ashahrior / Java-Short-Notes

Just for short reminders of the basic tics and toes of Java.

Written with StackEdit.

Table of Contents

Introduction
Syntax
Data types

Intrdocution

• Any java code is written for a particular Java Virtual Machine.
• The JVM compiles the code into intermediate code called Bytecode. Only JVM can understand the bytecode.
• JVM interprets the bytecode into machine code during runtime. For this interpretation JVM uses Just In Time (JIT) compiler.
• Java is both compiled and interpreted language.
• Bytecode - a set of instructions only understandable by JVM.
• Java code is not written for any particular machine but for particular JVMs. Different operating system runs different JVM. But all JVM can run the same java code.
• Just In Time (JIT) compiler processes the bytecode produced by the compiler. It is also called Dynamic Translator as it processes bytecode during runtime.
• JVM is inside of JRE. JRE is inside of JDK.
• Java platform has 4 subsets -
  • Java Standard Edition - JSE
  • Java Enterprise Edition - JEE
  • Java Micro Edition - JME
  • JavaFX
• Class loader - A special class as a part of java runtime system. It loads necessary java compiled classes during runtime.

Syntax

• A program must contain - function and data
• Package name always written in all lowercase letters.
• A java method has six parts-
  • Modifier
  • Return type
  • Method name
  • List of parameters
  • List of exceptions
  • Method body Example:

     public String concat(String val1, String val2) throws IllegalArgumentException
     {
     	if (val1==null) {
     		throw new IllegalArgumentException("val-1 is null");
     	}
     	if (val2==null) {
     		throw new IllegalArgumentException("val-2 is null");
     	}
     	return val1+val2;
     }

• Java object creation consists of the following steps-
  • Declaration
  • Instantiation
  • Initialization Example: Here,      Bicycle bike - Declaration      new - Instantiation      Bicycle() - Initialization

   Bicycle bike = new Bicycle();

• Class constructor has no return type.
• If class constructor is not written explicitly then java compiler creates it implicitly before compilation. It is called Default constructor.
• If a class contains several constructors then they are differentiated on the basis of their respective arguments.
• getter method also called accessor.
• setter method also called mutator.
• Java method 2 types -
  • Static method
    • It can't access non-static fields
    • It can't access object states
    • Doesn't require any object instantiation for access
    • Example:

     Integer.parseInt("26");

  • Non-static method
• Class import can be done in 2 ways-
  • Specific importing - Fully Qualified Name
  • Wildcard importing - using '*' sign

Data types

• Four types of variables in java-
  • Instance variables (Non-static field)
  • Class variable (Static field)
  • Local variable - declared inside of the method body or any code-block
  • Parameters variable Example:

   class Example {
   	int a;	\\ instance-variable
   	static int b;	\\ class-variable
   	void method(int c)	\\ 'c' is parameters variable
   	{
   		int d;	\\ local variable
   	}
   }

• Java has two data types-
  • Primitive types - boolean, byte, short, char, int, long, float, double
  • Reference types - Every primitive type has its own reference type. These are called Wrapper class of primitive types. Examples: Boolean, Byte, Short, Char, Integer, Long, Float, Double
• Primitive types and wrapper class objects can be used interchangeably. This is called Automatic Conversion.
• Primitive type -> Reference type => Autoboxing
• Reference type -> Primitive type => Unboxing
• char default value is '\u000'
• Initializing without instantiation is called literal. Example: int x = 5; here 5 is a value and a literal.
• Smaller data types -> Larger data types => Type conversion / Widening Primitive Conversion
• In Java there are 19 types of Type Conversions.
• Larger data types -> Smaller data types => Type casting / Narrowing Primitive Conversion
• In Java there are 22 types of Type Casting.
• In Java, array is a special type of object. Even though it has no class, it is created dynamically during runtime. Like any other class or object in Java, array also inherits java.lang.object class.
• Accessing array elements using index is called Array Access Expression.
• An array can be declared even with an Interface or Abstract Class. In this case, its elements are the objects of that interface's implemented class or the objects of that abstract class's concrete class. Array can store null value as well.
• Array has a public final field called length that keeps record of the number of elements in array.