How to sort by String value of object in Java

Question

I have Java bean class and I want to sort list of these beans by one bean attribute that is of String type. How can I do that?

Answer 1

Either make the type itself implement Comparable<Foo>, implementing the compareTo method by comparing the two strings, or implement a Comparator<Foo> which again compares by the strings.

With the first approach, you'd then just be able to use Collections.sort() directly; with the second you'd use Collections.sort(collection, new FooComparator());

For example:

public class Foo {
    public String getBar() {
        ...
    }
}

public class FooComparatorByBar implements Comparator<Foo> {
    public int compare(Foo x, Foo y) {
        // TODO: Handle null values of x, y, x.getBar() and y.getBar(),
        // and consider non-ordinal orderings.
        return x.getBar().compareTo(y.getBar());
    }
}

Answer 2

By using a custom Comparator?

import java.util.*;

class Bean {
    public final String name;
    public final int value;

    public Bean(final String name, final int value) {
        this.name = name;
        this.value = value;
    }

    @Override
    public String toString() {
        return name + " = " + value;
    }
}

public class SortByProp {
    private static List<Bean> initBeans() {
        return new ArrayList<Bean>(Arrays.asList(
            new Bean("C", 1),
            new Bean("B", 2),
            new Bean("A", 3)
        ));
    }

    private static void sortBeans(List<Bean> beans) {
        Collections.sort(beans, new Comparator<Bean>() {
            public int compare(Bean lhs, Bean rhs){ 
                return lhs.name.compareTo(rhs.name);
            }
        });
    }


    public static void main(String[] args) {
        List<Bean> beans = initBeans();
        sortBeans(beans);
        System.out.println(beans);
    }
}

Answer 3

Using Guava, it's just

Collections.sort(list, Ordering.natural().compose(Functions.toStringFunction()));