Here is a solution I found, it seems to be useful for all kind of Object to primitive types.

public static BigDecimal getBigDecimal( Object value ) {
    BigDecimal ret = null;
    if( value != null ) {
        if( value instanceof BigDecimal ) {
            ret = (BigDecimal) value;
        } else if( value instanceof String ) {
            ret = new BigDecimal( (String) value );
        } else if( value instanceof BigInteger ) {
            ret = new BigDecimal( (BigInteger) value );
        } else if( value instanceof Number ) {
            ret = new BigDecimal( ((Number)value).doubleValue() );
        } else {
            throw new ClassCastException("Not possible to coerce ["+value+"] from class "+value.getClass()+" into a BigDecimal.");
        }
    }
    return ret;
}

here is the link: http://www.java2s.com/Code/Java/Data-Type/ConvertObjecttoBigDecimal.htm

First, jsonObj.get is not a method, so surely that's the error.

Second, I don't think that casting any object to a BigDecimal is going to work like that.

Reading the API...

Going down from JsonReader to BigDecimal

JsonReader 
   > JsonObject
       > JsonNumber
          > BigDecimal 

You can do

jsonObj.getJsonNumber("productHeight").bigDecimalValue()

This assumes that productHeight is a numeric value, and not a string, though.

If it is a String, look at BigDecimal constructors. Specifically the one that takes a String value.

new BigDecimal(jsonObj.getString("productHeight"))

This is fine, remember that using the constructor of BigDecimal to declare a value can be dangerous when it's not of type String. Consider the below...

BigDecimal valDouble = new BigDecimal(0.35);
System.out.println(valDouble);

This will not print 0.35, it will infact be...

0.34999999999999997779553950749686919152736663818359375

I'd say your solution is probably the safest because of that.