Programming Tutorials

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Of Object, equals (), == and hashCode ()

By: Girish Manwani in Java Tutorials on 2023年05月04日 [フレーム]

In Java, the Object class is the root class of all classes, which means that every class in Java is derived from the Object class. The Object class provides several methods that can be overridden in the derived classes, including equals(), ==, and hashCode().

equals(): This method is used to compare two objects for equality. The default implementation of this method in the Object class compares the memory addresses of the objects. In order to compare the contents of the objects, this method must be overridden in the derived class. The equals() method takes an Object parameter, which represents the object to be compared, and returns a boolean value that indicates whether the objects are equal or not.

For example, let's say we have a Person class that has a name and an age field. We can override the equals() method in the Person class to compare two Person objects based on their name and age fields, like so:

public class Person {
 private String name;
 private int age;
 // constructor, getters, and setters
 @Override
 public boolean equals(Object obj) {
 if (obj == this) {
 return true;
 }
 if (!(obj instanceof Person)) {
 return false;
 }
 Person other = (Person) obj;
 return this.name.equals(other.getName()) && this.age == other.getAge();
 }
}

==: This operator is used to compare the memory addresses of two objects. It checks if two object references refer to the same object. If the two object references refer to the same object, the result is true. Otherwise, the result is false.

For example:

String s1 = "hello";
String s2 = "hello";
String s3 = new String("hello");
System.out.println(s1 == s2); // true
System.out.println(s1 == s3); // false

hashCode(): This method returns a hash code value for the object. The default implementation of this method in the Object class returns a hash code based on the memory address of the object. In order to provide a meaningful hash code for the object, this method must be overridden in the derived class.

For example, let's say we have a Person class as before. We can override the hashCode() method in the Person class to return a hash code based on the name and age fields, like so:

public class Person {
 private String name;
 private int age;
 // constructor, getters, and setters
 @Override
 public int hashCode() {
 int result = 17;
 result = 31 * result + name.hashCode();
 result = 31 * result + age;
 return result;
 }
}

In this example, we use the formula 31 * result + field.hashCode() to calculate the hash code. The numbers 31 and 17 are arbitrary values that are commonly used in hash code calculations. The formula ensures that the hash code is calculated in a consistent and predictable way.

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