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POHash code implementation in java
 

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    <pre><code>package MavenWeb.MavenWeb; import java.util.HashMap; import java.util.Map; public class StringEquality { public static void main(String[] args) { Person p1 = new Person("Naveen", 22, 1000); Person p2 = new Person("Naveen", 21, 2000); Person p3 = new Person("Naveen", 23, 3000); if(p1.equals(p2)){ System.out.println("P1 and p2 :" + p1.equals(p2)); } else{ System.out.println("P1 and p2 :" + p1.equals(p2)); } if(p1.equals(p3)){ System.out.println("P1 and p3 :" + p1.equals(p3)); } Map&lt;Person, Object&gt; map = new HashMap&lt;Person, Object&gt;(); map.put(p1, p1); map.put(p2, p2); System.out.println(map.get(new Person("Naveen", 21, 2000))); } } </code></pre> <p>...</p> <pre><code>class Person{ public Person(String name, int id, float salary){ this.name = name; this.id = id; this.salary = salary; } public Float getSalary() { return salary; } public void setSalary(Float salary) { this.salary = salary; } String name; Float salary; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getId() { return id; } public void setId(int id) { this.id = id; } int id; @Override public boolean equals(Object obj) { if(obj == null){ return false; } if(this == obj){ return true; } if(obj instanceof Person){ Person person = (Person)obj; if((person.getName().equals(name)) &amp;&amp; person.getId() == id &amp;&amp; person.getSalary() == salary){ return true; } } return false; } @Override public int hashCode() { int hashCode = 1; hashCode = 31 * hashCode + name.hashCode(); hashCode = 31 * hashCode + id; hashCode = 31 * hashCode + salary.intValue(); return hashCode; } @Override public String toString() { return "Name :" + name + ", id : " + id; } } </code></pre> <p>I have write the following program. Little bit confused regarding hashcode implementation,</p> <ol> <li>Why we use 31 in implementation</li> <li>According to equals method logic this program should return p2(Naveen, 21) instance but it is returning null. why?</li> </ol>
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    1. COFor the `31` part see http://stackoverflow.com/q/299304/72478
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