走近Guava(二): 函数式编程

函数式编程:

使用Function接口(jdk8中已经存在):

/**
 * 其功能就是将输入类型转换为输出类型
 */
public interface Function<F, T> {
  T apply(@Nullable F input);
}

比如一个简单的日期转换:

/**
 * 日期转换
 */
public class DateFormatFunction implements Function<Date, String> {
	@Override
	public String apply(Date input) {
		SimpleDateFormat dateFormat = new SimpleDateFormat("dd/mm/yyyy");
		return dateFormat.format(input);
	}
}

使用Functions类:

  • Functions.forMap()方法:
/**
 * 州类
 */
public class State {
	private String name;
	private String code;
	private Set<City> mainCities = new HashSet<City>();
}

现在你想在一个Map<String, State>(key为州的编号)对象中查找某个key, 你可以:

Map<String, State> states = new HashMap<String, State>();
Function<String, State> lookup = Functions.forMap(states);
System.out.println(lookup.apply(key));//key不存在会抛异常

//你也可以给不存在的key指定一个默认值
Function<String, State> lookup = Functions.forMap(states, null);
  • Functions.compose()方法
/**城市类**/
public class City {
	private String name;
	private String zipCode;
	private int population;

	@Override
	public String toString() {
		return name;
	}
}
/**
 * 将州的城市转换为字符串
 */
public class StateToCityString implements Function<State, String> {
	@Override
	public String apply(State input) {
		return Joiner.on(",").join(input.getMainCities());
	}
}

你可以通过组合Function,查找某州的城市列表

Function<String, State> lookup = Functions.forMap(states);
Function<State, String> stateFunction = new StateToCityString(); //州到城市的转换
Function<String, String> stateCitiesFunction = Functions.compose(stateFunction, lookup); //组合Function
System.out.println(stateCitiesFunction.apply(key));

等价于:

stateFunction.apply(lookup.apply(key));

使用Predicate接口(jdk8中已存在):

  • Predicate接口
public interface Predicate<T> {
     boolean apply(T input); //不同于Function.apply, 该apply用于过滤对象
}

如:

/**
 * 过滤人口小于500000的城市
 */
public class PopulationPredicate implements Predicate<City> {
	@Override
	public boolean apply(City input) {
		return input.getPopulation() <= 500000;
	}
}

使用Predicates类:

有两个过滤条件:

/**
 * 选择气候为TEMPERATE的城市
 */
public class TemperateClimatePredicate implements Predicate<City> {
	@Override
	public boolean apply(City input) {
		return input.getClimate().equals(Climate.TEMPERATE);
	}
}

/**
 * 选择雨量小于45.7的城市
 */
public class LowRainfallPredicate implements Predicate<City> {
	@Override
	public boolean apply(City input) {
		return input.getAverageRainfall() < 45.7;
	}
}

你可以运用下面的方法实现过滤组合等:

Predicates.and(smallPopulationPredicate,lowRainFallPredicate);//且
Predicates.or(smallPopulationPredicate,temperateClimatePredicate);//或
Predicate.not(smallPopulationPredicate);//非
Predicates.compose(smallPopulationPredicate,lookup);//组合转换再过滤

使用Supplier接口:

  • Supplier接口
public interface Supplier<T> {
       T get(); //用于创建对象
}

使用Suppliers类:

  • Suppliers.memorize()方法:
SupplyCity sc = new SupplyCity();
System.out.println(Suppliers.memoize(sc).get());
System.out.println(Suppliers.memoize(sc).get());//返回同一对象, 单例
  • Suppliers.memorizeWithExpiration()方法:
SupplyCity sc = new SupplyCity(); //超时再新建对象, 类似缓存
Supplier<City> supplier = Suppliers.memoizeWithExpiration(sc, 5, TimeUnit.SECONDS);
City c = supplier.get();
System.out.println(c); 
Thread.sleep(3000);
c = supplier.get();
System.out.println(c); //与之前相等
Thread.sleep(2000);
c = supplier.get();
System.out.println(c); //与之前不等

Guava函数式编程基础,后面集合处理中,将体现得更强大。

https://www.yeetrack.com/?p=1171

走近Guava(二): 函数式编程

走近Guava(三): 集合

走近Guava(四): 并发

走近Guava(五): 缓存

走近Guava(六): 事件总线EventBus

走近Guava(七): 文件操作

来源:http://my.oschina.net/indestiny/blog/215041

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