Develop Reference

Implementing a Doubly Linked List from a Sorted Array Bag (Data Structures)

it's my first time ever posting on StackOverFlow, because I'm truly desperate right now. I couldn't find an answer for my problem anywhere, so long story short, I have some kind of project for my Data Structures course. The project had 2 parts. The first part was implementing a Sorted Array Bag/ Sorted Collection for some problem. We are using java.
The second part is where I do actually have a lot of problems. So the main idea is implementing a doubly-linked list from the sorted-array bag/ sorted collection and in a way that I would just switch sorted array bag with doubly-linked list in my main and everything should work the way it was working before.
The main thing about the SortedArrayBag is as far as I understand using a Comparator when you declare the SortedArrayBag in your main, and it looks like this:
SortedBag<Grupe> al = new SortedArrayBag<>(new ComparatorVot());
al.add(new Grupe("gr1", 5));
al.add(new Grupe("gr2", 7));
The sorted collection/sorted array bag was implemented by my teacher because there is no such data structure in Java, here is her implementation:
public class SortedArrayBag<T> implements SortedBag<T> {
private ArrayList<T> elemente;
private Comparator<T> relatie;
public SortedArrayBag(Comparator<T> rel) {
this.elemente = new ArrayList<>();
this.relatie = rel;
}
public void add(T elem) {
int index = 0;
boolean added = false;
while (index < this.elemente.size() && added == false) {
T currentElem = this.elemente.get(index);
if (relatie.compare(currentElem, elem) < 0) {
index++;
} else {
this.elemente.add(index, elem);
added = true;
}
}
if (!added) {
this.elemente.add(elem);
}
}
public void remove(T elem) {
boolean removed = this.elemente.remove(elem);
}
public int size() {
return this.elemente.size();
}
public boolean search(T elem) {
return this.elemente.contains(elem);
}
public Iterator<T> iterator() {
return this.elemente.iterator();
}
}
And the SortedBag interface looks like this
public interface SortedBag<T> {
public void add(T elem);
public void remove(T elem);
public int size();
public boolean search(T elem);
public Iterator<T> iterator();
}
Also in case it helps, the comparator looks like this:
public class ComparatorVot implements Comparator<Grupe> {
public int compare(Grupe o1, Grupe o2) {
Grupe gr1 = (Grupe) o1;
Grupe gr2 = (Grupe) o2;
if (gr1.getNrPersoane() / 2 + 1 == gr2.getNrPersoane() / 2 + 1) {
return 0;
} else if (gr1.getNrPersoane() / 2 + 1 > gr2.getNrPersoane() / 2 + 1) {
return 1;
} else {
return -1;
}
}
}
So, I tried my best implementing doublyLinkedList using a SortedArrayBag, this is what I did, also if it helps making my code more clear, prim=first, ultim=last, urmator=next, anterior=previous
import java.util.Iterator;
public class LDI {
private Nod prim;
private Nod ultim;
//private int lungime;
public LDI() {
this.prim = null;
this.ultim = null;
//this.lungime = 0;
}
public class Nod {
private int elem;
private int frecventa;
private Nod urmator;
private Nod anterior;
public Nod(int e, int f) {
this.elem = e;
this.frecventa = f;
this.urmator = null;
this.anterior = null;
}
}
public void add(int elem, int frecventa) {
Nod nodNou = new Nod(elem, frecventa);
nodNou.elem = elem;
nodNou.frecventa = frecventa;
if (prim == null) {
this.prim = nodNou;
this.ultim = nodNou;
} else if (frecventa <= prim.frecventa) {
nodNou.urmator = prim;
this.prim.anterior = nodNou;
this.prim = nodNou;
} else if (frecventa >= prim.frecventa) {
nodNou.anterior = prim;
for (; nodNou.anterior.urmator != null; nodNou.anterior = nodNou.anterior.urmator) {
if (nodNou.anterior.urmator.frecventa > frecventa)
break;
}
nodNou.urmator = nodNou.anterior.urmator;
if (nodNou.anterior.urmator != null) {
nodNou.anterior.urmator.anterior = nodNou;
}
nodNou.anterior.urmator = nodNou;
nodNou.anterior = nodNou.anterior;
}
}
public void remove() {
if (this.prim != null) {
if (this.prim == this.ultim) {
this.prim = null;
this.ultim = null;
} else
this.prim = this.prim.urmator;
this.prim.anterior = null;
}
}
public int size() {
int count = 0;
for (Nod nodNou = prim; nodNou != null; nodNou = nodNou.urmator)
count++;
return count;
}
public class MyIterator {
private Nod curent;
public MyIterator() {
this.curent = prim;
}
public void urmator() {
this.curent = this.curent.urmator;
}
public int getElem() {
return this.curent.elem;
}
public boolean valid() {
if (this.curent != null) {
return true;
} else {
return false;
}
}
}
public Iterator iterator() {
return new MyIterator();
}
}
The thing is, it doesn't work, I have no idea how to make my data structure able to receive the Comparator I used and also the Iterator doesn't work. If you have any idea how to make this work, please do help me.

