Java is a high-level, general-purpose, object-oriented programming language particularly designed to enable developers to write computer applications that are able to run on any device that supports Java without requiring recompilation. It's well known for its "Write Once, Run Anywhere" principle, ensuring cross-platform compatibility. It is widely used for developing robust and secure standalone and web-based applications.

Understanding Java requires knowledge of its core concepts, including exception handling, multi-threading, synchronization, serialization, generics, and Java collection. Besides this, Java developer needs to understand the use of various Java frameworks and libraries such as Spring, Hibernate, JUnit, and Struts. Proficiency in Java EE (Enterprise Edition) adds valuable expertise to Java programming skills, enabling developers to build large-scale, distributed, and network-centric applications.

Knowledge of databases, SQL and NoSQL, enhances Java application development. Familiarity with the JVM (Java Virtual Machine) and understanding of JVM language, such as Scala or Groovy, is beneficial. Good grasp of design patterns, and UML (Unified Modeling Language) aids in designing efficient solutions.

Fundamental skills that serve as a foundation for learning or mastering Java include good analytical skills to understand problem statements and formulate logical solutions. An understanding of basic computer science principles, algorithms, and data structures is necessary. Prior experience in object-oriented programming (OOP) languages like C++ or Python can be very helpful in understanding Java faster.

Experience with integrated development environments (IDEs), such as Eclipse or IntelliJ Idea, is particularly beneficial as they are the primary tools used for Java development. A good understanding of version control systems, such as Git, helps manage different versions of code effectively. Understanding the principles of software testing, along with knowledge of Agile methodologies, can significantly improve code quality and collaboration.

In a nutshell, a Java developer must not only be proficient in Java, but also should understand related technologies and frameworks, databases, algorithm design and problem-solving, effective use of tools, and incorporate best practices in software development. This skill set is what companies usually look for when hiring Java developers.