Java - Language Fundamentals & Syntax

Source File Structure

Each public class is typically declared in its own .java file with the same name as the class.

package com.example.hello;        // 1) package declaration (optional but recommended)

import java.util.List;            // 2) imports (types you reference by simple name)

/**
 * 3) Type declarations (classes, interfaces, enums, records)
 */
public class Hello {              // filename should be Hello.java
  public static void main(String[] args) {
    System.out.println("Hello");
  }
}

Identifiers

• Names you choose for classes, methods, variables, packages.
• May contain letters, digits, _, and $; must not start with a digit.
• Are case-sensitive: value and Value are different.

int count = 0;           // ok
int Count = 0;           // different identifier
// int 2things = 0;      // invalid (starts with digit)

Keywords and Reserved Words

Keywords have special meaning (e.g., class, public, if, return) and cannot be used as identifiers.

Comments

• // single-line comments
• /* ... */ multi-line comments
• /** ... */ Javadoc comments for API docs

/** Adds two numbers. */
public static int add(int a, int b) { // a and b are integers
  return a + b; /* returns the sum */
}

Blocks and Scope

Blocks are delimited by { ... }. Variables declared inside a block are not visible outside that block.

if (true) {
  int x = 10;    // x lives only inside this block
}
// System.out.println(x); // error: x cannot be resolved here

Whitespace and Formatting

• Java ignores extra spaces and newlines but conventions matter for readability.
• Standard style: 2 or 4 spaces indentation; braces on the same line as statements.

if (condition) {
  doWork();
} else {
  fallback();
}

Naming Conventions

• Classes, Interfaces: PascalCase (e.g., UserService)
• Methods, Variables: camelCase (e.g., findUser, retryCount)
• Constants: UPPER_SNAKE_CASE (e.g., MAX_RETRIES) with final
• Packages: all lowercase dot-separated (e.g., com.example.api)

Unicode and Source Encoding

Modern builds standardize on UTF‑8. Non-ASCII identifiers are technically allowed but avoided in most codebases.

Try It: jshell

Java’s REPL helps experiment quickly:

jshell
jshell> int x = 40 + 2
x ==> 42
jshell> /exit

Summary Checklist

• One public top-level class per file, filename matches class.
• Use packages that match folders.
• Follow naming conventions to aid readability.