Collection Framework - Guided Exercise - CustomerRewardsSystem – Mastering Map, Set, and Stream Ops in Java #111

akash-coded · 2025-06-03T04:02:02Z

akash-coded
Jun 3, 2025
Maintainer

🧪 Exercise: `CustomerRewardsSystem – Mastering Map, Set, and Stream Ops in Java`

🏢 Context: Welcome to LambdaMart CRM Team

LambdaMart has launched a loyalty program for its repeat customers. You're building the RewardsManager system that uses HashMap, HashSet, and Stream API enhancements to manage:

Customer registrations
Unique mobile numbers
Reward points management
Duplicate detection
Fraud cleanup
High-value customer segmentation

🎯 Key Concepts Covered

HashMap<K,V>, LinkedHashMap, TreeMap
HashSet<T>, LinkedHashSet, TreeSet
Methods like computeIfAbsent, putIfAbsent, replaceAll, removeIf, forEach
Streams on maps/sets
Clean coding + real-world use cases

📦 Files to Create

Customer.java – model
RewardsManager.java – logic layer
Main.java – test file

🧩 Part 1: Define the `Customer` Class

`Customer.java`

public class Customer {
    private int id;
    private String name;
    private String mobile;
    private int points;
public Customer(int id, String name, String mobile, int points) {
    this.id = id;
    this.name = name;
    this.mobile = mobile;
    this.points = points;
}

// Getters and Setters
// TODO: Override toString() for pretty print
// TODO: Override equals() and hashCode() based on mobile

}

✅ Task A: Fill in the blanks

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Customer)) return false;
Customer c = (Customer) o;
return this.mobile.equals(c.__________); // 🔲 fill

}
@Override

public int hashCode() {

return __________.hashCode(); // 🔲 use correct field

}

🧩 Part 2: Customer Management with Map

`RewardsManager.java`

import java.util.*;
public class RewardsManager {
private Map&lt;Integer, Customer&gt; customerMap = new HashMap&lt;&gt;();

public void registerCustomer(Customer c) {
    customerMap.putIfAbsent(c.getId(), c); // Avoids overwriting
    System.out.println("✅ Registered: " + c);
}

public void displayAll() {
    customerMap.forEach((id, customer) -&gt; System.out.println(id + " -&gt; " + customer));
}

}

✅ Task B: Predict the output

Customer c1 = new Customer(101, "Ravi", "9876543210", 100);
Customer c2 = new Customer(101, "Ravi", "9876543210", 200);
manager.registerCustomer(c1);

manager.registerCustomer(c2);

manager.displayAll();

✍️ What will the final points for Ravi be?
Answer: __________
Why? __________

✅ Task C: Write a method to increase reward points

public void addPoints(int id, int points) {
    customerMap.computeIfPresent(id, (k, v) -> {
        v.setPoints(v.getPoints() + points);
        return v;
    });
}

✍️ Rewrite this using lambda block style and comment each step inline.

🧩 Part 3: Mobile Validation Using Set

✅ Task D: Ensure mobile numbers are unique

Create a Set<String> to track already-used mobile numbers.

private Set<String> mobileSet = new HashSet<>();
public void registerCustomerWithValidation(Customer c) {

if (!mobileSet.contains(c.getMobile())) {

customerMap.put(c.getId(), c);

mobileSet.add(c.getMobile());

System.out.println("✅ Registered: " + c);

} else {

System.out.println("⚠️ Duplicate mobile found: " + c.getMobile());

}

}

🧠 Challenge:

What if the same number is used but with a different customer ID?
How would you prevent duplicates without relying on IDs?

✍️ Write a plan in 2 lines:

1. 
2.

🧩 Part 4: Removing Suspicious or Inactive Entries

✅ Task E: Use `removeIf()` on `Set` or `Map.entrySet()`

// Remove all customers with less than 50 points
public void removeInactiveCustomers() {
    customerMap.entrySet().removeIf(entry -> __________); // 🔲 fill logic
}

✅ Task F: Use `replaceAll()` to boost high-value customers

// Double points for customers having 500+ points
public void boostHighValueCustomers() {
    customerMap.replaceAll((id, c) -> {
        if (__________) {
            c.setPoints(c.getPoints() * 2);
        }
        return c;
    });
}

🧩 Part 5: Sorting and Segmentation

✅ Task G: Migrate to TreeMap for sorted display

// Show customers in ascending ID order
private Map<Integer, Customer> customerMap = new TreeMap<>();

✅ Add this method:

public void printSorted() {
    customerMap.forEach((id, customer) -> {
        System.out.println(id + " => " + customer.getName() + ", Points: " + customer.getPoints());
    });
}

✅ Task H: Filter high-reward customers using stream

// Print only customers with > 1000 points
public void showPremiumCustomers() {
    customerMap.values().stream()
               .filter(__________) // 🔲 write lambda condition
               .forEach(System.out::println);
}

