Java Overrides equals(), hashCode(), compareTo()

Wednesday. October 24, 2018 - 2 mins

Two rule when dealing with object identification in Java

1. Override hashCode when you override equals
2. Make conpareTo consistent with euqals

Default equals is based on memory location of two objects, so we need to customize it to meet our need.

1. equals must conform to rules:
   • x.equals(x)
   • x.equals(y) then must y.equals(x)
   • x.equals(y) and y.equals(z) then x.equals(z)
   • mutiple invocations of x.equals(y) return same
   • if x is not null, x.equals(null) return false
2. Five-step for overridding equals()

    public class SampleEquals {
        private int val1;
        private int val2;
        ...
   
        public boolean equals (Object o) {
            //step 1: check if o is current object (== checks addresses in memory)
            if（this == o）{
                return true;
            }
   
            // step 2: instance check
            if (!(o instanceof SampleEquals)) {
                return false;
            }
   
            // step 3: cast
            SampleEquals other = (SampleEquals) o;
   
            // step 4: customized check
            return other.val1 == this.val1 && other.val2 == this.val2;
   
            // step 5: checks all the rules for equals() above
        }
    }
   

hashCode() method of objects is used when you insert them into a HashTable, HashMap or HashSet.

• When these data structure need to compare two objects, first they compare their hash codes, if they have same hash codes, then compare use equals.
• Default hashCode() hashes addresses of object.
• How HashTable get() works with hashCode()
  • hashCode() gets key’s hash code
  • retrieve the list of pairs stored in the bucket
  • scan through each entry until one key equals() current key
• Two ways of generate customized hash code
  • concatenate all fields into a string, and return hashed value
  • add each field’s hash code together

          public class SampleHashCode {
              private int val1;
              private int val2;
              ...
              public int hashCode1() {
                  int hash1 = Object.hashCode("" + val1 + val2);
                  return hash1;
              }
    
              public int hashCode2() {
                  int hash2 = Object.hashCode(val1) + Object.hashCode(val2);
                  return hash2;
              }
          }
    

compareTo() is used to implement natural sorting on Java classes

• rules of compareTo()
  • returns negative integer means less than
  • returns 0 means equal
  • returns positive integer means greater than
• costomize compareTo() by specifying how we want to order

    public class SampleCompareTo {
        private final Strint name;
        private final int id;
        ...
        public int compareTo(SampleCompareTo o) {
            int res = this.name.compareTo(o.name);
            if (res == 0) {
                res = Integer.valueOf(this.id).compareTo(o.id); 
            }
            return res;
        }
    }