Java - equals Method

Java | equals Method

equals Method

equals Method
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); }
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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)); }
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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); }
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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(); } }
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  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(); } }
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  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();
  }
  }