🧩 Part 6: Main Class for Testing

`Main.java`

public class Main {
    public static void main(String[] args) {
        RewardsManager manager = new RewardsManager();
    manager.registerCustomerWithValidation(new Customer(101, "Ravi", "9876543210", 100));
    manager.registerCustomerWithValidation(new Customer(102, "Seema", "9123456789", 550));
    manager.registerCustomerWithValidation(new Customer(103, "Tina", "9876543210", 70)); // should fail

    manager.addPoints(101, 50);
    manager.boostHighValueCustomers();
    manager.removeInactiveCustomers();

    manager.showPremiumCustomers();
    manager.printSorted();
}

}

✍️ Add 2 more customers manually in main, one of them having exactly 1000 points.

📘 Clean Code Practices Recap

Guideline	Applied Example
Use putIfAbsent() for safe inserts	Avoids overwriting
Use computeIfPresent() for logic updates	Elegant and safe
Separate ID and value validation	Set for mobile
Always override equals() and hashCode()	Used in HashSet
Avoid == for string comparison	Use .equals() or .equalsIgnoreCase()
Prefer streams over manual loops when filtering large sets	customerMap.values().stream().filter(...)

🧠 Final Reflection

What’s the difference between replace() and replaceAll()?
Why does HashSet require equals() and hashCode() to be overridden?
When would you use a TreeSet or TreeMap in an application?

🏁 Deliverables

Your submission must include:

✅ Completed Customer.java with overridden equals, hashCode, and toString
✅ Fully working RewardsManager.java:
- Safe customer registration
- Validation using Set
- Point management using Map
- Stream filters
- removeIf, replaceAll, computeIfPresent
✅ Test logic in Main.java

Submit screenshots of:

Output before and after removeIf()
Output of sorted map
showPremiumCustomers()

prasetha21-max · 2025-06-03T04:48:50Z

prasetha21-max
Jun 3, 2025

package com.practice.problems.java;
import java.util.*;
import java.util.stream.*;

class Customer {
    private int id;
    private String name;
    private String mobile;
    private int points;

    public Customer(int id, String name, String mobile, int points) {
        this.id = id;
        this.name = name;
        this.mobile = mobile;
        this.points = points;
    }

    public int getId() { return id; }
    public String getName() { return name; }
    public String getMobile() { return mobile; }
    public int getPoints() { return points; }
    public void setPoints(int points) { this.points = points; }

    @Override
    public String toString() {
        return "Customer{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", mobile='" + mobile + '\'' +
                ", points=" + points +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Customer)) return false;
        Customer c = (Customer) o;
        return this.mobile.equals(c.getMobile());
    }

    @Override
    public int hashCode() {
        return mobile.hashCode();
    }
}

class RewardsManager {
    private Map<Integer, Customer> customerMap = new TreeMap<>();
    private Set<String> mobileSet = new HashSet<>();

    public void registerCustomerWithValidation(Customer c) {
        if (!mobileSet.contains(c.getMobile())) {
            customerMap.put(c.getId(), c);
            mobileSet.add(c.getMobile());
            System.out.println("Registered: " + c);
        } else {
            System.out.println("Duplicate mobile found: " + c.getMobile());
        }
    }

    public void addPoints(int id, int points) {
        customerMap.computeIfPresent(id, (key, customer) -> {
            customer.setPoints(customer.getPoints() + points);
            return customer;
        });
    }

    public void boostHighValueCustomers() {
        customerMap.replaceAll((id, c) -> {
            if (c.getPoints() >= 500) {
                c.setPoints(c.getPoints() * 2);
            }
            return c;
        });
    }

    public void removeInactiveCustomers() {
        customerMap.entrySet().removeIf(entry -> entry.getValue().getPoints() < 50);
    }

    public void showPremiumCustomers() {
        System.out.println("Premium Customers:");
        customerMap.values().stream()
                .filter(c -> c.getPoints() > 1000)
                .forEach(System.out::println);
    }

    public void printSorted() {
        System.out.println("Sorted Customers:");
        customerMap.forEach((id, customer) ->
                System.out.println(id + " => " + customer.getName() + ", Points: " + customer.getPoints()));
    }

    public void displayAll() {
        customerMap.forEach((id, customer) -> System.out.println(id + " -> " + customer));
    }
}

public class Main {
    public static void main(String[] args) {
        RewardsManager manager = new RewardsManager();
        manager.registerCustomerWithValidation(new Customer(101, "Ravi", "9876543210", 100));
        manager.registerCustomerWithValidation(new Customer(102, "Seema", "9123456789", 550));
        manager.registerCustomerWithValidation(new Customer(103, "Tina", "9876543210", 70));

        manager.addPoints(101, 50);
        manager.boostHighValueCustomers();

