📘 Day 22 – Optional, Method References & Advanced Streams

✅ What I learned today

1. Optional Class

• Container object to avoid null checks

• Optional.ofNullable(), ifPresent(), orElse(), orElseThrow()

//Code

Optional opt = Optional.ofNullable(getName()); String name = opt.orElse("Guest");

2. Method References (::)

Shortcut for lambdas

Types: Static (Math::max), Instance (System.out::println), Constructor (Employee::new)

3. Advanced Streams – Grouping

• Collectors.groupingBy(Function) → groups into Map<K, List>

• Collectors.groupingBy(Function, downstream collector) → group + count/average

//Code

// Group employees by department

Map<String, List> byDept = employees.stream() .collect(Collectors.groupingBy(Employee::getDepartment));

// Count per department

Map<String, Long> counts = employees.stream() .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.counting()));

// Average salary per department Map<String, Double> avgSalary = employees.stream() .collect(Collectors.groupingBy(Employee::getDepartment, Collectors.averagingDouble(Employee::getSalary)));