Java Standard Libraries

java.lang supplies Object, String, Math, wrappers, threads, and system facilities and is automatically imported. java.util contains collections, Optional, random generators, and utilities; java.io and java.nio cover I/O; java.time models date/time.

Prefer standard abstractions

Instant now = Instant.now();
Duration timeout = Duration.ofSeconds(5);
List<String> copy = List.copyOf(source);
Objects.requireNonNull(value, "value");

Read Javadoc contracts for nullability, exceptions, mutability, complexity, and thread safety. Use modern focused APIs—java.time over legacy date types, NIO Paths over string path manipulation—and avoid reimplementing tested library behavior. API availability depends on the project’s minimum Java release.