函数式编程

主要是2个方面：

• Predicate
• Function

Predicate主要是用来判断条件是不是成立，而Function是用来执行实际的操作的。

Predicate和Predicates

Predicate是接口的定义，而Predicates是内在提供的一些常用实现。

常用的方法

• instanceOf(Class)
• assignableFrom(Class)
• contains(Pattern)
• in(Collection)
• isNull()
• alwaysFalse(), alwaysTrue()
• equalTo(Object)
• compose(Predicate, Function)
• and(Predicate...), or(Predicate...), not(Predicate)

简单使用

Predicate<CountryEnum> fromAsiaPredicate = new Predicate<CountryEnum>() {
        @Override
        public boolean apply(@Nullable CountryEnum country) {
            return country.getContinent().equals("ASIA");
        }
    };
    Predicate<CountryEnum> historyPredicate = new Predicate<CountryEnum>() {
        @Override
        public boolean apply(@Nullable CountryEnum country) {
            return country.getAge() > 1000;
        }
    };
    Predicate<CountryEnum> composedPredicate = Predicates.and(fromAsiaPredicate, historyPredicate);
    Iterable<CountryEnum> filteredCountries = Iterables.filter(CountryEnum.findCountries(), composedPredicate);
    Iterator iterator = filteredCountries.iterator();
    while (iterator.hasNext()) {
        System.out.println(iterator.next());
    }

Function和Functions

Function是接口的定义，而Functions是内在提供的一些常用实现。

常用的方法

• forMap(Map)
• compose(Function, Function)
• constant(T)
• identity()
• toStringFunction()

简单实用

Function<Country, String> upperCaseFunction = new Function<Country, String>() {
            @Override
            public String apply(@Nullable CountryEnum country) {
                if (country == null) {
                    return "";
                }
                return country.getCapital().toUpperCase();
            }
        };
        //倒排名称
        Function<String, String> reverseFunction = new Function<String, String>() {
            @Override
            public String apply(String s) {
                if (s == null) {
                    return null;
                }
                return new StringBuilder(s).reverse().toString();
            }
        };
        //混合方法
        Function<Country, String> composeFunction = Functions.compose(reverseFunction, upperCaseFunction);
        List<String> capitals = Lists.transform(countryList.findCountries(), composeFunction);