        System.out.println("\nBefore Removing Inactives:");
        manager.displayAll();

        manager.removeInactiveCustomers();

        System.out.println("\nAfter Removing Inactives:");
        manager.displayAll();

        manager.showPremiumCustomers();
        manager.printSorted();
    }
}

0 replies

Lohithr1975 · 2025-06-03T06:22:00Z

Lohithr1975
Jun 3, 2025

Lohith R

package org.example;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.*;

public class Practice {
public static void main(String[] args) throws ParseException {

    System.out.println("hello");


    RewardsManager manager = new RewardsManager();
    manager.registerCustomerWithValidation(new Customer(101, "Ravi", "9876543210", 100));
    manager.registerCustomerWithValidation(new Customer(102, "Seema", "9123456789", 550));
    manager.registerCustomerWithValidation(new Customer(103, "Tina", "9876543210", 70)); // should fail

    manager.addPoints(101, 50);
    manager.boostHighValueCustomers();
    manager.removeInactiveCustomers();

    manager.showPremiumCustomers();
    manager.printSorted();
}

}

class Customer {
private int id;
private String name;
private String mobile;
private int points;
public Customer(int id, String name, String mobile, int points) {
this.id = id;
this.name = name;
this.mobile = mobile;
this.points = points;
}

// Getters and Setters
// TODO: Override toString() for pretty print
// TODO: Override equals() and hashCode() based on mobile

 public int getId() {
     return id;
 }

 public void setId(int id) {
     this.id = id;
 }

 public String getName() {
     return name;
 }

 public void setName(String name) {
     this.name = name;
 }

 public String getMobile() {
     return mobile;
 }

 public void setMobile(String mobile) {
     this.mobile = mobile;
 }

 public int getPoints() {
     return points;
 }

 public void setPoints(int points) {
     this.points = points;
 }

 @Override
 public String toString() {
     return "Customer{" +
             "id=" + id +
             ", name='" + name + '\'' +
             ", mobile='" + mobile + '\'' +
             ", points=" + points +
             '}';
 }

 @Override
 public boolean equals(Object o) {
     if (this == o) return true;
     if (!(o instanceof Customer)) return false;
     Customer c = (Customer) o;
     return this.mobile.equals(c.mobile);




 }


 @Override
 public int hashCode() {
     return mobile.hashCode();
 }

}

class RewardsManager {

// private Map<Integer,Customer> customerMap = new HashMap<>();
private Set mobileSet = new HashSet<>();
private Map<Integer, Customer> customerMap = new TreeMap<>();

 public void registerCustomerWithValidation(Customer c) {
    if (!mobileSet.contains(c.getMobile())) {
        customerMap.put(c.getId(), c);
        mobileSet.add(c.getMobile());
        System.out.println("Registered: " + c);
    } else {
        System.out.println("⚠️ Duplicate mobile found: " + c.getMobile());
    }
}
public void registerCustomer(Customer c) {
    customerMap.putIfAbsent(c.getId(), c); // Avoids overwriting
    System.out.println("✅ Registered: " + c);
}

public void displayAll() {
    customerMap.forEach((id, customer) -> System.out.println(id + " -> " + customer));

}

public void addPoints(int id, int points) {
    customerMap.computeIfPresent(id, (k, v) -> {
        v.setPoints(v.getPoints() + points);
        return v;
    });
}

 public void removeInactiveCustomers() {
     customerMap.entrySet().removeIf(entry -> entry.getValue().getPoints()<50);
 }
 public void boostHighValueCustomers() {
     customerMap.replaceAll((id, c) -> {
         if (c.getPoints()>500) {
             c.setPoints(c.getPoints() * 2);
         }
         return c;
     });
 }
 public void printSorted() {
     customerMap.forEach((id, customer) -> {
         System.out.println(id + " => " + customer.getName() + ", Points: " + customer.getPoints());
     });
 }
 public void showPremiumCustomers() {
     customerMap.values().stream()
             .filter(c->c.getPoints()>1000) 
             .forEach(System.out::println);
 }

}

0 replies

Sdey0907 · 2025-06-03T13:02:50Z

Sdey0907
Jun 3, 2025

**

_Task 1 - CustomerRewardsSystem Sayan Dey_Completed

**

RewardsManagerSystem.zip

0 replies

vbhavana1 · 2025-06-03T15:45:51Z

vbhavana1
Jun 3, 2025

Bhavana

import java.util.;
import java.util.stream.;

class Customer {
private int id;
private String name;
private String mobile;
private int points;

public Customer(int id, String name, String mobile, int points) {
    this.id = id;
    this.name = name;
    this.mobile = mobile;
    this.points = points;
}

public int getId() { return id; }
public String getName() { return name; }
public String getMobile() { return mobile; }
public int getPoints() { return points; }
public void setPoints(int points) { this.points = points; }

