测试代码：
MapTest maptest = new MapTest(); List<Country> testdata = maptest.getCountry(); Map<String, Country> result1 = maptest.getCountryDataMap(testdata); Map<String, Country> result2 = maptest.getCountryDataMap2(testdata); Stream<Country> temp = testdata.stream(); Predicate<Country> match = country -> country.getCode().equals("US"); Stream<Country> result = temp.filter(match); System.out.println("Size 2: " + result.count()); List<Country> filterResult = testdata.stream().filter((country) -> country.getCode().equals("US")).collect(Collectors.toList()); System.out.println("size: " + filterResult.size()); filterResult.forEach(System.out::println); System.out.println(maptest.assertEqual2(result1, result2));
Predicate的源代码：
/** * Represents a predicate (boolean-valued function) of one argument. * * <p>This is a <a href="package-summary.html">functional interface</a> * whose functional method is {@link #test(Object)}. * * @param <T> the type of the input to the predicate * * @since 1.8 */
Predicate接受一个类型作为参数，返回布尔值。
因此代码里我就可以使用第113行这种箭头函数，country为箭头函数或者说Lambda Function里的形式参数(输入参数)，因为Predicate接收的泛型参数为T- Country，所以编译器能断定，形式参数的类型一定为Country.
最后用collect，传入Collectors.toList().
collect方法将传入的Collector施加到stream的元素内，
package java8.forHybrisCodeReview; import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; import java.util.function.Consumer; import java.util.function.Predicate; import java.util.stream.Collectors; import java.util.stream.Stream; class Country{ private String mCountryCode; private int mRank; public Country(String mCode, int rank){ this.mCountryCode = mCode; this.mRank = rank; } public String getCode(){ return this.mCountryCode; } @Override public boolean equals(Object other) { if( other == null) return false; if ( !(other instanceof Country) ) return false; Country country = (Country)other; return this.mCountryCode.equals(country.mCountryCode) && this.mRank == country.mRank; } @Override public int hashCode() { return this.mCountryCode.hashCode()*37 + this.mRank; } } public class MapTest{ public List<Country> getCountry(){ List<Country> countries = new ArrayList<Country>(); Country country = new Country("ZH", 1); countries.add(country); country = new Country("US", 2); countries.add(country); country = new Country("JP", 3); countries.add(country); return countries; } public Map<String, Country> getCountryDataMap(List<Country> countryList) { final Map<String, Country> countryDataMap = new HashMap<>(); for (final Country countryData : countryList){ countryDataMap.put(countryData.getCode(), countryData); } return countryDataMap; } public Map<String, Country> getCountryDataMap2(List<Country> countryList) { final Map<String, Country> countryDataMap = new HashMap<>(); Consumer<Country> consumer = c -> countryDataMap.put(c.getCode(), c); countryList.forEach(consumer); return countryDataMap; } // forEach之所以被称为内部遍历器，原因在于一旦它开始执行了，那么遍历操作就不能够被轻易中断。 public boolean assertEqual(Map<String, Country> map1, Map<String, Country> map2){ if( map1.size() != map2.size()) return false; final boolean equal = true; map1.forEach((key, value)-> { System.out.println("key of map1:" + key); Country country = map2.get(key); if( !value.equals(country)){ } // Void methods cannot return a value // return false; // equal = false; // cannot change final }); return equal; } public boolean assertEqual2(Map<String, Country> map1, Map<String, Country> map2){ if( map1.size() != map2.size()) return false; for (Map.Entry<String,Country> entry : map1.entrySet()) { String key = entry.getKey(); Country country = entry.getValue(); Country country2 = map2.get(key); if( !country.equals(country2)) return false; } return true; } public static void main(String[] arg){ MapTest maptest = new MapTest(); List<Country> testdata = maptest.getCountry(); Map<String, Country> result1 = maptest.getCountryDataMap(testdata); Map<String, Country> result2 = maptest.getCountryDataMap2(testdata); Stream<Country> temp = testdata.stream(); Predicate<Country> match = country -> country.getCode().equals("US"); Stream<Country> result = temp.filter(match); System.out.println("Size 2: " + result.count()); List<Country> filterResult = testdata.stream().filter((country) -> country.getCode().equals("US")).collect(Collectors.toList()); System.out.println("size: " + filterResult.size()); filterResult.forEach(System.out::println); System.out.println(maptest.assertEqual2(result1, result2)); } }
