how to remove arraylist duplicate values

问题

I am performing some maintenance tasks on an old system. I have an arraylist that contains following values:

a,b,12
c,d,3
b,a,12
d,e,3
a,b,12

I used following code to remove duplicate values from arraylist

ArrayList<String> arList;
  public static void removeDuplicate(ArrayList arlList)
  {
   HashSet h = new HashSet(arlList);
   arlList.clear();
   arlList.addAll(h);
  }

It works fine, if it finds same duplicate values. However, if you see my data carefully, there are some duplicate entries but not in same order. For example, a,b,12 and b,a,12 are same but in different order.

How to remove this kind of duplicate entries from arraylist?

Thanks

回答1:

Assuming the entries are String. Then you can sort each of the entry and then do the duplicate check. Then you can store the entry in a map and use the contains(key) to see if they exist.

EDIT: added a complete code example.

public class Test {

    /**
     * @param args
     */
    public static void main(String[] args) {
        Test test = new Test();
        List<String> someList = new ArrayList<String>(); 
        someList.add("d,e,3");
        someList.add("a,b,12");
        someList.add("c,d,3");
        someList.add("b,a,12");
        someList.add("a,b,12");
            //using a TreeMap since you care about the order
        Map<String,String> dupMap = new TreeMap<String,String>();
        String key = null;
        for(String some:someList){
            key = test.sort(some);
            if(key!=null && key.trim().length()>0 && !dupMap.containsKey(key)){
                dupMap.put(key, some);
            }
        }
        List<String> uniqueList = new ArrayList<String>(dupMap.values());
        for(String unique:uniqueList){
            System.out.println(unique);
        }

    }
    private String sort(String key) {
      if(key!=null && key.trim().length()>0){
        char[] keys = key.toCharArray();
        Arrays.sort(keys);
        return String.valueOf(keys);
      }
      return null;
   }
}

Prints:

a,b,12

c,d,3

d,e,3

回答2:

Wrap the element as "Foo" instead of "String", rest of code 'removeDuplicate' remains:

public class Foo {
    private String s1;
    private String s2;
    private String s3;

    public Foo(String s1, String s2, String s3) {
     this.s1 = s1;
     this.s2 = s2;
     this.s3 = s3;
    }

 @Override
    public int hashCode() {
     final int prime = 31;
     int result = 1;
     result = prime * result + ((s1 == null) ? 0 : s1.hashCode());
     result = prime * result + ((s2 == null) ? 0 : s2.hashCode());
     result = prime * result + ((s3 == null) ? 0 : s3.hashCode());
     return result;
    }

 @Override
    public boolean equals(Object obj) {
     if (this == obj)
      return true;
     if (obj == null)
      return false;
     if (getClass() != obj.getClass())
      return false;
     Foo other = (Foo) obj;
     //Notice here: 'a,b,12' and 'b,a,12' will be same
     if(fieldsAsList().containsAll(other.fieldsAsList())){
      return true;
     }

     return false;
    }

 private List<String> fieldsAsList(){
  ArrayList<String> l = new ArrayList<String>(3);
  l.add(s1);
     l.add(s2);
     l.add(s3);
     return l;
 }    
}

Then arList will be ArrayList < Foo>.

回答3:

Create a class to wrap around a row string (triplet) to provide your equality semantics. Implement the equals() and hashCode() methods. Then use the HashSet method to remove duplicates.

回答4:

Try this simple solution...(No Set interface used)

https://stackoverflow.com/a/19434592/369035

回答5:

In ArrayList we don't have a chance to remove duplicate elements directly. We can achieve it with sets, because sets don't allow duplicates, so, better to use HashSet or LinkedHashSet classes. See reference.

来源：https://stackoverflow.com/questions/4778260/how-to-remove-arraylist-duplicate-values

标签

java

arraylist

duplicates