Guava 精讲一
Function用于定义一类对象的方法，将一个集合转换为其他类型集合，主要用于Colllections2, Lists, Iterables, Iterators,等类的transform方法，Functions类提供很多预定义的方法能将一组对象组合成一个复杂的对象。
简单示例 - 将countries集合转换为capital cities集合
@Test
public void shouldPrintCountryWithCapitalCityUpperCase() throws Exception {
// given
Function<Country, String> capitalCityFunction = new Function<Country, String>() {
public String apply(@Nullable Country country) {
if (country == null) {
return "";
}
return country.getCapitalCity();
}
};
// when
Collection<String> capitalCities = Collections2.transform(Country.getSomeCountries(), capitalCityFunction);
// then
assertThat(capitalCities).contains("Warsaw", "Madrid");
}
Functions.compose() - 将两个或者多个functions对象组合成一个
@Test
public void shouldComposeTwoFunctions() throws Exception {
Function<Country, String> upperCaseFunction = new Function<Country, String>() {
public String apply(@Nullable Country country) {
if (country == null) {
return "";
}
return country.getCapitalCity().toUpperCase();
}
};
Function<String, String> reverseFunction = new Function<String, String>() {
public String apply(String string) {
if(string == null) {
return null;
}
return new StringBuilder(string).reverse().toString();
}
};
Function<Country, String> composedFunction = Functions.compose(reverseFunction, upperCaseFunction);
// when
Collection<String> reversedCapitalCities = Collections2.transform(Country.getSomeCountries(), composedFunction);
// then
assertThat(reversedCapitalCities).contains("WASRAW", "DIRDAM");
}
Functions.forMap() - 预定义方法提供一组key从map中取出value
@Test
public void shouldUseForMapFunction() throws Exception {
// given
Map<String, String> map = Maps.newHashMap();
map.put(Country.POLAND.getName(), Country.POLAND.getCapitalCity());
map.put(Country.BELGIUM.getName(), Country.BELGIUM.getCapitalCity());
map.put(Country.SPAIN.getName(), Country.SPAIN.getCapitalCity());
map.put(Country.ENGLAND.getName(), Country.ENGLAND.getCapitalCity());
// when
Function<String, String> capitalCityFromCountryName = Functions.forMap(map);
List<String> countries = Lists.newArrayList();
countries.add(Country.POLAND.getName());
countries.add(Country.BELGIUM.getName());
// then
Collection<String> capitalCities = Collections2.transform(countries, capitalCityFromCountryName);
assertThat(capitalCities).containsOnly(Country.POLAND.getCapitalCity(), Country.BELGIUM.getCapitalCity());
}
Functions.forMap() - 当没有找到对应的value，程序会抛出异常。这一种情况可以用设定默认值来改变。
@Test(expectedExceptions = IllegalArgumentException.class, expectedExceptionsMessageRegExp = "Key 'Belgium' not present in map")
public void shouldUseForMapFunctionWithNonExistingKey() throws Exception {
// given
Map<String, String> map = Maps.newHashMap();
map.put(Country.POLAND.getName(), Country.POLAND.getCapitalCity());
// we omit this one intentionally
// map.put(Country.BELGIUM.getName(), Country.BELGIUM.getCapitalCity());
map.put(Country.SPAIN.getName(), Country.SPAIN.getCapitalCity());
map.put(Country.ENGLAND.getName(), Country.ENGLAND.getCapitalCity());
// when
Function<String, String> capitalCityFromCountryName = Functions.forMap(map);
List<String> countries = Lists.newArrayList();
countries.add(Country.POLAND.getName());
countries.add(Country.BELGIUM.getName());
// then
Collection<String> capitalCities = Collections2.transform(countries, capitalCityFromCountryName);
assertThat(capitalCities).containsOnly(Country.POLAND.getCapitalCity(), Country.BELGIUM.getCapitalCity());
}
@Test
public void shouldUseForMapFunctionWithDefaultValue() throws Exception {
// given
Map<String, String> map = Maps.newHashMap();
map.put(Country.POLAND.getName(), Country.POLAND.getCapitalCity());
// we omit this one intentionally
// map.put(Country.BELGIUM.getName(), Country.BELGIUM.getCapitalCity());
map.put(Country.SPAIN.getName(), Country.SPAIN.getCapitalCity());
map.put(Country.ENGLAND.getName(), Country.ENGLAND.getCapitalCity());
// when
Function<String, String> capitalCityFromCountryName = Functions.forMap(map, "Unknown");
List<String> countries = Lists.newArrayList();
countries.add(Country.POLAND.getName());
countries.add(Country.BELGIUM.getName());
// then
Collection<String> capitalCities = Collections2.transform(countries, capitalCityFromCountryName);
assertThat(capitalCities).containsOnly(Country.POLAND.getCapitalCity(), "Unknown");
}