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xujiaji/Functional.java

Created May 15, 2018 02:05
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Java编程思想：泛型的一些实用性方法对象
Raw
Functional.java
interface Combiner<T>
{
T combine(T x, T y);
}
interface UnaryFunction<R, T>
{
R function(T x);
}
interface Collector<T> extends UnaryFunction<T, T>
{
T result();
}
interface UnaryPredicate<T>
{
boolean test(T x);
}
public class Functional
{
/**
* 结合所有元素
*/
public static <T> T reduce(Iterable<T> seq, Combiner<T> combiner)
{
Iterator<T> it = seq.iterator();
if (it.hasNext())
{
T result = it.next();
while (it.hasNext())
{
result = combiner.combine(result, it.next());
}
return result;
}
return null;
}
/**
* 通过方法对象调用每一个元素，忽略调用方法的返回结果
*/
public static <T> Collector<T> forEach(Iterable<T> seq, Collector<T> func)
{
for (T t : seq)
{
func.function(t);
}
return func;
}
/**
* 调用每一个方法，并通过List收集所有方法返回结果
*/
public static <R, T> List<R> transform(Iterable<T> seq, UnaryFunction<R, T> func)
{
List<R> result = new ArrayList<>();
for (T t : seq)
{
result.add(func.function(t));
}
return result;
}
/**
* 遍历每一个元素，并用List收集符合条件的元素
*/
public static <T> List<T> filter(Iterable<T> seq, UnaryPredicate<T> pred)
{
List<T> result = new ArrayList<>();
for (T t : seq)
{
if (pred.test(t))
{
result.add(t);
}
}
return result;
}
//创建方法对象去实现我的特殊需要
/**
* 相加
*/
static class IntegerAdder implements Combiner<Integer>
{
@Override
public Integer combine(Integer x, Integer y)
{
return x + y;
}
}
/**
* 相减
*/
static class IntegerSubtracter implements Combiner<Integer>
{
@Override
public Integer combine(Integer x, Integer y)
{
return x - y;
}
}
static class BigDecimalAdder implements Combiner<BigDecimal>
{
@Override
public BigDecimal combine(BigDecimal x, BigDecimal y)
{
return x.add(y);
}
}
static class BigIntegerAdder implements Combiner<BigInteger>
{
@Override
public BigInteger combine(BigInteger x, BigInteger y)
{
return x.add(y);
}
}
static class AtomicLongAdder implements Combiner<AtomicLong>
{
@Override
public AtomicLong combine(AtomicLong x, AtomicLong y)
{
return new AtomicLong(x.addAndGet(y.get()));
}
}
static class BigDecimalUlp implements UnaryFunction<BigDecimal, BigDecimal>
{
@Override
public BigDecimal function(BigDecimal x)
{
return x.ulp();
}
}
static class GreaterThan<T extends Comparable<T>> implements UnaryPredicate<T>
{
private T bound;
public GreaterThan(T bound)
{
this.bound = bound;
}
@Override
public boolean test(T x)
{
return x.compareTo(bound) > 0;
}
}
static class MultiplyingIntergerCollector implements Collector<Integer>
{
private Integer val = 1;
@Override
public Integer function(Integer x)
{
val *= x;
return val;
}
@Override
public Integer result()
{
return val;
}
}
public static void main(String[] args)
{
List<Integer> li = Arrays.asList(1, 2, 3, 4, 5, 6, 7);
Integer result = reduce(li, new IntegerAdder());
print(result);
result = reduce(li, new IntegerSubtracter());
print(result);
print(filter(li, new GreaterThan<>(4)));
print(forEach(li, new MultiplyingIntergerCollector()).result());
print(forEach(filter(li, new GreaterThan<>(4)), new MultiplyingIntergerCollector()).result());
}
}
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