Top Most Core Java Fundamentals Notes

datatype	Default value
int	0
char	‘\u0000’
float /double	0.0
Object reference	Null

Primitive datatype	Range
byte	-128 to 127
short	-32768 to 32767
int	-2147483648 to 2147483647
long	- 9223372036854775808 to 9223372036854775807
char	0 to 65535

1. following are valid declaration of package ,import and class declarations

·         package a;

import java.io.*;

class A{}

·         import java.io.*;

class A{}

2. following are invalid declarations ,

·         import …..

package ….

class ….

·         class…..

import ….

class….

3. the package or import statement at the beginning of the file is applicable to all the classes in the file.
4. following are valid main declarations

·         public static void main(String [] args){}

·         public static void main(String []XXX){}//in the place of XXX any thing can be placed

·         static public void main(String [] args){}

·         public static final void main(String [] args){}

·         public static synchronized void main(String [] args){}

·         public static strictfp void main(String [] args){}

5. following declarations of main method results in runtime error(not compile time error)

·         public void main(String [] args){}//static modifier missing

·         static void main(String [] args){}//public modifier missing

·         public static int main(String [] args){return 1;}//return type must be void

·         private static void main(String [] args){}//access modifier is private

·         protected static void main(String [] args){}//access modifier is protected

·         public static void main(String args){}//argument to main method is a String and not ‘String[]’ (String array)

·         public static void main(){}//argument String[] missing

6. Any class which implements a given interface must provide implementation to all the methods in the interface unless the class itself is abstract.

7. The Runnable interface has one and only one method whose prototype is

·         public void run();

8. Thread class implements Runnable interface.
9. When an array is created then the contents of the array are indexed from 0 to array.length-1 .

Java keywords

package	import	class	extends	interface
implements	public	private	protected	final
abstract	strictfp	super	this	byte
short	int	long	char	boolean
float	double	static	transient	volatile
void	return	native	synchronized	try
catch	finally	throw	throws	for
while	do	break	continue	switch
case	default	if	else	new
instanceof	const	goto	assert

10. A variable declared within a method will not get it’s default value and hence it if it is used in any expression then the compiler objects that the variable may not be initialized.
11. instance variables get their default value.
12. Array elements get default value whether it is declared inside a method or declared outside the method.
13. backslash characters are

‘\b’	‘\f’	‘\n’	‘\r’
‘\t’	‘\\’	‘\’’	‘\”’
‘\0’	‘\1’	‘\2’	‘\3’
‘\4’	‘\5’	‘\6’	‘\7’

In any String or char if  value present next to ‘\’(backslash) is other than those mentioned in the above table then it results in compiler error ,for eg

“\/santosh”//results in error

“/\santosh”//results in error

“//santosh”//no error

\\santosh //no error

14. given, \u000a = \n and \u000d = \r we cannot use these(\u000a and \u000d) in any 

string or char assignment statements. Hence the following will cause compiler error ,

String s1 = “\u000a”;

String s2 = “\u000d”;

These two are interpreted as line terminators and hence results in compiler error.