I want to make a generic method that determines whether the first parameter is null, and if it is the case returns the second which is the default, else returns the first.

Can I make this possible?

I don't know how to implement it properly.

 public static <T> ifNull(Object parameter, Object default, Class<T> type) {
        return type.cast(parameter == null ? default: parameter));
 }

Note: this code doesn't work for real, it's just an example.

CodePudding user response：

I think this link might be useful: is there a Java equivalent to null coalescing operator (??) in C#?

 public static <T> T coalesce(T one, T two) {
    return one != null ? one : two; }

or

 public static <T> T coalesce(T... params) {
     for (T param : params)
         if (param != null)
             return param;
     return null; }

CodePudding user response：

You can use something like this,

public static <T> T getValOrDefault(T value, T defaultValue) {
    return value != null ? value : defaultValue;
}

CodePudding user response：

I am not 100 % sure, but i think you want something like that:

public static <T> T ifNull(T parameter, T defaultValue) {
    return parameter == null ? defaultValue: parameter;
}

CodePudding user response：

If you are receiving two nullable objects of Object type and want to type-cast them dynamically to the given type by passing an in instance of Class<T> to your method, I think you were intending to achieve this:

public static <T> Optional<T> ifNull(Object first, Object second, Class<T> type) {
    return Optional.ofNullable(type.cast(first))
            .or(() -> Optional.ofNullable(type.cast(second)));
}

Class<T> object represents a class or interface

Method cast() will cast the given object to a particular type T, if it's the object is assignable to that type, otherwise a ClassCastException will be thrown. Note that compiler will not issue warnings for such way of downcasting (which is always potentially unsafe).

Optional is a special container-object introduced with Java 8 in order to be used a return type in cases when the resulting value could be null. Optional object might either contain a value or be empty, but it would never be null (if you are following good practices and not using optional as a field, method parameter and not storing it anywhere)

Optional.ofNullable() creates optional object containing the given value, if it's non-null, otherwise returns an empty optional.

Method or() will substitute the current optional object, if it is empty, with the provided optional.

The result of method execution will be the following:

Optional[first] --(first == null)--> Optional[second] --(second == null)--> empty optional

Note:

• In order to declare a generic method you have to place a generic type parameter like <T> or wild card <? extends Number> between modifiers and return type or keyword void (in the method above <T> is ageneric type parameter and Optional<T> is a return type).
• default is a keyword in Java.