Java-8-IntStream接口

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 。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));



    }
}