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. Any help would be great :-)

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

http://www.movable-type.co.uk/script...ridref-v1.html

Original JavaScript

Code:

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    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);
       
                          if (e100k<0 || e100k>6 || n100k<0 || n100k>12) return '';
       
                          // translate those into numeric equivalents of the grid letters
                          var l1 = (19-n100k) - (19-n100k)%5 + Math.floor((e100k+10)/5);
                          var l2 = (19-n100k)*5%25 + e100k%5;
       
                          // compensate for skipped 'I' and build grid letter-pairs
                          if (l1 > 7) l1++;
                          if (l2 > 7) l2++;
                          var letPair = String.fromCharCode(l1+'A'.charCodeAt(0), l2+'A'.charCodeAt(0));
       
                          // strip 100km-grid indices from easting & northing, and reduce precision
                          e = Math.floor((e%100000)/Math.pow(10,5-digits/2));
                          n = Math.floor((n%100000)/Math.pow(10,5-digits/2));
       
                          var gridRef = letPair + ' ' + e.padLz(digits/2) + ' ' + n.padLz(digits/2);
       
                          return gridRef;
                        }

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Attempted Port

Code:

---

      public void gridrefNumToLet(int e, int n, int digits) {
              // get the 100km-grid indices
              double e100k = Math.floor(e/100000), n100k = Math.floor(n/100000);
             
              if (e100k<0 || e100k>6 || n100k<0 || n100k>12) return '';
   
              // translate those into numeric equivalents of the grid letters
              double l1 = (19-n100k) - (19-n100k)%5 + Math.floor((e100k+10)/5);
              double l2 = (19-n100k)*5%25 + e100k%5;
   
              // compensate for skipped 'I' and build grid letter-pairs
              if (l1 > 7) l1++;
              if (l2 > 7) l2++;
              double letPair = String.fromCharCode(l1+'A'.charCodeAt(0), l2+'A'.charCodeAt(0));
   
              // strip 100km-grid indices from easting & northing, and reduce precision
              e = Math.floor((e%100000)/Math.pow(10,5-digits/2));
              n = Math.floor((n%100000)/Math.pow(10,5-digits/2));
   
              double gridRef = letPair + e.padLZ(digits/2) + n.padLZ(digits/2);
   
              return gridRef;
            }

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