CODEWITHSUNDEEP
javagenericswildcard
Vipin
Vipin

Reputation: 5223

Java method parameter gives compilation error "Unexpected bound"

Out of these 2 methods inspect1 shows compilation error "Unexpected bound" and inspect2 works fine, why ?

public <U> void inspect1(List<U extends Number> u){
    System.out.println("T: " + t.getClass().getName());
    System.out.println("U: " + u.getClass().getName());
}

public <U> void inspect2(List<? extends Number> u){
    System.out.println("T: " + t.getClass().getName());
    System.out.println("U: " + u.getClass().getName());
}

Upvotes: 2

Views: 1830

Answers (1)

Nikolas
Nikolas

Reputation: 44398

To make it work, this should be defined in the generic type method signature saying that U can be any type that is a subclass of Number:

public <U extends Number> void inspect1(List<U> u) {
    // body
}

Also, note in the second method, the parameter U is never used and should be as below. This says that any list of a subclass of Number is accepted. No generic parameter U is involved.

public void inspect2(List<? extends Number> u) {
    // body
}

Is there going to be any difference between these 2 methods (inspect1 and inspect2) you have provided in the answer, or they are going to work exactly the same?

They practically do the same since you cannot extend Number and the only types extending this class are (hope I got them all):

  • Double, Float, Short, Integer and Long

Upvotes: 3

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