@Override
public String toString() {
    return "Customer{" +
            "id=" + id +
            ", name='" + name + '\'' +
            ", mobile='" + mobile + '\'' +
            ", points=" + points +
            '}';
}

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Customer)) return false;
    Customer c = (Customer) o;
    return this.mobile.equals(c.getMobile());
}

@Override
public int hashCode() {
    return mobile.hashCode();
}

}

class RewardsManager {
private Map<Integer, Customer> customerMap = new TreeMap<>();
private Set mobileSet = new HashSet<>();

public void registerCustomerWithValidation(Customer c) {
    if (!mobileSet.contains(c.getMobile())) {
        customerMap.put(c.getId(), c);
        mobileSet.add(c.getMobile());
        System.out.println("Registered: " + c);
    } else {
        System.out.println("Duplicate mobile found: " + c.getMobile());
    }
}

public void addPoints(int id, int points) {
    customerMap.computeIfPresent(id, (key, customer) -> {
        customer.setPoints(customer.getPoints() + points);
        return customer;
    });
}

public void boostHighValueCustomers() {
    customerMap.replaceAll((id, c) -> {
        if (c.getPoints() >= 500) {
            c.setPoints(c.getPoints() * 2);
        }
        return c;
    });
}

public void removeInactiveCustomers() {
    customerMap.entrySet().removeIf(entry -> entry.getValue().getPoints() < 50);
}

public void showPremiumCustomers() {
    System.out.println("Premium Customers:");
    customerMap.values().stream()
            .filter(c -> c.getPoints() > 1000)
            .forEach(System.out::println);
}

public void printSorted() {
    System.out.println("Sorted Customers:");
    customerMap.forEach((id, customer) ->
            System.out.println(id + " => " + customer.getName() + ", Points: " + customer.getPoints()));
}

public void displayAll() {
    customerMap.forEach((id, customer) -> System.out.println(id + " -> " + customer));
}

}

public class Main {
public static void main(String[] args) {
RewardsManager manager = new RewardsManager();
manager.registerCustomerWithValidation(new Customer(101, "Ravi", "9876543210", 100));
manager.registerCustomerWithValidation(new Customer(102, "Seema", "9123456789", 550));
manager.registerCustomerWithValidation(new Customer(103, "Tina", "9876543210", 70));

    manager.addPoints(101, 50);
    manager.boostHighValueCustomers();

    System.out.println("\nBefore Removing Inactives:");
    manager.displayAll();

    manager.removeInactiveCustomers();

    System.out.println("\nAfter Removing Inactives:");
    manager.displayAll();

    manager.showPremiumCustomers();
    manager.printSorted();
}

}

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Collection Framework - Guided Exercise - CustomerRewardsSystem – Mastering Map, Set, and Stream Ops in Java #111

Uh oh!

akash-coded Jun 3, 2025 Maintainer

🧪 Exercise: CustomerRewardsSystem – Mastering Map, Set, and Stream Ops in Java

🏢 Context: Welcome to LambdaMart CRM Team

🎯 Key Concepts Covered

📦 Files to Create

🧩 Part 1: Define the Customer Class

Customer.java

✅ Task A: Fill in the blanks

🧩 Part 2: Customer Management with Map

RewardsManager.java

✅ Task B: Predict the output

✅ Task C: Write a method to increase reward points

🧩 Part 3: Mobile Validation Using Set

✅ Task D: Ensure mobile numbers are unique

🧠 Challenge:

🧩 Part 4: Removing Suspicious or Inactive Entries

✅ Task E: Use removeIf() on Set or Map.entrySet()

✅ Task F: Use replaceAll() to boost high-value customers

🧩 Part 5: Sorting and Segmentation

✅ Task G: Migrate to TreeMap for sorted display

✅ Task H: Filter high-reward customers using stream

🧩 Part 6: Main Class for Testing

Main.java

📘 Clean Code Practices Recap

🧠 Final Reflection

🏁 Deliverables

Replies: 4 comments

Uh oh!

prasetha21-max Jun 3, 2025

Uh oh!

Lohithr1975 Jun 3, 2025

Uh oh!

Sdey0907 Jun 3, 2025

**

Uh oh!

vbhavana1 Jun 3, 2025

akash-coded
Jun 3, 2025
Maintainer

🧪 Exercise: `CustomerRewardsSystem – Mastering Map, Set, and Stream Ops in Java`

🧩 Part 1: Define the `Customer` Class

`Customer.java`

`RewardsManager.java`

✅ Task E: Use `removeIf()` on `Set` or `Map.entrySet()`

✅ Task F: Use `replaceAll()` to boost high-value customers

`Main.java`

prasetha21-max
Jun 3, 2025

Lohithr1975
Jun 3, 2025

Sdey0907
Jun 3, 2025

vbhavana1
Jun 3, 2025