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Java: Core Concepts & OOP

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Core Java syntax and object-oriented programming fundamentals.

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Java

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1Java
A high-level, general-purpose, memory-safe, object-oriented programming language
2WORA
Write Once, Run Anywhere - compiled Java code can run on all platforms that support Java without recompiling
3Java bytecode
An intermediate representation of Java code, intended to be executed by a JVM rather than directly by hardware
4JVM
Java Virtual Machine - translates Java bytecode into the platform's machine language
5What was Java originally designed for?
Interactive television (but it was too advanced for the digital cable TV industry at the time)
6Java type system
Static, strong, safe, nominative, and manifest
7How does Java handle memory?
Automatic garbage collection - the runtime recovers memory once objects are no longer in use
8Five primary goals of Java
1) Simple, object-oriented, familiar 2) Robust and secure 3) Architecture-neutral and portable 4) High performance 5) Interpreted, threaded, and dynamic
9Four Java editions
Java Card (smart cards), Java ME (limited resources), Java SE (workstations), Java EE (enterprise)
10Is Java syntax similar to C and C++?
Yes, but Java was built almost exclusively as an object-oriented language
11Are all data items objects in Java?
No, primitive data types (int, float, boolean, char) are not objects for performance reasons
12Does Java support operator overloading?
No
13Does Java support multiple inheritance for classes?
No, but multiple inheritance is supported for interfaces
14Three comment styles in Java
// (single line), /* */ (multi-line), /** */ (Javadoc)
15Javadoc
A comprehensive documentation system that generates documentation from /** */ comments
16Traditional main method signature
public static void main(String[] args)
17Java applet
Programs embedded in other applications (mainly web pages). API deprecated in Java 9 (2017)
18Java servlet
Server-side Java EE components that generate responses (usually HTML) to HTTP requests
19JSP
JavaServer Pages - server-side components that generate HTML using special delimiters <% %>
20Swing
A graphical user interface library for Java SE with pluggable look and feel
21JavaFX
A platform for creating desktop and rich web applications, intended to replace Swing
22Why were generics added to Java?
To enable compile-time type checking for container classes without creating many similar classes
23Are generics type-safe in Java?
No, the type system was proven unsound in 2016 - it's possible to cause ClassCastExceptions at runtime
24JRE
Java Runtime Environment - contains parts of Java SE required to run Java programs (intended for end users)
25JDK
Java Development Kit - includes development tools (compiler, Javadoc, Jar, debugger) for software developers
26OpenJDK
An open-source Java SE implementation licensed under GNU GPL, the official Java reference implementation as of Java SE 7
27Default garbage collector since Java 9
G1GC (Garbage First Garbage Collector)
28What other garbage collectors are available?
ZGC (Java 11+), Shenandoah GC (Java 12+), Parallel GC (default in Java 8)
29Does Java support pointer arithmetic?
No, to ensure type safety, security, and enable garbage collector to relocate objects
30Where are primitive types stored?
Directly in fields (for objects) or on the stack (for methods) rather than on the heap