Question

I tried a code

double temp=0;
List list=new ArrayList();
list.add(1.1);
temp+=list.get(0); //error occurs here

Error that occurred is as follows

bad operand types for binary operator ‘+’

first type: double

second type: Object

What should I do to add the temp value and the double value in the list?

The programming language that I preferred is Java.

Answer 1

You should always use Generic type for your Collections type like List, ArrayList, so on, when programming in Java 1.5+.

When you use a raw type List, then the method List.get(0) will always give you Object type, which you would need to do some casting to get it to work.

So, either you can add a casting to Double in your current code, which is absolutely a bad idea.

Or, you should use generic version of List as below: –

List<Double> list=new ArrayList<Double>();
list.add(1.1);

Note that, the value 1.1 is a double, still it can be added directly, without the need to convert it to Double. This is because of the feature of Java called Auto Boxing.

In Java, when you store a double value in a Double reference, then the double value is automatically boxed into Double. While in reverse case, it is called Unboxing, i.e. from Double to double.

That is why, when you do: –

temp+=list.get(0);

the value of list.get(0) is of type Double. And since you are performing an operation with double primitive type, that vlaue is automatically boxed to double, and addition operation is performed.