根据Java中属性的值查找ArrayList中对象的索引

Question

I am using Java 7. Is it possible to get the index of an Object in an ArrayList depending on a particular attribute value. eg:

class Abc{
  String first_name;
  String last_name;
  // getter and setters...
}

Now

List<Abc> abcList = new ArrayList();
Abc abcObj = new Abc();
abcObj.setFirst_name("Jeet");
abcObj.setLast_name("Adhikari");
abcList.add(abcObj);

Abc abcObj2 = new Abc();
abcObj2.setFirst_name("John");
abcObj2.setLast_name("Something");
abcList.add(abcObj2);

Now is there any better way without iterating to get the index of the object in abcList where name = "John" .

Answer 1

No, but you could use a HashMap instead with first name as key and the complete object as value

HashMap<String, Abc> abcMap = new HashMap<String, Abc>();
Abc abcObj = new Abc();
abcObj.setFirst_name("Jeet");
abcObj.setLast_name("Adhikari");
abcMap.put(abcObj.getFirst_name(), abcObj);

Abc abcObj2 = new Abc();
abcObj2.setFirst_name("John");
abcObj2.setLast_name("Something");
abcMap.put(abcObj2.getFirst_name(), abcObj2);

And then you can simply get the full object by calling for eg

Abc objectByKey = abcMap.get("John");

I hope this alternative solution is also ok for you

Answer 2

As your list is an ArrayList, it can be assumed that it is unsorted. Therefore, there is no way to search for your element that is faster than O(n).

If you can, you should think about changing your list into a Set (with HashSet as implementation) with a specific Comparator for your sample class.

Another possibility would be to use a HashMap. You can add your data as Sample (please start class names with an uppercase letter) and use the string you want to search for as key. Then you could simply use

Sample samp = myMap.get(myKey);

If there can be multiple samples per key, use Map>, otherwise use Map<String, Sample> . If you use multiple keys, you will have to create multiple maps that hold the same dataset. As they all point to the same objects, space shouldn't be that much of a problem.

Answer 3

You could use the indexOf . You must override the equals method in your Abc class.

Answer 4

There are 2 solutions:

• Using a iteration like arrayList.indexOf(value)
• Using a Hash based Data structure like a java.util.HashSet instead of java.util.ArrayList

Answer 5

我不知道你的解决方案背后的要求，但我会考虑使用LinkedHashMap而不是ArrayList。