Java remove duplicate objects in ArrayList [duplicate]

问题

This question already has answers here:

How do I remove repeated elements from ArrayList? (38 answers)

Closed 6 years ago.

I have a very lengthy ArrayList comprised of objects some of them however, are undoubtedly duplicates. What is the best way of finding and removing these duplicates. Note: I have written a boolean-returning compareObjects() method.

回答1:

Example

List<Item> result = new ArrayList<Item>();
Set<String> titles = new HashSet<String>();

for( Item item : originalList ) {
    if( titles.add( item.getTitle() ) {
        result.add( item );
    }
}

Reference

Set
Java Data Structures

回答2:

You mentioned writing a compareObjects method. Actually, you should override the equals method to return true when two objects are equal.

Having said that, I would just return a new list that contains unique elements from the original:

ArrayList<T> original = ...
List<T> uniques = new ArrayList<T>();
for (T element : original) {
  if (!uniques.contains(element)) {
    uniques.add(element);
  }
}

This only works if you override equals. See this question for more information.

回答3:

Hashset will remove duplicates. Example:

Set< String > uniqueItems = new HashSet< String >();
uniqueItems.add("a");
uniqueItems.add("a");
uniqueItems.add("b");
uniqueItems.add("c");

The set "uniqueItems" will contain the following : a, b, c

来源：https://stackoverflow.com/questions/20433692/java-remove-duplicate-objects-in-arraylist