Inner Classes Review

Recall inner classes, which were classes within a class or method.

Each of the classes below implements Comparator<Planet> and its compare(Planet, Planet) method.

• Member Class - at field or method level.

  public class Outer {
    class OrbitComparator implements Comparator<Planet> {
      @Override
      public int compare(Planet a, Planet b) {
        return a.getOrbit() > b.getOrbit() ? 1 : -1;
      }
    }
  }
  

• Local Class - within a method.

  public void aMethod() {
    class OrbitComparator implements Comparator<Planet> {
      @Override
      public int compare(Planet a, Planet b) {
        return a.getOrbit() > b.getOrbit() ? 1 : -1;
      }
    };
  }
  

• Anonymous Class - definition and instantiation together.

  Comparator<Planet> comp = new Comparator<Planet>() {
    @Override
    public int compare(Planet o1, Planet o2) {
      return a.getOrbit() > b.getOrbit() ? 1 : -1;
    }
  };
  

Passing Code with Inner Classes

Before Java 8, if you wanted to pass functionality to a method, you would have to wrap it in an inner class.

class OrbitComparator implements Comparator<Planet> {
  public int compare(Planet a, Planet b) {
    return a.getOrbit() > b.getOrbit() ? 1 : -1;
  }
}

// Use an instance of the inner class
Collections.sort(planets, new OrbitComparator());

• The Collections.sort() method, for example, takes an instance of a Comparator as an argument to describe the comparison logic.

• The syntax is clumsy, especially when you use anonymous inner classes.

package com.example.lambdas.examples;

import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class PrintSortedPlanets {
  public static void main(String[] args) {
    List<Planet> planets = PlanetUtilities.getPlanets();

    // Passing anonymous instance
    Collections.sort(planets, new Comparator<Planet>() {
      public int compare(Planet a, Planet b) {
        return a.getOrbit() > b.getOrbit() ? 1 : -1;
      }
    });
    for (Planet planet : planets) {
      System.out.println(planet);
    }
  }
}

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