?? ex27(1).java
字號:
// generics/Ex27.java
// TIJ4 Chapter Generics, Exercise 27, page 680
// Show that covariancd doesn't work with Lists using Numbers and
// Integers, then introduce wildcards.
import java.util.*;
public class Ex27 {
public static void main(String[] args) {
// compile error: incompatible types:
// List<Number> lnum = new ArrayList<Integer>();
List<? extends Number> nlist = new ArrayList<Integer>();
// compile error: can't add Integer:
// nlist.add(new Integer(0));
nlist.add(null); // can add null
Number x = nlist.get(0); // can get Number (null)
System.out.println(nlist);
}
}
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