帶有Java集合的泛型和通配符

Question

在使用AssertJ的測試類中，我的代碼類似於以下內容：

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);
        }
    };
}

我對has和doesNotHave的調用出現了編譯錯誤：

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>>).

我是新來的Java，我不明白的問題： java.util.List是一個超級型java.util.List和java.lang.String延伸java.lang.String ，不是嗎？

Answer 1

在你的情況下， has和doesNotHave方法采用Condition<? super List<? extends T> Condition<? super List<? extends T> Condition<? super List<? extends T>條件，而不是Condition<? super List<T>> 當您從Condition<List<T>> sameNamesAs方法返回時， Condition<? super List<T>> 。

您需要Condition<List<? extends String>>的實例 Condition<List<? extends String>> type（它是原始類型Condition<? super List<? extends String>>的子類）：

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

我嘗試使用以下代碼段來說明這一點：

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);