Interview question

What is the difference between Comparable and Comparator in Java? Java में Comparable और Comparator में क्या अंतर है?

Answer

Aspect	Comparable	Comparator
Package	java.lang	java.util
Method	compareTo(Object o)	compare(Object o1, Object o2)
Sorting logic location	Inside the class being sorted	Separate class or lambda
Number of orderings	Only ONE natural ordering per class	MULTIPLE different orderings possible

// Comparable - defines natural ordering INSIDE the class
class Employee implements Comparable<Employee> {
    String name;
    int age;
    Employee(String name, int age) { this.name = name; this.age = age; }

    @Override
    public int compareTo(Employee other) {
        return this.age - other.age;  // natural order = by age
    }
}

List<Employee> employees = new ArrayList<>();
employees.add(new Employee('John', 30));
employees.add(new Employee('Jane', 25));
Collections.sort(employees);  // uses compareTo() - sorted by age

// Comparator - defines ordering OUTSIDE the class, multiple possible
Comparator<Employee> byName = (e1, e2) -> e1.name.compareTo(e2.name);
Comparator<Employee> byAgeDesc = (e1, e2) -> e2.age - e1.age;

employees.sort(byName);      // sort by name
employees.sort(byAgeDesc);   // sort by age, descending

// Chaining comparators (Java 8+)
Comparator<Employee> byNameThenAge = Comparator
    .comparing((Employee e) -> e.name)
    .thenComparing(e -> e.age);

पहलू	Comparable	Comparator
Package	java.lang	java.util
Method	compareTo()	compare()
Sorting logic कहां	Class के अंदर	अलग class/lambda
Orderings की संख्या	सिर्फ एक natural ordering	Multiple orderings संभव

class Employee implements Comparable<Employee> {
    String name;
    int age;
    Employee(String name, int age) { this.name = name; this.age = age; }

    @Override
    public int compareTo(Employee other) {
        return this.age - other.age;
    }
}

List<Employee> employees = new ArrayList<>();
employees.add(new Employee('John', 30));
employees.add(new Employee('Jane', 25));
Collections.sort(employees);  // age से sort

Comparator<Employee> byName = (e1, e2) -> e1.name.compareTo(e2.name);
Comparator<Employee> byAgeDesc = (e1, e2) -> e2.age - e1.age;

employees.sort(byName);
employees.sort(byAgeDesc);

Comparator<Employee> byNameThenAge = Comparator
    .comparing((Employee e) -> e.name)
    .thenComparing(e -> e.age);

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