java - 深入比较数组中的每个元素

Question

我有以下3个文件，

A.java:

class A {

    private float b;    

    public A(float b) {
        this.b = b;
    }

    public float getB() {
        return b;
    }

    public String toString() {
        return "A(b = " + b + ")";    
    }

}

C.java:

import java.util.Arrays;

class C {

    private A[] d;
    private int i = 0;

    public C() {
        d = new A[5];
    }

    public void addB(A b) {
        d[i++] = b;
    }

    public String toString() {
        return "C(b = " + Arrays.toString(d) + ")";    
    }

    public void duplicate() {
        A temp[] = Arrays.copyOf(d, d.length);
        for (int cur = 0; cur < d.length; cur++) {
            if (d[cur] == null) continue;
            float currB = d[cur].getB();
            for (int nxt = cur + 1; nxt < d.length; nxt++) {
                if(d[nxt] == null) continue;
                if(currB == d[nxt].getB()) {
                    temp[i++] = new A(currB * 0.5f);
                }
            }
        }
        d = temp;
    }
}

D.java:

class D {

    public static void main(String[] args) {
        C c = new C();
        c.addB(new A(3));
        c.addB(new A(5));
        c.addB(new A(3));
        System.out.println(c.toString()); // C(b = [A(b = 3.0), A(b = 5.0), A(b = 3.0), null, null])
        c.duplicate();
        System.out.println(c.toString()); // C(b = [A(b = 3.0), A(b = 5.0), A(b = 3.0), A(b = 1.5), null])

    }

}

这符合我的预期，即用一半的 b 将另一个项目添加到数组中如果两个元素从 A.getB() 返回相同的 float .但是，我尝试使用花哨的 Java 8 流方法和 lambda 函数来实现它，如下所示:

Arrays.stream(d).anyMatch(cur -> {
    if (cur == null) return false;
    Arrays.stream(d).anyMatch(nxt -> {
        if (nxt == null) return false;
        System.out.println("Checking " + cur.getB() + " with " + nxt.getB());
        return false;
    });
    return false;
});

这输出:

Checking 3.0 with 3.0 Checking 3.0 with 5.0 Checking 3.0 with 3.0 Checking 5.0 with 3.0 Checking 5.0 with 5.0 Checking 5.0 with 3.0 Checking 3.0 with 3.0 Checking 3.0 with 5.0 Checking 3.0 with 3.0

如您所见，这遵循 O(n²) 算法，这不是我想要的。在我的原始代码中，我“跳过”了我已经使用外部嵌套 for 循环中的索引检查过的元素。所以我的问题是，是否有办法在嵌套的 <Stream>.anyMatch(...) 中以某种方式实现它？我尝试过。或者有更简洁的方法吗？

Answer 1

您可以使用 Stream API 复制 duplicate 方法，如下所示:

Stream<A> result = 
       IntStream.range(0, d.length)
                .filter(cur -> d[cur] != null)
                .flatMap(cur -> IntStream.range(cur + 1, d.length)
                        .filter(nxt -> d[nxt] != null)
                        .filter(nxt -> d[cur].getB() == d[nxt].getB())
                        .map(i -> cur))
                .mapToObj(cur -> new A(d[cur].getB() * 0.5f));

d = Stream.concat(Arrays.stream(d), result)
          .toArray(A[]::new);