java - Which part of the equals() general contract does my equals() not satisfy -

i'm new java , trying head around understanding @override of equals() , hashcode() methods.
know equals method correct needs be:

1. reflexive: a.equals(a)
2. symmetric: a.equals(b) then b.equals(a)
3. transitive: a.equals(b) && b.equals(c) then a.equals(c)
4. not null: ! a.equals(null)

i struggling pinpoint of above properties , not satisfying when writing overide of equals method.

i aware eclipse can generate these me, haven't yet gotten concept fully, writing out helps me learn.

i have written out think correct way it, when check eclipse generated version seem 'missing' aspects.

example:

public class people {      private name first; //invariants --> !null, !=last     private name last;  // !null, !=first     private int age;    // !null, ! <=0     ... } 

what wrote:

public boolean equals(object obj){     if (obj == null){         return false;     }     if (!(obj instanceof people)){          return false;     }     people other = (people) obj;     if (this.age != other.age){         return false;     }     if (! this.first.equals(other.first)){         return false;     }     if (! this.last.equals(other.last)){         return false;     }     return true; } 

vs eclipse generated

public boolean equals(object obj) {     if (this == obj)         return true;     if (obj == null)         return false;     if (getclass() != obj.getclass())         return false;      people other = (people) obj;     if (first == null) {         if (other.first != null)             return false;     } else if (!first.equals(other.first))         return false;      if (age != other.age)         return false;     if (last == null) {         if (other.last != null)             return false;     } else if (!last.equals(other.last))         return false;      return true; } 

i missing:

• if (this == obj)     return true; 

• if (getclass() != obj.getclass())     return false; 

• and each variable:

  if (first == null) {     if (other.first != null)         return false; } else if (!first.equals(other.first))     return false; 

i'm not sure getclass() , implmentation incorrect?

first piece of code:

if (this == obj)     return true; 

this improves performance in case compare object reference against itself. example: a.equals(a);.

second piece of code:

if (getclass() != obj.getclass())     return false; 

this compares if class of reference being compared same class of this. difference between using approach , instanceof it's more restrictive when comparing against sub class. example:

public class foo { } public class bar extends foo { }  //... foo foo = new bar(); system.out.println(foo instanceof bar); //prints true system.out.println(foo instanceof foo); //prints true foo foo2 = new foo(); system.out.println(foo.getclass() == foo2.getclass()); //prints false 

which 1 should choose? there's no or bad approach, depend on desired design.

third piece of code:

if (first == null) {         if (other.first != null)             return false;     } else if (!first.equals(other.first))         return false; //for each variable. 

this null check each object reference field in class. note if this.first null doing this.first.equals(...) throw nullpointerexception.