In this post, we will see how to override equals and hashCode method in Java.
The general contract for overriding equals is proposed in item 8 of Josh Bloch’s Effective Java. Ideally equals method should satisfy the following conditions. It should be:
• Reflexive: A non-null object should be equal to itself i.e. x.equals(x) == true.
• Symmetric: If the first object is equal to a second object, then the second object should be equal to the first object. i.e. x.equals(y) == true if and only if y.equals(x) == true.
• Transitive: If the first object is equal to a second object and the second object is equal to a third object, then the first object should be equal to the third object. i.e. x.equals(y) == true and y.equals(z) == true, then x.equals(z) == true.
• Consistent: Multiple calls to equals (x.equals(y) or y.equals(x)) must return the same value at any point of time unless either x or y is modified.
Item 9 in Josh Bloch’s Effective Java always asks us to override hashCode() method if the class overrides equals(). Otherwise, the class object will not behave properly on hash-based collections like HashMap, HashSet, and Hashtable (see why?). Also, keep in mind that
1. If equals() returns true for two objects, then hashCode() method should also return the same value. i.e. if a.equals(b) == true and b.equals(a) == true, then a.hashCode() == b.hashCode().
2. If equals() returns false for two objects, then hashCode() method should return different values. i.e. if a.equals(b) == false or b.equals(a) == false, then a.hashCode() != b.hashCode().
Now let’s discuss various ways to override equals and hashCode method in Java.
1. Java 6 and less
Here’s recommended process to compute hashCode manually before Java 7:
1. Initialize hashcode by a non-zero value, ideally a prime number, say 17.
2. For all relevant fields from the object (which are used in equals method for equality), compute the hashcode c by following below rules and append c into the result using prime multiplier 37 as
result = 31 * result + c.
• For primitve fields, compute (int)f for byte, char or short, (f ? 1 : 0) for a boolean,
(int)(f ^ (f >>> 32)) for a long (>>> is unsigned right shift operator) and Float.floatToIntBits(f) and Double.doubleToLongBits(f), for float and double respectively.
• Recursively invoke hashCode on field that is an object reference.
• If the field is an array, invoke Arrays.hashCode() method or compute hashCode for each element of the array by applying above rules.
• If the value of the field is null, return 0.
We should always choose a prime number in hashCode() method that results in a good hash function which produces unequal hash codes for unequal objects and uniformly distribute all possible hash values across the hash table to avoid collision.
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class Person { private String name; private int age; private String gender; // Constructor Person(String name, int age, String gender) { this.name = name; this.age = age; this.gender = gender; } @Override public boolean equals(Object ob) { if (ob == this) return true; if (ob == null || ob.getClass() != getClass()) { return false; } Person p = (Person) ob; return p.name.equals(name) && p.age == age && p.gender.equals(gender); } @Override public int hashCode() { int result = 17; // 31 is used as it is a prime and also offers better performance as: // 31 * result == (result << 5) - result result = 31 * result + (name == null ? 0 : name.hashCode()); result = 31 * result + age; result = 31 * result + (gender == null ? 0 : gender.hashCode()); return result; } // Program to override equals and hashCode in Java 6 and less public static void main (String[] args) { Person p1 = new Person("John", 20, "Male"); Person p2 = new Person("John", 20, "Male"); Person p3 = new Person("Carol", 16, "Female"); System.out.println(p1.equals(p2)); // true System.out.println(p1.equals(p3)); // false } } |
Output:
true
false
Another option is to place all the input values into an array, and calculate hash of that array using Arrays.hashCode(Object[]) method which was introduced in Java 5 as shown below:
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@Override public int hashCode() { return Arrays.hashCode(new Object[] { name, age, gender }); } |
2. Java 7 and above
Java 7 introduced Objects class that provides static utility methods for computing the hash code of an object and comparing two objects.
This should be preferred over implementing our own hash function. For objects containing multiple fields, we can use Objects.hash(…) method that generates a hash code for a sequence of input values which internally uses Arrays.hashCode(Object[]) method. For Object equality, we should use Objects.equals(...) method or Objects.deepEquals(...) method to check for deep equality.
