You can run java programs on a wide variety of computer using a range of operating system. Your java programs will run just as well on a PC running Windows 95/98/NT/2000 as it will on Linux or a Sun Solaris workstation. This is possible because a java program does not execute directly on your computer. It runs on a standardized hypothetical computer that is called the java virtual machine or JVM. Which is emulated inside your computer by a program.

A java compiler converts the java source code that you write into a binary program consisting of byte codes. Byte codes are machine instructions for the java virtual machine. When you execute a java program, a program called the java interpreter inspects and deciphers the byte codes for it, checks it out to ensure that it has not been tampered with and is safe to execute, and then executes the actions that the byte codes specify within the java virtual machine. A java interpreter can run stand-alone, or it be invoked automatically to run applets in a Web page.

Because your java program consists of byte codes rather than native machine instructions, it is completely insulated from the particular hardware on which it is run. Any computer that has the java environment implemented will handle your program as any other, and because the java interpreter sits between your program and the physical machine, it can prevent unauthorized actions in the program from being executed

Variables of this type can have values from -2147483648 to 2147483647 and occupy 4 bytes (32 bits) in memory.

public static void main (String[] args)

public : This specifies that main() is accessible from outside of the class.

Static : This specifies that main() exists without any objects being defined.

Void : This specifies that main() does not return any value .

return strError;
}

function isInteger(s){
var i;
for (i = 0; i < s.length; i++){
// Check that current character is number.
var c = s.charAt(i);
if (((c < "0") || (c > "9"))) return false;
}
// All characters are numbers.
return true;
}

function stripCharsInBag(s, bag){
var i;
var returnString = "";
// Search through string's characters one by one.
// If character is not in bag, append to returnString.
for (i = 0; i < s.length; i++){
var c = s.charAt(i);
if (bag.indexOf(c) == -1) returnString += c;
}
return returnString;
}

function daysInFebruary (year){
// February has 29 days in any year evenly divisible by four,
// EXCEPT for centurial years which are not also divisible by 400.
return (((year % 4 == 0) && ( (!(year % 100 == 0)) || (year % 400 == 0))) ? 29 : 28 );
}
function DaysArray(n) {
for (var i = 1; i <= n; i++) {
this[i] = 31
if (i==4 || i==6 || i==9 || i==11) {this[i] = 30}
if (i==2) {this[i] = 29}
}
return this
}

function isDate(dtStr){
var daysInMonth = DaysArray(12)
var pos1=dtStr.indexOf(dtCh)
var pos2=dtStr.indexOf(dtCh,pos1+1)
var strMonth=dtStr.substring(0,pos1)
var strDay=dtStr.substring(pos1+1,pos2)
var strYear=dtStr.substring(pos2+1)
strYr=strYear
if (strDay.charAt(0)=="0" && strDay.length>1) strDay=strDay.substring(1)
if (strMonth.charAt(0)=="0" && strMonth.length>1) strMonth=strMonth.substring(1)
for (var i = 1; i <= 3; i++) {
if (strYr.charAt(0)=="0" && strYr.length>1) strYr=strYr.substring(1)
}
month=parseInt(strMonth)
day=parseInt(strDay)
year=parseInt(strYr)
if (pos1==-1 || pos2==-1){
alert("The date format should be : mm/dd/yyyy")
return false
}
if (strMonth.length<1 || month<1 || month>12){
alert("Please enter a valid month")
return false
}
if (strDay.length<1 || day<1 || day>31 || (month==2 && day>daysInFebruary(year)) || day > daysInMonth[month]){
alert("Please enter a valid day")
return false
}
if (strYear.length != 4 || year==0 || year<minYear || year>maxYear){
alert("Please enter a valid 4 digit year between "+minYear+" and "+maxYear)
return false
}
if (dtStr.indexOf(dtCh,pos2+1)!=-1 || isInteger(stripCharsInBag(dtStr, dtCh))==false){
alert("Please enter a valid date")
return false
}
return true
}

function ValidateForm(){
var dt=document.frmSample.txtDate
if (isDate(dt.value)==false){
dt.focus()
return false
}
return true
}

</script>

<form name="frmSample" method="post" action="" onSubmit="return ValidateForm()">
<p>Enter a Date <font color="#CC0000"><b>(mm/dd/yyyy)</b></font>
:
<input type="text" name="txtDate" maxlength="10" size="15">
</p>
<p>
<input type="submit" name="Submit" value="Submit">
</p>
</form>