Java Fundamentals

Typical .java layout

• Optional package → import statements → class declaration → fields → constructors → methods → main if this class is the entry point.
• java.lang is imported automatically (String, System, Object, Math).

public class FundamentalsDemo {
    public static void main(String[] args) {
        int count = 10;
        System.out.println("count = " + count);
    }
}

Tokens

• The compiler breaks source into tokens: keywords, identifiers, literals, operators, and separators (;, ,, ., (), {}, []).
• Literals include integers (42), longs (42L), doubles (3.14), floats (3.14f), chars ('A'), strings ("hi"), booleans, and null.
• Java is free-form whitespace; statements end with ; (except class/method/block declarations).

Keywords and identifiers

• Identifiers start with a letter, $, or _, then letters, digits, $, or _. They cannot be keywords.
• Convention: PascalCase types, camelCase methods/fields, UPPER_SNAKE constants, lowercase packages.
• Java 17 keywords/reserved words you will meet early: class, public, static, void, int, if, else, for, while, new, return, this, super, extends, implements, try, catch, finally, throw, throws, import, package, boolean, char, byte, short, long, float, double, null/true/false (literals, not keywords in the same way, but reserved).

Compilation: javac and java

• javac Hello.java reads source, type-checks it, and writes Hello.class bytecode.
• java Hello starts a JVM, loads the class, and invokes public static void main.
• Errors at compile time (missing ;, type mismatch) never reach the JVM. Runtime errors (NPE, / by zero) happen after launch.

Comments, Unicode, and source rules

• // line comments, /* */ block comments, and /** */ Javadoc. Nested /* */ is not allowed.
• A file may contain at most one public top-level type; that type’s name is the file name.
• var (Java 10+) is not a reserved keyword in all contexts — it is a reserved type name for local-variable type inference.
• Underscores in numeric literals (1_000_000) are allowed since Java 7; a lone _ is no longer a valid identifier (Java 9+).

Tokens and a simple class structure

public class TokensDemo {
    static final int MAX = 3; // constant identifier

    public static void main(String[] args) {
        int score = 10; // identifier + literal
        score += MAX;   // operator
        System.out.println("score = " + score);
    }
}