Miscellaneous and Handy Java Functions

Object Ordering/Comparing:

1. A List l can be sorted as follows: Collections.sort(l). If the List consists of built-in class objects, e.g. String, Integer, Long, Date, Character, File etc., then the objects will be sorted in a way of natural ordering of that class. But for objects from custom classes, our class needs to either implement Comparable interface or it can implement Comparator interface to provide a custom comparator for Collections.sort(list, comparator) method. The following example is adapted from Oracle’s tutorial page.

import java.util.*;
public class EmpSort {
  //we can either define a comparator here
    static final Comparator<Employee> SENIORITY_ORDER =
                                        new Comparator<Employee>() {
            public int compare(Employee e1, Employee e2) {
                return e2.hireDate().compareTo(e1.hireDate());
            }
    };

    // Employee database
    static final Collection<Employee> employees = ... ;

    public static void main(String[] args) {
        List<Employee> employeeList = new ArrayList<Employee>(employees);
        //Collections.sort(employeeList,  SENIORITY_ORDER);
        // Or we can define comparator in-line with the sort method, as shown below
        @Override
        Collections.sort(employeeList, new Comparator<Employee>() {
            public int compare(Employee e1, Employee e2) {
                return e2.hireDate().compareTo(e1.hireDate());
            }
    });
        System.out.println(employeeList);
    }
}

2. Similar to List, java.util.Arrays also provides convenient sort() method for ordering primitive data types. For example, the following example sorts an integer array in O(n log(n)). The sort method here implements a [dual-pivot Quicksort](https://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#sort(int[]) algorithm.

import java.util.Arrays;

public class SortExample
{
    public static void main(String[] args)
    {
        int[] arr = {13, 77, 609, 45, 21, 9, 101, 102, -101, 30};

        Arrays.sort(arr);

        System.out.printf("Modified arr[] : %s", Arrays.toString(arr));
    }
}

Conversion

1. String to Character array:

String string = "Hello world!";
char[] stringToCharArray = string.toCharArray();