Comparable interface in java.md

Java Comparable interface is used to compare objects and sort them according to the natural order.

Natural ordering is referred to as its compareTo() function. The String objects are sorted lexicographically, and the wrapper class objects are sorted according to their built-in compareTo() function (like how Integers objects are sorted in ascending order)

compareTo() The compareTo() function compares the current object with the provided object.This function is already implemented for default wrapper classes and primitive data types; but, this function also​ needs to be implemented for user-defined classes. It returns:

1. a positive integer, if the current object is greater than the provided object.

2. a negative integer, if the current object is less than the provided object.

3. zero, if the current object is equal to the provided object.

Example Sorting string

ArrayList<String> list = new ArrayList<>();

list.add("E");

list.add("A");

list.add("C");

list.add("B");

list.add("D");

Collections.sort(list);

System.out.println(list); // [A, B, C, D, E]

Sorting user defined objects

class Employee implements Comparable<Employee> {
	private String name;
	private int age;

	Employee(String name, int age) {
		this.name = name;
		this.age = age;
	}

	@Override
	public int compareTo(Employee employee) {
		if (this.age == employee.age)
			return 0;
		else if (this.age > employee.age)
			return 1;
		else
			return -1;
	}
	
	@Override
    public String toString() {
        return "[name=" + this.name + ", age=" + this.age +"]";
    }
}

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List<Employee> employeeList = new ArrayList<Employee>();
employeeList.add(new Employee("Emp1", 25));
employeeList.add(new Employee("Emp2", 10));
System.out.println(employeeList);	// [[name=Emp1, age=25], [name=Emp2, age=10]]
Collections.sort(employeeList);	
System.out.println(employeeList); // [[name=Emp2, age=10], [name=Emp1, age=25]]