Java equals hashCode Contract

Java equals hashCode Contract

• 1.
  Java Equals HashCodeContract Sujit Kumar Zenolocity LLC © 2013-2023
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  The Contract • 2equal objects should have the same hashCode. • The reverse may or may not be true => 2 objects with the same hashCode may or may not be equal.
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  Default implementations inObject class • The default implementation of hashCode() in Object class returns distinct integers for different objects. • The default implementation of equals() in Object class is reference equality, and therefore two instances are the same if and only if they are the same instance.
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  Class with equals()but no hashCode() public class Banana { private String color; public Banana (String color) { this.color = color; } public boolean equals(Object obj) { if (!(obj instanceof Banana)) return false; if (obj == this) return true; return this.color.equals(((Banana) obj).color); } }
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  equals() provided withouthashCode() Banana a1 = new Banana("green"); Banana a2 = new Banana(“yellow”); //hashMap stores Banana type and its quantity HashMap<Banana, Integer> m = new HashMap<Banana, Integer>(); m.put(a1, 10); m.put(a2, 20); // Returns null even though the map contains a green Banana System.out.println(m.get(new Banana("green")));
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  Add hashCode() methodto Class public int hashCode() { return 29 * this.color.hashCode(); } // Returns the green Banana now ! System.out.println(m.get(new Banana("green")));
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  Hash Collisions • putmethod – calculates the hashCode on the key and places the key-value pair in a bucket for that hashCode value. • The next put may result in the same hashCode if the hashCode method is not good enough to produce a unique value. This results in a hash collision. • Hash Collision results in multiple entries (k-v pairs) put in the same bucket in the form of a list. • When you retrieve using the get method, calculate the hashCode, go to the bucket & retrieve from the list in the bucket by invoking the equals method for each key in the list. • Hash Collisions degrade performance.
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  Techniques to makea hashCode unique • Invoke the hashCode on each instance variable, multiply with a prime number like 29 and keep adding the results. • Return the final result.