Java Collections and Java 8+: Streams, Optionals, Functional Interfaces

Java Collections and Java 8+

Backend Java code relies on collections, streams, and modern language APIs for clean and reliable business logic.

Collection Types

• List: ordered collection with duplicates.
• Set: unique elements.
• Map: key-value pairs.
• Queue: FIFO or priority processing.
• Deque: double-ended queue.

Common Implementations

• ArrayList: fast random access, resizable array.
• LinkedList: fast insert/remove at ends, poor random access.
• HashMap: average O(1) lookup, unsorted.
• LinkedHashMap: maintains insertion order.
• TreeMap: sorted keys, O(log n) operations.
• HashSet: backed by HashMap, unique elements.
• TreeSet: sorted set.

HashCode and Equals

• Implement equals() and hashCode() consistently for objects used in hashed collections.
• Violating the contract breaks HashMap, HashSet, and caching semantics.

Comparable and Comparator

• Comparable defines natural ordering with compareTo().
• Comparator defines external ordering strategies.
• Use comparators for sorting by different fields without changing object classes.

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Streams and Functional Programming

Lambda Expressions

• Use lambdas for concise behavior passed into stream operations and callbacks.

Functional Interfaces

• A functional interface has a single abstract method.
• Common examples: Function, Predicate, Consumer, Supplier.
• Predicate<T> evaluates a boolean condition.
• Consumer<T> consumes a value without returning one.
• Supplier<T> produces a value without input.

Streams

• Use Stream for declarative collection processing.
• Prefer streams for transformations and filters, but avoid them for simple loops when performance matters.

Optional

• Optional<T> represents a value that may or may not be present.
• Use it for return values, not for fields or parameters.

Method References

• Use Class::method or instance::method to make lambda expressions shorter and clearer.