java - Java 中集合的泛型和通配符

Question

在使用 AssertJ 的测试类中，我有类似于以下的代码:

public void someTest() {
    assertThat(getNames()).has(sameNamesAs(getExpectedNames()));
    assertThat(getNames()).doesNotHave(sameNamesAs(getOtherNames()));
}

private List<String> getNames() {
    return null;
}
private List<String> getExpectedNames() {
    return null;
}
private List<String> getOtherNames() {
    return null;
}

private Condition<List<String>> sameNamesAs(List<String> rhs) {
    return new Condition<List<String>>("same names as " + rhs) {
        @Override
        public boolean matches(final List<String> lhs) {
            return lhs.containsAll(rhs) && rhs.containsAll(lhs);
        }
    };
}

我在调用 has 和 doesNotHave 时遇到编译错误:

has/doesNotHave
(org.assertj.core.api.Condition<? super java.util.List<? extends java.lang.String>>)
in AbstractListAssert cannot be applied
to
(org.assertj.core.api.Condition<java.util.List<java.lang.String>>).

我是 Java 的新手，我不明白这个问题:java.util.List 是 java.util.List 和 java.lang.String extends java.lang.String，不是吗？

Answer 1

在你的例子中，has和 doesNotHave方法采用 Condition<? super List<? extends T>条件，不是 Condition<? super List<T>>当你从你的 Condition<List<T>> sameNamesAs 返回时方法。

您需要一个 Condition<List<? extends String>> 的实例类型(它是原始类型 Condition<? super List<? extends String>> 的子类):

private Condition<List<? extends String>> sameNamesAs(List<String> rhs) {
    return new Condition<List<? extends String>>("same names as " + rhs) { ... };
}

我试着用下面的片段来说明这一点:

List<String> list = getNames();

// ELEMENT = String, ACTUAL = List<? extends ELEMENT>
ListAssert<String> assertThat = assertThat(list);

// by the signature, we have to pass Condition<? super ELEMENT> or Condition<? super ACTUAL>
// Condition<? super ACTUAL> = Condition<? super List<? extends String>>
Condition<List<? extends String>> condition = sameNamesAs(list);

// Condition<List<? extends String>> extends Condition<? super List<? extends String>>
assertThat.has(condition);