Java Language Basics

Java is a statically and strongly typed object-oriented language. Source is compiled not to native machine code but to portable bytecode that the Java Virtual Machine (JVM) runs on any platform — "write once, run anywhere". The JVM manages memory itself through garbage collection, so there are no raw pointers here, only references.

But Java is not a pure object language: alongside objects live primitives (int, char, boolean) that are not objects. This split between primitives and objects is exactly what produces most beginner traps — autoboxing, the Integer cache, the difference between == and equals(). The layers below take the language from its execution model down to those traps.

Topic map

• Key Java Features — compilation to bytecode, static typing, automatic memory management.
• The main Method — why the entry point is declared public static void main(String[] args).
• static vs Instance — class member vs object member; why a static method has no this.
• Class Initialization Order — static, then field initializers, then constructor; parent before child.
• Wrapper Classes — how Integer and Double give a primitive an object form for collections and generics.
• Autoboxing — the hidden valueOf and intValue, NullPointerException when unboxing null.
• Integer Cache — why == on wrappers is only true within the -128..127 range.
• final vs finally vs finalize — three similarly spelled but unrelated concepts.
• Why Java Is Not Pure OO — primitives are not objects, and wrappers bridge the gap.

Common traps

Interview relevance

Core language basics come up on almost every junior interview — but the check is your mental model, not recall: where data lives and when things run. A candidate who explains autoboxing as "the compiler inserts Integer.valueOf, which caches -128..127" stands out from "well, int becomes Integer".

Typical checks:

• How bytecode differs from machine code and what makes Java platform-independent.
• Why main is declared public static void and what String[] args is.
• The difference between static and instance; why a static method has no this.
• Initialization order: static → field initializers → constructor, parent before child.
• Primitives vs wrappers, autoboxing and its cost, the Integer cache and why == is unreliable.

Common wrong answer: "== compares wrapper values." That opens the discussion that == compares references, and the Integer cache makes the result sometimes-true and otherwise-false for equal numbers — and that wrapper values must be compared with equals().