Java Tutorial Flashcards
summary
What is JAVA ?
Java is a high level , popular, object oriented programming language , it’s purpose is : WORA (Write Once , Run Anywhere) , that means compiled java code can run in any plateform without the need to recompile it again
Java Features
- Object Oriented : Abstraction , encapsulation , polymorphism , inheritance , object,Class.
- Platform Independant (interpreted also) : Java compiler compile code into byte code this byte code run by JVM on any platform.
- Easy to learn / simple: if you understand OOP , and if you already know about c and c++ , java inherit features from these languages and remove its complex feature like pointers , operator overloading ,multiple inheritence and explicit memory allocation (provides garbage collection).
- Secure : a developer is not required to interact with memory or operating system , Java provides automatic garbage collection so developer don t have to worry about memory leaks / management …
- Architectural neutral : jre handles automatically java byte code execution on any processor
- Portable : the same code executed on any platform
- Robust : compile-time error check ,strict type checking and runtime error handling
- Multithreading : multiprocess, thread handle , deadlock handle …
it enables you to run multiple threads at the same time. - High performance : JIT(just in time) compiler improve performance
- Distributed : developping internet based applications.
- Dynamic : more dynamic than c and c++ because it can adapt to an evolving environment.
Java’s release date
1995
Java’s original name
oak
Java Platforms (editions)
- JAVA SE ( java standard edition ) : develop and deploy portable code for desktop and server environments
- JAVA EE ( java enterprise edition ) : to develop web apps
- JAVA ME ( java micro edition ) : to develop mobile apps
- JavaFx : to develop light-weighted user interface for rich internet apps.
multitasking vs multiprocessing vs multithreading
multitasking : the ability of OS to execute multiple task simultaneously . there two type of multitask :
*multiprocessing : you can run multiple process and programs that doesn t share the same cpu and memory space (more isolated than threads) which make multiprocess a resource-intensive . it s good for CPU-BOUND where each process run in parallel on a different CPU cores.
*mutithreading : run mutliple thread with the same memory space and cpu (single process) which make communication / switch between threads more easier and lightweight.
=> Java have built-in support multihreading with Thread Class and Runnable Interface
BUT doesn t have built-in support multiprocessing , you can run java applications separately or use libraries for inter-process communication (IPS) if needed.
C++ Features
- middle level language : it can be used for systems development and large-scale app consumer.
- Execution speed : C++ is a compiled language and use procedures extensively.Garbage collection and dynamic typing and other modern features impede program execution.
- Object Oriented Programming : Growing code makes procedural code harder to handle. that’s the key advantage between c and C++.
*Extensive Libraries : C++ has a vast libraries like 3rd party which make development faster
Java vs C++
- platform independent + interpreted + compiled :
jAVA : can run on any platform (because java interpreted compiled)
C++ : can t (only compiled) - Memory management :
Java : GC manage it automatically
C++ : manage it manually (with help of ‘new’ and ‘delete’).
*Libraries:
Java:don t support it directly, but we can configure it using 3rd party libraries.
C++:support direct system library calls.
*Multiple inheritance:
JAVA: support only single inheritance.
C++ : support multiple inheritance because of the ‘virtual’ keyword
*Mutithreading:
JAVA: support it.
C++:don t , but can by using 3rd party libraries.
*Pass by value/reference :
Java : Java support pass by value only , both in primitive type and also in object reference.
C++: support pass by value , and pass by reference using the pointer &.
JVM Architecture
- Class Loader:
has 3 types : Bootstrap , Extension , Application Class Loader.
JVM manages the process of loading,linking and initializing classes and interfaces in a dynamic manner.
During the loading process, JVM find binary representation of a class and creates it.
In the linking process, loaded classes combined with run-time state of JVM so they can be executed during initialiazing phase.
During initializing, execution of linked classes
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JVM Memory/Runtime Data Areas: - Method Area:
- JVM Language Stacks:
- Native Method Stacks:
- PC (Program counter) Register:
- Heap:
___________________________________________ - Execution Engine : comprises GC , Compiler JIT and Interpreter.
=>JIT Compiler : is a part of JRE and it compiles byte code to machine code at runtime. - Native Method Interface :
- Native Method Libraries : collection of c and c++ libraries which are essential for execution.
What JDK, JRE and JVM ?
- JDK (java development kit) : it include all the tools ,executables and binaries for compile , debug and execute a java program.
It is platform dependent (there are different installers for windows, mac, unix)
JDK contains JRE and JVM.
the version of JDK define Java version
______________________________ - JRE (java runtime environment): it is JVM’s implementation , it is responsible for code execution
platform dependent
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*JVM (java virtual machine): it is a specification / something abstract.
Convert byte code to machine code and can also run code.
platform independent
How much variables java has ? and what is it ?
- Local variables: are always inside a block ( constructor , method … ), created in block , destroyed after skip block.
no default value.
can t use access modifiers
*static/class variables: outside block, inside class , have a default value
can access without creating a new object (className.varName)
created when program start , destroyed when program end
there are only one class variable copy per class , regardless of how many objects we create from it.
*Instance variables (non-static variables):outside block, inside class , have a default value.
created when object created , destroyed when object destroyed
How much modifiers java has ? and what is it ?
- Access Modifiers:
-default : Visible to package , no modifiers required
-public : visible to world
-private : visible to class only
-protected : visible to all subclasses and package - Non-access modifiers: final, abstract, strictfp
Java Data types
- Primitive:int, long , short, float , double , byte …
- Object/Reference
Java type casting (type conversion)
- Widening: also implicit type casting where compiler automatically cast a smaller type to a larger type.
- Narrowing: also explicit type where programmer manually cast type from large to small.
Java Generics
Are methods and classes that we use for different types without writing it again.
