使用Comparator和Inner类进行Java排序

Question

Given this combination of Java classes:

public class OuterClass
    {
        public String           field01;
        public int              field02;
        public InnerClass       innerField
        // ...getters, setters

    public class InnerClass
        {
            public int              innerField01;
            public BigDecimal       innerField02;
            // ...getters, setters

I want to order them by outer and inner fields. Now, given a List<OuterClass> list , I can easily sort that by eg, field01 with:

Collections.sort(list, Comparator.comparing(OuterClass::getField01));

But which way may I sort by InnerClass.innerfield.innerfield01 ? I tried

Collections.sort(list, Comparator.comparing(OuterClass::InnerField::innerField01));

and few other ways, but with no success. Maybe I should use in some way OuterClass::new , but I don't know how.

Answer 1

Let both outer and inner class implement Comparable and add a compareTo method to both or just let outer class implement it and handle all comparison there. Which way is best depends on how you want to compare, in the below example Outer is sorted first, then Inner

public class OuterClass implements Comparable{
    public String           field01;
    public int              field02;
    public InnerClass       innerField;

    public class InnerClass implements Comparable {
        public int              innerField01;
        public BigDecimal       innerField02;

        @Override
        public int compareTo(Object o) {
            //...
        }
    }

    @Override
    public int compareTo(Object o) {
        OuterClass obj = (OuterClass)o;
        int res = field01.compareTo(obj.field01);

        if (res != 0) {
            return res;
        }
        return this.innerField.compareTo(obj.innerField);
    }
}

Answer 2

You could use Comparator.comparing(Function, Comparator) like this:

Collections.sort(list, 
  Comparator.comparing(OuterClass::getInnerField, 
    Comparator.comparingInt(InnerClass::getInnerField01)));

The first argument is the function used to extract the key on which you want to sort. In your case: the Innerfield from the OuterClass .

The second argument is the Comparator used to compare the sort key. In your case: a Comparator for one of the fields of the InnerField ( innerField01 in the example above).

Answer 3

Collections.sort(list, Comparator.comparing(obj -> obj.getInnerField().getInnerField01()));