When implementing the equals method we should make sure that it's:

• reflexive (x <-> x)
  
  
• symmetric (x <-> y)
  
  
• transitive (x <-> y && y <-> z => x <-> z)
  
  
• consistent (either always true or always false)

The call to equals method with null value as argument should return false.

The equals method is defined in the java.lang.Object class:

public boolean equals(Object obj) {
    return (this == obj);
}

The java.util.Objects class provides utility methods to check if two objects are equals or deeply equals (in case of arrays):

public static boolean equals(Object a, Object b) {
    return (a == b) || (a != null && a.equals(b));
}

public static boolean deepEquals(Object a, Object b) {
    if (a == b)
        return true;
    else if (a == null || b == null)
        return false;
    else
        return Arrays.deepEquals0(a, b);
}

The java.util.Arrays class provides utility methods to check if two arrays are equals (including specific overloaded methods for each primitive type). It also provides a utility method to check if two arrays are deeply equals (array of arrays).

The following is a sample code that shows how to implement the equals method.
The code provides an implementation of the equals method for a superclass "P" and subclass "C".
The code also has a custom method (same) that provides a check if two instances of the same class are equal.

• Superclass "P":
  

  class P {
      Integer id = 1;
  
      public Integer getId() {
          return id;
      }
  
      @Override
      public boolean equals(Object obj) {
          if (this == obj)
              return true;
  
          if (obj == null)
              return false;
  
          if (getClass() != obj.getClass())
              return false;
  
          P p = (P) obj;
  
          return new EqualsBuilder().append(this.getId(), p.getId()).build();
      }
  
      public boolean same(Object obj) {
          if (this == obj)
              return true;
  
          if (obj == null)
              return false;
  
          if (!(obj instanceof P))
              return false;
  
          P p = (P) obj;
  
          return new EqualsBuilder().append(this.getId(), p.getId()).build();
      }
  }

• Subclass "C":
  

  class C extends P {
      String code = "default";
  
      public String getCode() {
          return code;
      }
  
      @Override
      public boolean equals(Object obj) {
          if (!super.equals(obj))
              return false;
  
          C c = (C) obj;
  
          return new EqualsBuilder().append(this.getCode(), c.getCode()).build();
      }
  
      @Override
      public boolean same(Object obj) {
          if (!super.same(obj))
              return false;
  
          if (!(obj instanceof C))
              return false;
  
          C c = (C) obj;
  
          return new EqualsBuilder().append(this.getCode(), c.getCode()).build();
      }
  }