Sort an ArrayList based on an object field [duplicate]

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Closed 14 years ago.

Possible Duplicate:
Sorting an ArrayList of Contacts

I am storing DataNode objects in an ArrayList. The DataNode class has an integer field called degree. I want to retrieve DataNode objects from nodeList in the increasing order of degree. How can I do it.

List<DataNode> nodeList = new ArrayList<DataNode>();

• Alex Jasmin: Possible duplicate of stackoverflow.com/questions/2784514/…
• Alex Jasmin: On a second tought stackoverflow.com/questions/1814095/… may be closer to this
• Alex Jasmin: See also stackoverflow.com/questions/2535124/…

Answer 1

Use a custom comparator:

Collections.sort(nodeList, new Comparator<DataNode>(){
 public int compare(DataNode o1, DataNode o2){
 if(o1.degree == o2.degree)
 return 0;
 return o1.degree < o2.degree ? -1 : 1;
 }
});

• blitzkriegz: how about return(o1.degree - o2.degree);?
• camickr: The proper way to do this is the way Mark coded it. Yes the simple one line of code will work 99.9 percent of the time. But you will have a problem if the result of the subtraction is a large number causing the high order bit to overflow. For example you would expect (Integer.MAX_VALUE - (-10)) to be postive, but it isn't.
• Ben Aaronson: @camickr What about o1.degree.compare(o2.degree)?
• Kazaag: Java 8 update: Collection.sort(nodeList, Comparator.comparing(DataNode::getDegree)) or Collection.sort(nodeList, Comparator.comparingDouble(DataNode::getDegree)) if degree is an double instead of an Double
• yair: @Kazaag your comment should be the correct answer! :) The Comparator.comparing... methods can also be used with the Streams API as in nodesList.stream().sorted(Comparator.comparing(DataNode::getDegree)).collect(Collector.toList())

Answer 2

Modify the DataNode class so that it implements Comparable interface.

public int compareTo(DataNode o)
{
 return(degree - o.degree);
}

then just use

Collections.sort(nodeList);

• shmosel: Beware of overflow! Much safer to return Integer.compare(this.degree, o.degree);
• Christian Westbrook: For some reason I get an error if I don't input an object
• ACV: this will fail because there are no checks for null. Also you don't need the brackets
• Matt Westlake: I'm getting an issue where I have to use Object as the type in the method call, and I can't cast that to my object type (DataNode in this example)
• Luke B: Your class has to implement Comparable<YourClass> and not just implement Comparable.

Answer 3

You can use the Bean Comparator to sort on any property in your custom class.