Question

我无法理解，如何为stream构建谓词。
上课：

public class User {
    @Getter
    @Setter
    private String firstName;

    @Getter
    @Setter
    private String secondName;
}

另外，我有谓词课程：

public final class Predicates {

    private Predicates() {
    }

    /**
     * Returns a predicate, Not
     */
    public static <T> Predicate<T> not(Predicate<T> predicate) {
        return predicate.negate();
    }

    /**
     * Returns a predicate, isNull
     */
    public static <T> Predicate<T> isNull(Object o) {
        return (o == null)
                ? ObjectPredicate.IS_NULL.callback()
                : ObjectPredicate.ALWAYS_FALSE.callback();
    }

    //Enum for predicate object
    enum ObjectPredicate implements Predicate<Object> {
        IS_NULL {
            @Override
            public boolean test(Object o) {
                return o == null;
            }
        },
        ALWAYS_FALSE {
            @Override
            public boolean test(Object o) {
                return false;
            }
        };

        <T> Predicate<T> callback() {
            return (Predicate<T>) this;
        }
    }
}

然后我想用这样的东西：

List<User> users = getSomeUserList();
Booleant check = users.stream().anyMatch(Predicate.isNull(User::getSecondName));

帮助我理解，我做错了什么。

Answer 1

您可以使用map将Stream<User>映射到Stream<String>，然后使用您的谓词搜索匹配项：

Boolean check = users.stream()
                     .map(User::getSecondName)
                     .anyMatch(Predicates::isNull);

Answer 2

尝试以下方法：

function updateMetaThemeColor(theme) {
    var themeColor;
    if(theme == 'theme-1') {
        themeColor = '#f4dcdc'
    } else if(theme == 'theme-2') {
        themeColor = '#f0e8dd';
    } else if(theme == 'theme-3') {
        themeColor = '#d5e7f0';
    } else if(theme == 'theme-4') {
        themeColor = '#f6efb9';
    } else if(theme == 'theme-5') {
        themeColor = '#eaeae8';
    } else if(theme == 'theme-6') {
        themeColor = '#f8e3c7';
    } else if(theme == 'theme-7') {
        themeColor = '#cde8e8';
    }

    //remove the current meta
    $('meta[name=theme-color]').remove();
    //add the new one
    $('head').append('<meta name="theme-color" content="'+themeColor+'">');
}

在流Java 8中使用谓词

2 个答案: