Question

我编写了一段代码来了解有关java集合Comparator函数的更多信息。我有两组，每组有3个元素。我想比较一下。我在下面发布我的代码并输出计数器变量。谁能解释为什么变量i给出了这个奇怪的输出？我无法理解流程。

public class TestProductBundle {
public static void main(String args[]) {

    TestProductBundle productBundle = new TestProductBundle();

    Set<ClassA> hashSetA = new HashSet<ClassA>() {
        {
            add(new ClassA("name", 1, "desc"));
            add(new ClassA("name", 2, "desc"));
            add(new ClassA("name", 3, "desc"));
        }
    };

    Set<ClassA> hashSetB = new HashSet<ClassA>() {
        {
            add(new ClassA("name1", 2, "desc1"));      //"name" & "desc" are different than previous 
            add(new ClassA("name2", 1, "desc2"));
            add(new ClassA("name3", 3, "desc3"));
        }
    };

    if (productBundle.compareCollection(hashSetA, hashSetB)) {
        System.out.println("Equal set of tree");
    } else {
        System.out.println("Unequal set of tree");
    }
}

@SuppressWarnings("serial")
public boolean compareCollection(Set<ClassA> collection1, Set<ClassA> collection2) {

    TreeSet<ClassA> treeSetA = new TreeSet<ClassA>(new CompareID()) {
        {
            addAll(collection1);
        }
    };

    TreeSet<ClassA> treeSetB = new TreeSet<ClassA>(new CompareID()) {
        {
            addAll(collection2);
        }
    };

    if (treeSetA.containsAll(treeSetB) && treeSetB.containsAll(treeSetA))
        return true;
    else
        return false;
    }
}

ClassA的代码和实现Comparator的类。

class ClassA {
String name;
int id;
String desc;

public ClassA(String name, int id, String desc) {
    this.name = name;
    this.id = id;
    this.desc = desc;
}

int getId() {
    return id;
    }
}

＆安培;

class CompareID implements Comparator<ClassA> {
    int i = 0;

    @Override
    public int compare(ClassA o1, ClassA o2) {
        System.out.println(i++);                   // Counter variable
        if (o1.getId() > o2.getId())
            return 1;
        else if (o1.getId() < o2.getId())
            return -1;
        else
            return 0;

    }
}

输出（也在调试器中交叉验证）

0
1
2
3
0   // why started from 0 again ?
1      
2
3
4
5
6
7
8
4   // What the hell !!!
5
6
7
8
Equal set of tree  // is that correct output ?

Answer 1

我不确定你发现什么是奇怪的。您有两个 TreeSet实例，每个实例都有自己的CompareID实例作为Comparator，每个CompareID实例都维护它自己的计数器。

因此，看到每个计数器值（0,1,2等等）出现两次并不奇怪。至于计数器值的出现顺序，这取决于TreeSet的内部实现。

至于

树的等号//是正确的输出吗？

是的，两个集都包含相同的元素。订单无关紧要。为了澄清 - contains的方法containsAll和TreeSet考虑将x包含在TreeSet if compare(x,y)==0中的某个元素{ {1}}的{1}}，其中y是所提供的TreeSet的{{1}}方法。因此，在您的示例中，只有compare属性确定两个元素是否相等。

以下方案解释了输出：

compare

编辑：首先，您要向第一个Comparator添加元素（因此连续多次调用第一个id的{{1}}），然后您将元素添加到第二个0 // compare method of 1st CompareID object executed 1 // compare method of 1st CompareID object executed 2 // compare method of 1st CompareID object executed 3 // compare method of 1st CompareID object executed 0 // compare method of 2nd CompareID object executed 1 // compare method of 2nd CompareID object executed 2 // compare method of 2nd CompareID object executed 3 // compare method of 2nd CompareID object executed 4 // compare method of 1st CompareID object executed 5 // compare method of 1st CompareID object executed 6 // compare method of 1st CompareID object executed 7 // compare method of 1st CompareID object executed 8 // compare method of 1st CompareID object executed 4 // compare method of 2nd CompareID object executed 5 // compare method of 2nd CompareID object executed 6 // compare method of 2nd CompareID object executed 7 // compare method of 2nd CompareID object executed 8 // compare method of 2nd CompareID object executed TreeSet（因此连续多次调用第二个compare的{{1}}），然后调用Comparator，这会导致第一个TreeSet compare Comparator要连续多次调用，最后调用treeSetA.containsAll(treeSetB)，这会导致第二个compare的{{1}}连续多次被调用。

Answer 2

这里变量i将初始化为两个比较器对象，你正在初始化i = 0，根据你的观点，这里将对两个集合执行比较方法，而不是按顺序执行，所以最新发生的是你期望序列输出但实际上它不是

0// for set1(collection1)
1
2
3  // remember this value for set 1
0   // for set2(collection2)
1      
2
3
4
5
6
7
8
4   for set1(collection1 which was 3 and now increment to 1 )
5
6
7
8

Answer 3

添加到@Erans答案。

将代码更改为以下内容，您将看到

时输出的编译器

class CompareID implements Comparator<ClassA> {
    int i = 0;
    String a;
    public CompareID(String input) {
        a = input;
    }
    @Override
    public int compare(ClassA o1, ClassA o2) {
        // Output comperator "id"
        System.out.println(a+ "   "  + i++); // Counter variable
        if (o1.getId() > o2.getId())
            return 1;
        else if (o1.getId() < o2.getId())
            return -1;
        else
            return 0;

    }
}

... 

    TreeSet<ClassA> treeSetA = new TreeSet<ClassA>(new CompareID("A")) {
        {
            addAll(collection1);
        }
    };

    TreeSet<ClassA> treeSetB = new TreeSet<ClassA>(new CompareID("B")) {
        {
            addAll(collection2);
        }
    };

输出将更改为以下内容。评论包括产生输出的内容：

// This is addALL treeSetA
A   0
A   1
A   2
A   3
// this is addAll treeSetB
B   0
B   1
B   2
B   3
// this is treeSetA.containsAll(treeSetB)
A   4
A   5
A   6
A   7
A   8
// this is treeSetB.containsAll(treeSetA)
B   4
B   5
B   6
B   7
B   8

基本上两个comperators只输出

0-8
0-8

比较器java中计数器变量的奇怪输出

3 个答案: