Java In Brief

Java and Platform Independence

Build once, Run anywhere
Java is one of the most popular platform independent languages. Once we compile a java program and build a jar, we can run the jar (compiled java program) in any Operating System where a JVM is installed.
What is ByteCode?
The executable instructions are different in different operating systems. So, there are different JVM s for different operating systems. A JVM for windows is different from a JVM for mac.

JDK vs JVM VS JRE

JVM
- Virtual machine that run the Java bytecode.
- Makes java portable.
JRE
- JVM + Libraries + Other Components (to run applets and other java applications)
JDK
- JRE + Compilers + Debuggers

Classloaders and Types

A Java program is made up of a number of custom classes (written by programmers like us) and core classes (which come pre-packaged with Java). When a program is executed, JVM needs to load the content of all the needed class. JVM uses a ClassLoader to find the classes.
System Class Loader : Loads all classes from CLASSPATH
Extension Class Loader : Loads all classes from extension directory
Bootstrap Class Loader : Loads all the Java core files
When JVM needs to find a class, it starts with System Class Loader. If it is not found, it checks with Extension Class Loader. If it not found, it goes to the Bootstrap Class Loader. If a class is still not found, a ClassNotFoundException is thrown.

Language Basics

Why do we need Wrapper Classes?

Example 1
A wrapper class wraps (encloses) around a data type and gives it an object appearance
Wrapper: Boolean,Byte,Character,Double,Float,Integer,Long,Short
Primitive: boolean,byte,char ,double, float, int , long,short
Examples of creating wrapper classes are listed below.
- Integer number = new Integer(55);//int;
- Integer number2 = new Integer("55");//String
- Float number3 = new Float(55.0);//double argument
- Float number4 = new Float(55.0f);//float argument
- Float number5 = new Float("55.0f");//String
- Character c1 = new Character('C');//Only char constructor
- Boolean b = new Boolean(true);
Reasons
- null is a possible value
- use it in a Collection
- Object like creation from other types.. like String

Are instances of Wrapper Classes Immutable?

What is Immutability?
Wrapper classes are final and immutable.

What is Auto Boxing?

	// Auto Boxing
	Integer ten = 10;//new Integer(10);
	ten++;// allowed. Java does had work behind the screen for us

Boxing and new instances - Auto Boxing helps in saving memory by reusing already created Wrapper objects. However wrapper classes created using new are not reused.

		// Two wrapper objects created using new are not same object
		Integer nineA = new Integer(9);
		Integer nineB = new Integer(9);
		System.out.println(nineA == nineB);// false
		System.out.println(nineA.equals(nineB));// true

		// Two wrapper objects created using boxing are same object
		Integer nineC = 9;
		Integer nineD = 9;
		System.out.println(nineC == nineD);// true
		System.out.println(nineC.equals(nineD));// true

Strings

Are String's immutable?

		//Strings are immutable
		String str3 = "value1";
		str3.concat("value2");
		System.out.println(str3); //value1
		
		//The result should be assigned to a new reference variable (or same variable) can be reused.
		String concat = str3.concat("value2");
		System.out.println(concat); //value1value2

Where are string literals stored in memory?

All strings literals are stored in "String constant pool". If compiler finds a String literal,it checks if it exists in the pool. If it exists, it is reused. Following statement creates 1 string object (created on the pool) and 1 reference variable.

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