Java hashcode() with example

Java hashCode():

Java Object hashCode() is a native method and returns the integer hash code value of the object. The general contract of hashCode() method is:

• Multiple invocations of hashCode() should return the same integer value, unless the object property is modified that is being used in the equals() method.
• An object hash code value can change in multiple executions of the same application.
• If two objects are equal according to equals() method, then their hash code must be same. For more info Java Online Classes
• If two objects are unequal according to equals() method, their hash code are not required to be different. Their hash code value may or may-not be equal.

Usage of hashCode() in Data Structures:

The simplest operations on collections can be inefficient in certain situations.

For example, this triggers a linear search which is highly ineffective for lists of huge sizes:

List<String> words = Arrays.asList("Welcome", "to", "Java");
if (words.contains("Java")) {
    System.out.println("Java is in the list");
}

Java provides a number of data structures for dealing with this issue specifically – for example, several Map interface implementations are hash tables.

When using a hash table, these collections calculate the hash value for a given key using the hashCode() method and use this value internally to store the data – so that access operations are much more efficient. For more additional info Java Online Training

Understanding How hashCode() Works

Simply put, hashCode() returns an integer value, generated by a hashing algorithm.

Objects that are equal (according to their equals()) must return the same hash code. It’s not required for different objects to return different hash codes.

The general contract of hashCode() states:

• Whenever it is invoked on the same object more than once during an execution of a Java application, hashCode() must consistently return the same value, provided no information used in equals comparisons on the object is modified. This value needs not remain consistent from one execution of an application to another execution of the same application
• If two objects are equal according to the equals(Object) method, then calling the hashCode() method on each of the two objects must produce the same value
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, developers should be aware that producing distinct integer results for unequal objects improves the performance of hash tables

public int hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

The general contract of hashCode is:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

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