In this example, we will discuss the hashCode method in Java. This is one of the methods that all objects have since it is defined in the Object class, which is the base for all Java classes.
This method’s functionality is to digest the properties of an object into a single, 32-bit integer value. The hashCode() method should return a unique value for every object, and two objects must not have the same integer hash value, unless they are equal as the equals() method says.
The programmer is not obligated to override this method, since the base method defined in the Object class works perfectly (i.e. returns different values for different objects).
1.What is hashCode and why we use it?
hashCode is an integer value generated by an algorithm based on hashing. The hashcode of equal objects will be the same. Different objects might have a different value. This is typically equal to the integer representation of the internal address of the object.
2. Example of hashCode() usage in Java
With the following example, I will show that the hash value returned by hashCode() is the same for objects that equals() returns true; and it differs when the equals() returns false. Create a class called BasicHashCodeExample with the following source code:
package com.javacodegeeks.examples;
public class BasicHashCodeExample {
public static void main(String[] args) {
String name1 = "John";
String name2 = "Jane";
String name3 = "John";
System.out.println("name1.equals(name2): "+name1.equals(name2));
System.out.println("name1.hashCode() = "+name1.hashCode());
System.out.println("name2.hashCode() = "+name2.hashCode());
System.out.println("name1.equals(name3): "+name1.equals(name3));
System.out.println("name1.hashCode() = "+name1.hashCode());
System.out.println("name3.hashCode() = "+name3.hashCode());
}
}When executing this, the output will be:
name1.equals(name2): false name1.hashCode() = 2314539 name2.hashCode() = 2301262 name1.equals(name3): true name1.hashCode() = 2314539 name3.hashCode() = 2314539 You can notice here thatname1andname3have the same hash value, whilename1andname2don't!
3. Java hashCode() in action
Normally, hash values are used in hash tables or other data structures that use hash values for indexing individual objects in the collection. Let’s take a collection example, like a list of people names. You could store them in a simple String array, or in an ArrayList, or better yet in a TreeSet. To search a name in the list, you would use a searching algorithm, like linear search, or binary search in the case of the TreeSet, but they would take too much time if the list of names is big. In terms of big-O, linear search and binary search have complexities of O(n) and O(log(n)), respectively. But, if you decide to store the list of people names in a hash table for example, the complexity of searching would be O(1)! And that’s really faster!
To show how to search in a HashMap, an implementation of a hash table based on the Map interface, check the following example:
package com.javacodegeeks.examples;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
public class HashMapExample {
public static void main(String[] args) {
Map hashTable = new HashMap();
Random rnd = new Random();
String baseName = "Base_Name";
//loading values into the hash table
for (int i=0;i<100;i++) {
String tmpName = baseName+i;
int rndAge = rnd.nextInt(70);
Person p = new Person(tmpName, rndAge);
hashTable.put(tmpName.hashCode(), p);
}
//now let's find the person with name Base_Name75
String name = "Base_Name75";
Person person = hashTable.get(name.hashCode());
System.out.println(person.getName());
}
static class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return this.name;
}
public int getAge() {
return this.age;
}
}
}
Initially, we create a HashMap instance to store instances of a Person class using an integer as key. On the for loop, we create an instance of Person class with a random age and a name that is “Base_Name” plus an index. Then we store this instance into the hash table, using the hash value of the name as key.
Next, we use the hash value of the name we want to get the person instance from the hash table.
The output will be like this:
Base_Name75
4. HashCode() Implementation
Hashcode method can be implemented by overriding hashCode method. This is done when the class overrides equals. These two methods need to have the same set of fields. Hashcode of two equal objects will be the same. Hashcode is used for caching and lazy initialization when the object can be modified or immutable.
Let us look at the following example:
HashCode Implementation
public class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return this.name;
}
public int getAge() {
return this.age;
}
@Override
public boolean equals(Object object) {
if (this == object) {
return true;
}
if (!(object instanceof Person)) {
return false;
}
Person person = (Person)object;
return name.equals(person.name)
&& (age == person.age);
}
@Override
public int hashCode() {
int result = age+1;
result = 31 * result + name == null ? 0 : name.hashCode();
return result;
}
public static void main(String[] args)
{
Person person = new Person("Jack",34);
System.out.println(person.hashCode());
Person person2 = new Person("George",44);
System.out.println(person2.hashCode());
System.out.println(person.equals(person2));
}
}
The output for the code executed above is shown below:
Output
2300927 2129364991 false
5. More about hashCode()
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() 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.
- As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)
6. Handling Hash Collisions
Hash collisions might happen as unequal objects can have equal hashcode. Hash codes can point to one bucket but can have different hash keys. A separate chaining method is used for avoiding hash collisions in HashMap. There are different methods to avoid hash collisions. Hashtable is an array of list of objects in a linked list. HashMap implementation changed in Java 8 and the linked list was changed to treemap.
7.Importance of hashcode method
The hashcode method helps in presenting the contents of the object in a form to differentiate for identification. The Hash table has the keys which are generated from a hash function. Typically the internal memory address is the key value used in the hashtable. The hash function needs to generate a unique value for every object. One way to avoid collisions is to switch from the primary hash function to the secondary for differentiation.
8. Download the Source Code
You can download the full source code of this example here : Java hashCode method Example
Last updated on Feb. 09th, 2021.
