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 show how to implement the equals method. The code provide an implementation of the equals method for a superclass "P" and subclass "C". The code also an implementation of a method (called here same) that check if a superclass and possible a subclass are equals.

• 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(); } }