List使用Stream 分組求和groupingBy， collect、reduce方法流數據處理

jdk8 流式數據處理太靈活，一下了還不太好掌握 ，下面記錄一些操作，方便使用的時候參考

 

 public void testCodeSum() {

        Foo foo1 = new Foo(1, 2, 5);
        Foo foo2 = new Foo(2, 23, 6);
        Foo foo3 = new Foo(2, 6, 7);
        List<Foo> list = new ArrayList<>(4);
        list.add(foo1);
        list.add(foo2);
        list.add(foo3);
        Map<Integer, IntSummaryStatistics> collect = list.stream().collect(
                Collectors.groupingBy(Foo::getCode, Collectors.summarizingInt(Foo::getCount)));
        // 輸出求結果的和
        Map slist = list.stream().collect(Collectors.groupingBy(foo -> foo.getCode(), Collectors.summingInt(fs -> fs.getCount())));
        System.out.println("sumCount: " + slist.toString());

        Foo fooobj = list.stream().reduce(
                (x, y) -> new Foo((x.getCode() + y.getCode()), (x.getCount() + y.getCount()), (x.getId() + y.getId()))).orElse(
                new Foo(1, 2, 3));
        System.out.println("reduce: " + fooobj.getId() + ", " + fooobj.getCount() + ", " + fooobj.getCode());

        IntSummaryStatistics statistics2 = collect.get(2);
        System.out.println(statistics2.getSum());
        System.out.println(statistics2.getAverage());
        System.out.println(statistics2.getMax());
        System.out.println(statistics2.getMin());
        System.out.println(statistics2.getCount());

    }


// @Test  列表中的多個元素的屬性求和並返回
    public void test() {
        List<A> list = new ArrayList<A>();
        list.add(new A(1, 2));
        list.add(new A(100, 200));
        A a = list.stream().reduce((x, y) -> new A((x.getPrincipal() + y.getPrincipal()), (x.getFee() + y.getFee()))).orElse(new A(0, 0));
        System.out.println(a);
    }

 

代碼1

class A {
    int principal = 0;
    int fee       = 0;

    public A(int principal, int fee) {
        super();
        this.principal = principal;
        this.fee = fee;
    }

    public A() {
        super();
        // TODO Auto-generated constructor stub
    }

    public int getPrincipal() {
        return principal;
    }

    public void setPrincipal(int principal) {
        this.principal = principal;
    }

    public int getFee() {
        return fee;
    }

    public void setFee(int fee) {
        this.fee = fee;
    }

    @Override
    public String toString() {
        return "A [principal=" + principal + ", fee=" + fee + "]";
    }

}

代碼2

class Foo {
    private int code;

    private int count;

    private int id;

    public Foo(int code, int count, int id) {
        this.code = code;
        this.count = count;
        this.id = id;
    }

    public int getCode() {
        return code;
    }

    public void setCode(int code) {
        this.code = code;
    }

    public int getCount() {
        return count;
    }

    public void setCount(int count) {
        this.count = count;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

}

 

 

 

 

簡單使用， 以後再詳細實踐