Background

There are two methods to implement sorting for Custom Class in Java.

• Comparable< T >

  • It is an interface

  • < T > - the type of objects that this object may be compared to

  • It has only method:

    int compareTo(T o) {
      "do something"
    }
    

  • Java doc reference

• Comparator< T >

  • It is an interface

  • < T > - the type of objects that may be compared by this comparator

  • We usually use one method

    int compare(T o1, T o2) {
      "do something"
    }
    

  • Java doc reference

Why Use It?

• 我们需要对自定义的类排序时,但是Java内部没有实现这样的接口

• 比如我们自定义一个类叫Dog, 要求根据dog的年龄进行排序

• public class Dog {
      String name;
      int age;
        
      public Dog(String name, int age) {
          this.name = name;
          this.age = age;
      }
        
      @Override
      public String toString() {
          return "Dog{" +
                  "name='" + name + '\'' +
                  ", age=" + age +
                  '}';
      }
  }
  

• Output 会报错:

  public class Main {
      public static void main(String[] args) {
          Dog[] dogs = {new Dog("David", 3), new Dog("Bill", 1)};
          Arrays.sort(dogs);
          System.out.println(Arrays.toString(dogs));
      }
  }
  /*
  Exception in thread "main" java.lang.ClassCastException: com.algoxi.my_error.comparable.Dog cannot be cast to java.lang.Comparable
    at java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:320)
    at java.util.ComparableTimSort.sort(ComparableTimSort.java:188)
    at java.util.Arrays.sort(Arrays.java:1246)
    at com.algoxi.Main.main(Main.java:12)
  */
  

How To Use It?

1. 用comparable

   • Dog类直接实现 comparable
   • Dog类里面也要实现compareTo的方法

   public class Dog implements Comparable<Dog> {
       String name;
       int age;
      
       public Dog(String name, int age) {
           this.name = name;
           this.age = age;
       }
      
       @Override
       public String toString() {
           return "Dog{" +
                   "name='" + name + '\'' +
                   ", age=" + age +
                   '}';
       }
      
       @Override
       public int compareTo(Dog o) {
           if (this.age == o.age) {
               return 0;
           }
           return this.age < o.age ? -1 : 1;
       }
   }
   

   • Output:

   public class Main {
       public static void main(String[] args) {
           Dog[] dogs = {new Dog("David", 3), new Dog("Bill", 1)};
           Arrays.sort(dogs);
           System.out.println(Arrays.toString(dogs));
       }
   }
      
   /*
   [Dog{name='Bill', age=1}, Dog{name='David', age=3}]
   */
   

2. 用Comparator

   • 原来的Dog保持不变

   public class Dog {
       String name;
       int age;
      
       public Dog(String name, int age) {
           this.name = name;
           this.age = age;
       }
      
       @Override
       public String toString() {
           return "Dog{" +
                   "name='" + name + '\'' +
                   ", age=" + age +
                   '}';
       }
   }
   

   • 创建一个DogComparator类

   import java.util.Comparator;
      
   public class DogComparator implements Comparator<Dog> {
      
       @Override
       public int compare(Dog o1, Dog o2) {
           if (o1.age == o2.age) {
               return 0;
           }
           return o1.age < o2.age ? -1 : 1;
       }
   }
   

   • Output:
   • 要对comparator进行new的操作

   public class Main {
       public static void main(String[] args) {
           Dog[] dogs = {new Dog("David", 3), new Dog("Bill", 1)};
           Arrays.sort(dogs, new DogComparator());
           System.out.println(Arrays.toString(dogs));
       }
   }
   /*
   [Dog{name='Bill', age=1}, Dog{name='David', age=3}]
   */
   

Anonymous Inner Class

• Anonymous Example

• PriorityQueue<Dog> minHeap = new PriorityQueue<>(new Comparator<Dog>() {
    @Override
    public int compare(Dog o1, Dog o2) {
      if (o1.age == o2.age) {
        return 0;
      }
      return o1.age < o2.age ? -1 : 1;
    }
  });
  

• Normal Example

• PriorityQueue<Dog> minHeap = new PriorityQueue<>(new DogComparator());
  

• public class DogComparator implements Comparator<Dog> {
      @Override
      public int compare(Dog o1, Dog o2) {
          if (o1.age == o2.age) {
              return 0;
          }
          return o1.age < o2.age ? -1 : 1;
      }
  }
  

• Another Tips:

  • We can use Collections.reverseOrder() in PriorityQueue’s constructor to implement maxHeap.

“==” and equals

• For Object

  • check value: o1.equals(o2)

  • check address: o1 == o2

  • public static void main(String[] args) {
      String s1 = new String("abc");
      String s2 = new String("abc");
      System.out.println(s1 == s2);
      // false
      System.out.println(s1.equals(s2));
      //true
    }
    

    • because constant pool, they are created in same address

  • public static void main(String[] args) {
      String s1 = "abc";
      String s2 = "abc";
      System.out.println(s1 == s2);
      // true
      System.out.println(s1.equals(s2));
      //true
    }
    

• For Primitive Type

  • check value: i1 == i2

end.