Get unique values from arraylist in java

Question

I have an ArrayList with a number of records and one column contains gas names as CO2 CH4 SO2 etc.Now i want to retrieve different gas names(unique) only withou

Answer 1

When I was doing the same query, I had hard time adjusting the solutions to my case, though all the previous answers have good insights.

Here is a solution when one has to acquire a list of unique objects, NOT strings. Let's say, one has a list of Record object. Record class has only properties of type String, NO property of type int. Here implementing hashCode() becomes difficult as hashCode() needs to return an int.

The following is a sample Record Class.

public class Record{

    String employeeName;
    String employeeGroup;

    Record(String name, String group){  
        employeeName= name;
        employeeGroup = group;    
    }
    public String getEmployeeName(){
        return employeeName;
    }
    public String getEmployeeGroup(){
        return employeeGroup;
    }

  @Override
    public boolean equals(Object o){
         if(o instanceof Record){
            if (((Record) o).employeeGroup.equals(employeeGroup) &&
                  ((Record) o).employeeName.equals(employeeName)){
                return true;
            }
         }
         return false;
    }

    @Override
    public int hashCode() { //this should return a unique code
        int hash = 3; //this could be anything, but I would chose a prime(e.g. 5, 7, 11 )
        //again, the multiplier could be anything like 59,79,89, any prime
        hash = 89 * hash + Objects.hashCode(this.employeeGroup); 
        return hash;
    }

As suggested earlier by others, the class needs to override both the equals() and the hashCode() method to be able to use HashSet.

Now, let's say, the list of Records is allRecord(List allRecord).

Set distinctRecords = new HashSet<>();

for(Record rc: allRecord){
    distinctRecords.add(rc);
}

This will only add the distinct Records to the Hashset, distinctRecords.

Hope this helps.