如何从具有泛型类型的方法输入和返回值？

Question

I'm trying to create the method deNull() to check input values. These input value might be String, int, double, Timestamp. Therefore, I create the method with generics types.

If the input value is null, then it will return "";

If the input value is not null, then return the original value.

My code is as below:

public static <T> T deNull(T value){
    if(value == null ){
        return "";
    } else {
        return value;
    }
}   

However, this method freaks out at line 3 and shows Type mismatch: Cannot convert from String to T.

How should I amend this method to make it run as expected ?

Answer 1

int and double can not be null anyway, that leaves Timestamp and String as the only two options.

In my opinion just write both methods without generics, not worth the hassle.

If you actually need it for more than two types, i would suggest changing the method to.

public static <T> T deNull(T value, T orElse){
    if(value == null ){
        return orElse;
    } else {
        return value;
    }
}   

String x = deNull( string, "");
Integer y = deNull( integer, 0);

(I would also suggest to change the method name to valueOrElse )

Answer 2

Since the return value is always a String type, decouple it from parameter type T.

public static <T> String deNull(T value) {
    if (value == null) {
        return "";
    } else {
        return value.toString();
    }
}

Answer 3

If the 'T' is String your line 3 is valid else it is fail.

return "";

If the T is Integer string cannot cast to Integer

Integer a = 5;
String s = (String)a;//You cannot do like this