Java Guava过滤两个不同类型的集合

Question

我有两个收藏：

ArrayList<B> currentB = new ArrayList<B>();
{
    currentB.add(new B(new A("1")));
    currentB.add(new B(new A("2")));
    currentB.add(new B(new A("7")));
    currentB.add(new B(new A("3")));
    currentB.add(new B(new A("4")));
}
ArrayList<A> newA = new ArrayList<A>();
{
    newA.add(new A("1"));
    newA.add(new A("5"));
    newA.add(new A("2"));
    newA.add(new A("6"));
    newA.add(new A("7"));
    newA.add(new A("8"));

}

集合有以下类型：

class A {
    private String id;

    public String getId() {
        return id;
    }

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        A nodeRef = (A) o;

        return !(id != null ? !id.equals(nodeRef.id) : nodeRef.id != null);

    }

    public A() {
    }

    public A(String id) {
        this.id = id;
    }

    @Override
    public String toString() {
        return "NodeRef{" +
                "id='" + id + '\'' +
                '}';
    }
}

class B {
    private A a;

    public A a() {
        return a;
    }

    public A getA() {
        return a;
    }

    public void setA(A a) {
        this.a = a;
    }

    @Override
    public String toString() {
        return "B{" +
                "a=" + a +
                '}';
    }

    public B(A a) {
        this.a = a;
    }

    public B() {
    }
}

我要创建（最好的结果是修改newA和currentB）两个列表：

• 第一个应该有对象，在另一个中没有表示
  一个（例如 - a(3)、a(4)）；
• 第二个逻辑相同，反之亦然：(a(5), a(6), a(8))。

我可以用番石榴做，但需要创建 3 个集合：

Function<B, A> BtoA = new Function<B, A>() {
    public A apply(final B b) {
        return b.getA();
    }
};
Collection<A> currentA = Collections2.transform(currentB, BtoA);

java.util.Collection<A> idToDelete = Collections2.filter(currentA, Predicates.not(Predicates.in(newA)));
java.util.Collection<A> idToAdd = Collections2.filter(newA, Predicates.not(Predicates.in(currentA)));

System.out.println("Old B" + idToDelete);
System.out.println("New A" + idToAdd);

有没有办法摆脱 Collection.transform 甚至是最好的方法？

Answer 1

使用 java 8 流 API 看起来会很不错：

java.util.Collection<String> idToDelete =
        currentB.stream() //get the stream
        .filter(b -> !newA.contains(b.getA())) // filter those b, whose A is in newA
        .map(b -> b.getA().id) // map transform to get just an Id (you can use just getA() here)
        .collect(Collectors.toList()); // finally transform back to list

java.util.Collection<String> idToAdd =
        newA.stream() // again get stream
        .filter(
                // this is a little bit fancy...
                // only leave those A, for which currentB doesn't contain element, that has getA() equals to that A
                a -> currentB.stream().noneMatch(
                        b -> b.getA().equals(a)
                )
        )
        .map(a -> a.id) // again get id
        .collect(Collectors.toList()); // transform to list

[编辑：]

如果您查看 guava 的源代码，您会看到，transform 只是用一个转换函数包装了您现有的源代码。所以番石榴基本上就像java 8流一样工作。所以你实际上可以像以前一样使用变换。如果绝对不想这样做，这是番石榴的完整示例：

Function<B, A> BtoA = new Function<B, A>() {
    public A apply(final B b) {
        return b.getA();
    }
};
Function<A, String> aToId = new Function<A, String>() {
    public String apply(final A a) {
        return a.getId();
    }
};

java.util.Collection<B> bToDelete = Collections2.filter(currentB, Predicates.compose(Predicates.not(Predicates.in(newA)), BtoA));

//without transform, looks ugly
java.util.Collection<A> aToAdd = Collections2.filter(newA, new Predicate<A>() {
    @Override
    public boolean apply(final A a) {
        return !Iterables.any(currentB, new Predicate<B>() {
            @Override
            public boolean apply(B b) {
                return b.getA().equals(a);
            }
        });
    }
});
// this is essentially the same, you can safely use transform
//java.util.Collection<A> aToAdd = Collections2.filter(newA, Predicates.not(Predicates.in(Collections2.transform(currentB, BtoA))));

java.util.Collection<String> idToDelete = Collections2.transform(bToDelete, Functions.compose(aToId, BtoA));
java.util.Collection<String> idToAdd = Collections2.transform(aToAdd, aToId);

System.out.println("Old B: " + idToDelete);
System.out.println("New A: " + idToAdd);