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import java.util.Arrays; import java.util.Objects; class Person { private String name; private int age; private String gender; // Constructor Person(String name, int age, String gender) { this.name = name; this.age = age; this.gender = gender; } @Override public boolean equals(Object ob) { if (ob == this) return true; if (ob == null || ob.getClass() != getClass()) { return false; } Person p = (Person) ob; return Objects.equals(name, p.name) && p.age == age && Objects.equals(gender, p.gender); } @Override public int hashCode() { return Objects.hash(name, age, gender); // This is equivalent to // return Arrays.hashCode(new Object[] { name, age, gender }); } // Program to override equals and hashCode in Java 7 and above public static void main (String[] args) { Person p1 = new Person("John", 20, "Male"); Person p2 = new Person("John", 20, "Male"); Person p3 = new Person("Carol", 16, "Female"); System.out.println(p1.equals(p2)); // true System.out.println(p1.equals(p3)); // false } } |
Output:
true
false
3. Guava Library
Guava Library provides Objects class that has helper functions Objects.hashCode(...) and Objects.equal(...) that works in similar way as Java 7 Objects class. It can be used before Java 7.
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import com.google.common.base.Objects; class Person { private String name; private int age; private String gender; // Constructor Person(String name, int age, String gender) { this.name = name; this.age = age; this.gender = gender; } @Override public boolean equals(Object ob) { if (ob == this) return true; if (ob == null || ob.getClass() != getClass()) { return false; } Person p = (Person) ob; return Objects.equal(name, p.name) && Objects.equal(p.age, age) && Objects.equal(gender, p.gender); } @Override public int hashCode() { return Objects.hashCode(name, age, gender); } // Program to override equals and hashCode in Java using Guava public static void main (String[] args) { Person p1 = new Person("John", 20, "Male"); Person p2 = new Person("John", 20, "Male"); Person p3 = new Person("Carol", 16, "Female"); System.out.println(p1.equals(p2)); // true System.out.println(p1.equals(p3)); // false } } |
Output:
true
false
4. Apache Commons Lang
Apache Commons Lang library also provides useful helper classes EqualsBuilder and HashCodeBuilder, which strictly follows the rules laid out in Josh Bloch’s Effective Java.
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import org.apache.commons.lang.builder.EqualsBuilder; import org.apache.commons.lang.builder.HashCodeBuilder; class Person { private String name; private int age; private String gender; // Constructor Person(String name, int age, String gender) { this.name = name; this.age = age; this.gender = gender; } @Override public boolean equals(Object ob) { if (ob == this) return true; if (ob == null || ob.getClass() != getClass()) { return false; } if (ob == this) return true; if (!(ob instanceof Person)) { return false; } Person p = (Person) ob; return new EqualsBuilder() .append(age, p.age) .append(name, p.name) .append(gender, p.gender) .isEquals(); } @Override public int hashCode() { return new HashCodeBuilder(17, 37) .append(name) .append(age) .append(gender) .toHashCode(); // or use reflection // return HashCodeBuilder.reflectionHashCode(this); } // Program to override equals and hashCode in Java using Apache Commons public static void main (String[] args) { Person p1 = new Person("John", 20, "Male"); Person p2 = new Person("John", 20, "Male"); Person p3 = new Person("Carol", 16, "Female"); System.out.println(p1.equals(p2)); // true System.out.println(p1.equals(p3)); // false } } |
Output:
true
false
5. Using a Java IDE
It tedious and error-prone to implement equals() and hashCode() by hand, especially for large POJO’s. Most Java IDEs provide option to auto-generate equal() and hashCode() methods where we can also easily skip fields we don’t want to consider them for equality and hash code calculation.
For instance, below code is auto-generated by eclipse using Source > Generate equal() and hashCode():
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@Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + age; result = prime * result + ((name == null) ? 0 : name.hashCode()); result = prime * result + ((gender == null) ? 0 : gender.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Person other = (Person) obj; if (age != other.age) return false; if (name == null) { if (other.name != null) return false; } else if (!name.equals(other.name)) return false; if (gender == null) { if (other.gender != null) return false; } else if (!gender.equals(other.gender)) return false; return true; } |
Similarly, below code is auto-generated by IntelliJ IDEA using Code > Generate > equal() and hashCode():
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@Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; Person person = (Person) o; if (age != person.age) return false; if (name != null ? !name.equals(person.name) : person.name != null) return false; if (gender != null ? !gender.equals(person.gender) : person.gender != null) return false; return true; } @Override public int hashCode() { int result = name != null ? name.hashCode() : 0; result = 31 * result + age; result = 31 * result + (gender != null ? gender.hashCode() : 0); return result; } |
Please note that these generated methods are not automatically updated when we change any field in the POJO. So we might need to regenerate the methods.
References: Josh Bloch’s Effective Java
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