Java equals hashCode Contract
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The document discusses the contract between equals() and hashCode() methods in Java classes. It states that 2 equal objects must have the same hashCode, but the reverse may not be true. It then provides an example class Banana that implements equals() but not hashCode(), causing issues when used as a key in a HashMap. Finally, it discusses how to implement hashCode() to avoid collisions and improve performance.
Java equals hashCode Contract
- 1. Java Equals HashCode Contract Sujit Kumar Zenolocity LLC © 2013-2023
- 2. The Contract • 2 equal 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.
- 3. Default implementations in Object 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.
- 4. 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); } }
- 5. equals() provided without hashCode() 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")));
- 6. Add hashCode() method to Class public int hashCode() { return 29 * this.color.hashCode(); } // Returns the green Banana now ! System.out.println(m.get(new Banana("green")));
- 7. Hash Collisions • put method – 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.
- 8. Techniques to make a 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.