particularly good candidate problem for a stream as streams do not prioritize or consider state. btw, your Item object (particularly because of your declared definition of 'duplicate') should override equals and hashCode – g00se Commented Nov 19, 2024 at 10:33
• 2 @g00se it's the id (the map key as an Integer) that is duplicate, not the Item type itself, so no need for equals/hashCode (although it's generally a good idea to have them). – k314159 Commented Nov 19, 2024 at 10:37
• 2 why not use Map<Integer, Item> instead. You are using itemId as key right? You will have the required info – Anubhav Sharma Commented Nov 19, 2024 at 10:38
• 2 @k314159 An object that has an id attribute that doesn't uniquely identify that object, should probably name its attribute differently ;) – g00se Commented Nov 19, 2024 at 10:39

Add a comment  | 

1 Answer 1

Sorted by: Reset to default 7

You may use

Map<Integer, String> itemMap = items.stream()
    .collect(Collectors.groupingBy(
        Item::getId,                                         // Key mapper
        Collectors.collectingAndThen(
            Collectors.reducing((existing, incoming) -> {    // Merge function
            // Print info about the duplicate
            System.out.println("Duplicate key " + existing.getId()
                + " found. Existing value: " + existing.getName()
                + ", Incoming value: " + incoming.getName());
            return incoming; // Keep the incoming value
        }), o -> o.get().getName()                           // Value mapper
    )));

Duplicate key 1 found. Existing value: Item A, Incoming value: Item C
Resulting Map: {1=Item C, 2=Item B, 3=Item D}

But you may also consider a tradional loop:

Map<Integer, String> itemMap = new HashMap<>();
for(Item i: items) {
    String old = itemMap.put(i.getId(), i.getName());
    if(old != null) {
        System.out.println("Duplicate key " + i.getId()
          + " found. Existing value: " + old + ", Incoming value: " + i.getName());
    }
}

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