创建泛型类列表

Question

public static final <T> List<T> list(T t) {
    final List<T> rv = new ArrayList<>();
    rv.add(t);
    return rv;
}

final Class<?> aClass = Integer.class;
final List<Class<?>> trivialListOfClasses = list(aClass);

[javac] /some/path/Foo.java:41: error: incompatible types
[javac]             final List<Class<?>> trivialListOfClasses = list(aClass);
[javac]                                                                  ^
[javac]   required: List<Class<?>>
[javac]   found:    List<Class<CAP#1>>
[javac]   where CAP#1 is a fresh type-variable:
[javac]     CAP#1 extends Object from capture of ?
[javac] 1 error

Answer 1

能够在javac 1.8.0_144上使用-source7-target7进行复制。

如果您只打算从列表中删除一些内容，那么一个可行的方法就是使用？扩展类<？>:

Class<?> aClass = Integer.class;
List<? extends Class<?>> trivialListOfClasses = list(aClass);
Class<?> bClass = trivialListOfClasses.get(0); // works
// trivialListOfClasses.add(String.class); // doesn't work, though

同样有效的是在对list的调用中手动提供泛型类型：

Class<?> aClass = Integer.class;
List<Class<?>> trivialListOfClasses = Util.<Class<?>>list(aClass);
Class<?> bClass = trivialListOfClasses.get(0); // works
trivialListOfClasses.add(String.class); // also works