java.lang.ClassCastException：java.lang.Integer无法强制转换为java.lang.Double

Question

I'm Facing Issues at the Run time to compile this piece of code Which is throwing me an error that java.lang.Integer cannot be cast to Java.lang.Double. I'll more than happy if anyone help me out to correct this code

 double x;

 public Double getMethod() {

     HashMap hashmap= new HashMap();

     hashmap = SumCal();

     List  listabc = (List) hashmap.get("abclist");
     int total=(Integer) hashmap.get("all_total");
     x = (Double) listabc.get(0)*100/total;
     return x;
    }

Answer 1

You can do as below, But I would advice you to go with Generics .

x = ((Integer) listabc.get(0) * 100 / total);

If you had used the Generics like below you dont need any casting.

List<Integer> listabc 
HashMap<String, Integer> hashmap 
x = listabc.get(0) * 100 / total;

In that case you dont need any casting. One of the reason Integer, Double etc wrapper classes were introduced is to avoid casting.

Answer 2

A slight change in your code will grant success:

x = (double) listabc.get(0)*100/total;

Integer and Double are not cast compatible, but int and double are widening compatible. In your current code, the right hand side (RHS) of your expression is getting autoboxed up to an Integer, then you specify a cast of that result to Double . This cast fails.

By instead specifying a widening conversion, the Integer result gets unboxed into an int , then converted to a double . Then finally this line:

return x;

Boxes up the value of x into a Double and returns it as the result of the method.

Answer 3

First, you should not use a generic HashMap , because then the compiler can't help you with types. Try declaring the following:

HashMap<String, Integer> hashmap = new HashMap();

I would not recommend storing a HashMap of differently-valued types, as it just invites programming errors. Also, there's no need to cast an int to a Double . You can do any of the following:

x = listabc.get(0) * 100.0 / total; // multiply by a floating-point number

x = (double) listabc.get(0) * 100 / total; // cast with (double)