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

• reflexive (x <-> x): x.equals(x) must return true.
  
  
• symmetric (x <-> y): x.equals(y) returns true if and only if y.equals(x) returns true.
  
  
• transitive (equals()x <-> y && y <-> z => x <-> z): if x.equals(y) and y.equals(z) return true, then x.equals(z) must return true.
  
  
• consistent (either always true or always false): multiple invocations must consistently return the same result

Calling the equals() method with null as the argument should return false.

Important: When overriding equals(), you must also override hashCode() to maintain the general contract that equal objects must have equal hash codes.

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 (nested 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".

• Superclass "P":

  import java.util.Objects;
  
  class P {
      private Integer id = 1;
  
      public Integer getId() {
          return id;
      }
  
      @Override
      public boolean equals(Object obj) {
          if (this == obj) {
              return true;
          }
  
          if (!(obj instanceof P)) {
              return false;
          }
  
          P p = (P) obj;
  
          return Objects.equals(this.getId(), p.getId());
      }
  
      @Override
      public int hashCode() {
          return Objects.hash(id);
      }
  }

• Subclass "C":

  class C extends P {
      private String code = "default";
  
      public String getCode() {
          return code;
      }
  
      @Override
      public boolean equals(Object obj) {
          if (this == obj) {
              return true;
          }
  
          if (obj == null) {
              return false;
          }
  
          if (getClass() != obj.getClass()) {
              return false;
          }
  
          C c = (C) obj;
  
          if (!super.equals(obj)) {
              return false;
          }
  
          return Objects.equals(this.getCode(), c.getCode());
      }
  
      @Override
      public int hashCode() {
          return Objects.hash(super.hashCode(), code);
      }
  }

Notes:

• Use !(obj instanceof ClassName) to allow subclass equality.
• Use getClass() != obj.getClass() for strict type checking (no subclass equality).

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".

• Superclass "P":

  import org.apache.commons.lang3.builder.EqualsBuilder;
  
  class P {
      Integer id = 1;
  
      public Integer getId() {
          return id;
      }
  
      @Override
      public boolean equals(Object obj) {
          if (this == obj) {
              return true;
          }
  
          if (!(obj instanceof P)) {
              return false;
          }
  
          P p = (P) obj;
  
          return new EqualsBuilder().append(this.getId(), p.getId()).isEquals();
      }
  
      @Override
      public int hashCode() {
          return Objects.hash(id);
      }
  }

• 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()).isEquals();
      }
  
      @Override
      public int hashCode() {
          return Objects.hash(super.hashCode(), code);
      }
  }