Java-8-IntStream接口
interface IntStream extends BaseStream<Integer, IntStream>
下面这段代码计算菜单的热量:
int calories = menu.stream()
.map(Dish::getCalories)
.reduce(0, Integer::sum);
这段代码有一个问题，它有一个暗含的装箱成本。每个 Integer 都必须拆箱成一个原始类型，再进行求和
要是可以直接像下面这样调用 sum 方法，岂不是很好:
int calories = menu.stream()
.map(Dish::getCalories)
.sum();
但这是不可能的。问题在于 map 方法会生成一个 Stream<T> 。虽然流中的元素是 Integer 类型，但 Streams 接口没有定义 sum 方法
Java 8引入了三个原始类型特化流接口来解决这个问题： IntStream 、 DoubleStream 和LongStream ，分别将流中的元素特化为 int 、 long 和 double ，从而避免了暗含的装箱成本。每个接口都带来了进行常用数值归约的新方法，比如对数值流求和的 sum ，找到最大元素的 max 。此外还有在必要时再把它们转换回对象流的方法
生成流
- IntStream of(int t)
-
IntStream of(int... values)
-
IntStream iterate(final int seed, final IntUnaryOperator f)
-
IntStream generate(IntSupplier s)
使用
public class M1 {
public static void main(String[] args) {
IntStream intStream1 = IntStream.of(1,2,3,4,5,6,7,8,9);
intStream1.forEach(System.out::println);
System.out.println("***********************************************");
int[] d = {1,2,3,4,5,6,7,8,9};
IntStream intStream2 = IntStream.of(d);
intStream2.forEach(System.out::println);
System.out.println("***********************************************");
IntStream intStream3 = IntStream.iterate(1,x->x+3).limit(10);
intStream3.forEach(System.out::println);
System.out.println("***********************************************");
IntStream intStream4 = IntStream.generate(()->100).limit(10);
intStream4.forEach(System.out::println);
}
}
转换流
- asLongStream
-
asDoubleStream
-
toArray
使用
public class M2 {
public static void main(String[] args) {
IntStream intStream1 = IntStream.of(1,2,3,4,5,6,7,8,9);
LongStream longStream1 = intStream1.asLongStream();
longStream1.forEach(System.out::println);
System.out.println("***********************************************");
DoubleStream doubleStream1 = intStream1.asDoubleStream();
doubleStream1.forEach(System.out::println);
System.out.println("***********************************************");
int[] d = IntStream.of(1,2,3,4,5,6,7,8,9).toArray();
for (int i : d){
System.out.println(i);
}
}
}
数据汇总
// * and is equivalent to:
// * <pre>{@code
// * return reduce(0, Integer::sum);
// * }</pre>
int sum();
// * and is equivalent to:
// * <pre>{@code
// * return reduce(Integer::min);
// * }</pre>
OptionalInt min();
// * and is equivalent to:
// * <pre>{@code
// * return reduce(Integer::max);
// * }</pre>
OptionalInt max();
// * equivalent to:
// * <pre>{@code
// * return mapToLong(e -> 1L).sum();
// * }</pre>
long count();
OptionalDouble average();
IntSummaryStatistics summaryStatistics();
使用
public class M1 {
public static void main(String[] args) {
List<Employee> employees = Employee.supply_Ems();
IntStream intStream1 = IntStream.of(1,2,3,4,5,6,7,8,9);
int sum = intStream1.sum();
System.out.println(sum);
System.out.println("---------------------------------------");
//统计员工工资
IntStream intStream2 = employees.stream()
.mapToInt(Employee::getSalary);
// intStream2.forEach(System.out::println);
//统计工资总数
// int sum1 = intStream2.sum();
// System.out.println(sum1);
System.out.println("---------------------");
// 找出最低工资
// OptionalInt optionalInt = intStream2.min();
//
// System.out.println(optionalInt.getAsInt());
System.out.println("---------------------");
// 找出最高工资
// OptionalInt optionalInt2 = intStream2.max();
//
// System.out.println(optionalInt2.getAsInt());
// long num = intStream2.count();
//// 数据个数
// System.out.println(num);
System.out.println("---------------------");
// 平均工资
// OptionalDouble d1 = intStream2.average();
//
// System.out.println(d1.getAsDouble());
System.out.println("---------------------");
IntSummaryStatistics intSummaryStatistics = intStream2.summaryStatistics();
System.out.println(intSummaryStatistics.toString());
/*
IntSummaryStatistics{count=30, sum=173068, min=1621, average=5768.933333, max=9000}
*/
}
}
range && rangeClosed
// * @apiNote
// * <p>An equivalent sequence of increasing values can be produced
// * sequentially using a {@code for} loop as follows:
// * <pre>{@code
// * for (int i = startInclusive; i < endExclusive ; i++) { ... }
// * }</pre>
static IntStream range(int startInclusive, int endExclusive)
// * @apiNote
// * <p>An equivalent sequence of increasing values can be produced
// * sequentially using a {@code for} loop as follows:
// * <pre>{@code
// * for (int i = startInclusive; i <= endInclusive ; i++) { ... }
// * }</pre>
static IntStream rangeClosed(int startInclusive, int endInclusive)
使用
public class M1 {
public static void main(String[] args) {
// IntStream.range(1,100).forEach(System.out::println);
System.out.println("----------------------------------");
// IntStream.rangeClosed(1,100).forEach(System.out::println);
/***************************************/
int[] a1 = IntStream.rangeClosed(1,10)
.map(i -> i + i)
.toArray();
System.out.println(Arrays.toString(a1));
}
}
boxed
如果想转换回Integer流，可以使用这个方法
public class M3 {
public static void main(String[] args) {
int[] d1 = Data.supply();
IntStream i1 = IntStream.of(d1);
Stream<Integer> integerStream = i1.boxed();
integerStream.sorted(Integer::compareTo).forEach(System.out::println);
}
}
其它
public class M2 {
public static void main(String[] args) {
IntStream i1 = IntStream.of(1,1,1,2,5,8,9,55,5,5,5,8);
i1.distinct().forEach(System.out::println);
System.out.println("-----------------------------------");
int[] a1 = Data.supply();
System.out.println(Arrays.toString(a1));
System.out.println("-----------------------------------");
int[] sort = IntStream.of(a1).sorted().toArray();
System.out.println(Arrays.toString(sort));
}
}
网友评论