Stuck On Porting Small Segment Of JavaScript To Java

I'm fairly good at Java, but somewhat at a loss when the some of the JavaScript below is mentioned. I've attempted to port the below code, but I'm finding it somewhat difficult.

Parts of JavaScript I'm stuck at porting:

1. I'm stuck on the return statement on the 3rd line.

2. Similarly the lines utilizing "charCodeAt", as I keep getting error "Cannot invoke charCodeAt(int) on the primitive type char"

3. Also on the second to last line, I keep getting the error "Cannot invoke padLZ(int) on the primitive type int".

Link to original javascript

Convert between Latitude/Longitude & OS National Grid Reference points

Original JavaScript

Java Code:

OsGridRef.prototype.toString = function(digits) {
digits = (typeof digits == 'undefined') ? 10 : digits;
e = this.easting, n = this.northing;
if (e==NaN || n==NaN) return '??';
// get the 100km-grid indices
var e100k = Math.floor(e/100000), n100k = Math.floor(n/100000);

[Code] ....