使用集合引用作为if语句条件而不进行比较？

Question

I'm a little embarrassed to be asking this because I'm sure this is a very basic question but having searched and thought about it I still can't work this out.

The code is from a class the tutor walked through developing during a tutorial on collections for my course. The class represents a crime sheet of districts with associated crime codes. The sheet is implemented as a SortedMap where the keys are of type String (representing the district) and the values are an ArrayList of Integers (representing the crime codes).

There is one method that we didn't manage to finish during the tutorial that the tutor sent on a copy of and I can't follow the logic. The code for the method is below

/**
 * displays districts affected by given crime code
 * 
 *     @param Integer crime code
 *     @return     none
 */
public void displayAffectedDistricts(Integer crimeCode)
{
    ArrayList<Integer> searchCode;
    for(String eachDistrict:  crimeTable.keySet())
    {
        searchCode = new ArrayList<Integer>();
        searchCode.add(crimeCode);
        if(!(searchCode.retainAll(crimeTable.get(eachDistrict))))
        {
           System.out.println("Code "+crimeCode+" found in district "+eachDistrict);
         } 
    }
}

I've run the code and can see that this works but I can't follow the logic of the if condition.

My understanding is that searchCode.retainAll(crimeTable.get(eachDistrict)) will evaluate to the reference to the list searchCode and that, at that point, searchCode will contain the single value given by the crimeCode argument if the Map entry for eachDistrict has crimeCode in it's set of values or will otherwise be empty.

As far as I'm aware that's not a boolean value so can't be used as an if condition without a comparison operator.

Could someone please explain what I'm missing?

Answer 1

It's pretty bad code. But, here's how it works:

The retainAll() method returns true if the list changed because of the operation. A list is created that contains the target code, and if the other list contains it, the list will not change.

I would severely mark down the person who wrote this if I was marking it, because it's obtuse. Here's a better implementation, uses much less code, is straightforward and easy to understand and performs better too:

public void displayAffectedDistricts(Integer crimeCode) {
    for (String eachDistrict : crimeTable.keySet()) {
        if (crimeTable.get(eachDistrict).contains(crimeCode)) {
           System.out.println("Code "+crimeCode+" found in district "+eachDistrict);
        } 
    }
}

But even this can be improved upon, by using even less code and performs much better:

public void displayAffectedDistricts(Integer crimeCode) {
    for (Map.Entry<String, ArrayList<Integer>> entry : crimeTable.entrySet()) {
        if (entry.getValue().contains(crimeCode)) {
           System.out.println("Code "+crimeCode+" found in district "+entry.getKey());
        } 
    }
}

This last version avoids all the numerous calls to get() . It is the most elite way of coding it. Suggest this to your teacher, who by the way should find another job.

Answer 2

The javadoc of retainAll() says:

Returns:
true if this list changed as a result of the call

The method could also be designed to return the resulting list. However, this would obscure that the change was not performed on a copy, but the original list. Moreover, it is pretty handy to know if the set operation has caused a change.

Answer 3

retainAll根据http://docs.oracle.com/javase/1.5.0/docs/api/java/util/AbstractCollection.html#retainAll(java.util.Collection）返回一个布尔值

Answer 4

retainAll returns true if this list changed as a result of the call.

So

!(searchCode.retainAll(crimeTable.get(eachDistrict)))

is equals to

crimeTable.get(eachDistrict).contains(crimeCode)

Answer 5

retainAll is defined in java.util.List and return a boolean as return value. So this value can be used as the condition of the if statement.