&username=john&password=12345&email=john@go.com

for (set in array)
{
   params.add(new BasicNameValuePair(set.key, set.value));
}

String all = "&username=john&password=12345&email=john@go.com";
//Split across all instances of the 'and' symbol
String[] keyValueConcat = all.split("&");
Map<String, String> kvPairs = new HashMap<String, String>();
for (String concat : keyValueConcat) {
    if (concat.contains("=") {
        //For any string in the split that contains an equals sign
        //Split over the equals sign and add to the map
        String[] keyValueSplit = concat.split("=", 2);
        kvPairs.put(keyValueSplit[0], keyValueSplit[1];
    }
}

String exp="&username=john&password=12345&email=john@go.com"
String[] pairs=exp.split("&");
String[] temp;
Map<String, String> myMap= new HashMap<String, String>();

for(i=0;i<pairs.length;i++){
temp=pairs[i].split("=");
myMap.put(temp[0],temp[1]);

}

String string = "&username=john&password=12345&email=john@go.com";
Map<String, String> map= Stream.of(string.split("&")).map(p->p.split("=")).filter(p->p!=null && p.length==2).collect(Collectors.toMap(p->p[0], p->p[1]));