Data Structures and Algorithms implementation-Dictionary

I have implemented Dictionary with Vector(Array). In array i store a String data. Now i have get position Method. But i want to retrieve data at some position. what will be the method? Thank you.
private int findpositionProfile(String smkey){
DictionaryProfile p = new DictionaryProfile(smkey,null);
return data.getposi(p);
}
public Profile getProfile(int i){
// DictionaryProfile p = new DictionaryProfile(null,null);
return data.get(i);
this is not working
public class Dictionary {
private Vector data;
private Vector data1;
public Dictionary() {
data = new Vector(100);
data1 = new Vector(100);
}
public void addProfile(String smkey, Profile smvalue) {
DictionaryProfile d = new DictionaryProfile(smkey, smvalue);
if (data.getposi(d) == -1) {
data.addLast(d);
}
data.replace(d);
}
public void addCorporate(String smkey, CorporateProfile smvalue) {
DictionaryCorporate d = new DictionaryCorporate(smkey, smvalue);
if (data1.getposi(d) == -1) {
data1.addLast(d);
}
data1.replace(d);
}
private int findpositionProfile(String smkey) {
DictionaryProfile p = new DictionaryProfile(smkey,null);
return data.getposi(p);
}
public CorporateProfile getCorporate(int i){
return data.get(i);
}
public Profile getProfile(int i){
DictionaryProfile p = new DictionaryProfile(null,null);
return data.get(i);
}
My dictionaryPair::
public class DictionaryProfile implements Comparable
{
private String userName ;
private Profile completeProfile ;
public DictionaryProfile ( String name,Profile p){
userName = name;
completeProfile = p;
}
public String getUserName(){
return userName;
}
public Profile getProfile(){
return completeProfile;
}
public void setUsename ( String newname ){
userName= newname;
}
public void setProfile ( Profile pro ){
completeProfile = pro;
}
public int compareTo(Object obj){
DictionaryProfile dp = (DictionaryProfile) obj;
return (this.getUserName()).compareTo(dp.getUserName());
}
}

No one should be using the JDK 1.0 vintage Vector class. This doesn't look like a generic Dictionary ADT to me.
This method makes no sense whatsoever:
public Profile getProfile(int i){
DictionaryProfile p = new DictionaryProfile(null,null);
return data.get(i);
}
The local variable p is instantiated, never used, and eligible for GC when it goes out of scope. Data is a Vector holding type Object. Where do you expect to get a Profile from?
This code makes no sense.
This will work, unless you pass an index that's out of bounds.
public Profile getProfile(int i){
return (Profile) data.get(i);
}
None of this describes how a Dictionary works. It's a synonym for a Map, which has a key/value pair. Your code isn't doing that. Doesn't use generics for key or value. Why would you do this instead of just using a Map<K, V>?
I think you should start with this:
package collections;
public interface Dictionary<K, V> {
V get(K key);
V put(K key, V value);
boolean containsKey(K key);
int size();
}
Your keys ought to be immutable.
This is what I would consider the minimal interface for a proper Dictionary.
Here's an implementation that uses backing ArrayList:
package collections;
import java.util.ArrayList;
import java.util.List;
/**
* Implementation of a Dictionary interface
* Created by Michael
* Creation date 12/30/2015.
* #link https://stackoverflow.com/questions/34538520/data-structures-and-algorithms-implementation-dictionary/34538668?noredirect=1#comment56819702_34538668
*/
public class DictionaryImpl<K, V> implements Dictionary<K, V> {
private List<K> keys;
private List<V> values;
public DictionaryImpl() {
this.keys = new ArrayList<>();
this.values = new ArrayList<>();
}
#Override
public V get(K key) {
V value = null;
if (this.keys.contains(key)) {
int index = this.getIndex(key);
if (index != -1) {
value = this.values.get(index);
}
}
return value;
}
#Override
public V put(K key, V value) {
V previousValue = null;
if (this.keys.contains(key)) {
previousValue = this.get(key);
}
this.keys.add(key);
this.values.add(value);
return previousValue;
}
#Override
public boolean containsKey(K key) {
return this.keys.contains(key);
}
#Override
public int size() {
return this.keys.size();
}
private int getIndex(K keyToFind) {
int index = -1;
if (this.keys.contains(keyToFind)) {
for (K key : this.keys) {
++index;
if (key.equals(keyToFind)) {
break;
}
}
}
return index;
}
}
Here's a Junit test to prove that it's all working:
package collections;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
/**
* Junit test for Dictionary
* Created by Michael
* Creation date 12/30/2015.
* #link https://stackoverflow.com/questions/34538520/data-structures-and-algorithms-implementation-dictionary/34538668?noredirect=1#comment56819702_34538668
*/
public class DictionaryTest {
private Dictionary<String, Integer> testDictionary;
#Before
public void setUp() {
this.testDictionary = new DictionaryImpl<>();
this.testDictionary.put("foo", 17);
this.testDictionary.put("bar", 23);
this.testDictionary.put("baz", 31);
this.testDictionary.put("bat", 41);
}
#Test
public void testContainsKey_True() {
String [] keys = { "foo", "bar", "baz", "bat" };
for (String key : keys) {
Assert.assertTrue(String.format("Should have contained key '%s'", key), this.testDictionary.containsKey(key));
}
}
#Test
public void testContainsKey_False() {
String [] keys = { "dopey", "sleepy", "doc", "sneezy" };
for (String key : keys) {
Assert.assertTrue(String.format("Should not have contained key '%s'", key), !this.testDictionary.containsKey(key));
}
}
#Test
public void testGet_Success() {
String [] keys = { "foo", "bar", "baz", "bat" };
Integer [] values = { 17, 23, 31, 41 };
for (int i = 0; i < keys.length; ++i) {
Assert.assertEquals(String.format("Should have returned value %d for key '%s'", values[i], keys[i]), values[i], this.testDictionary.get(keys[i]));
}
}
#Test
public void testGet_NoSuchKey() {
String [] keys = { "dopey", "sleepy", "doc", "sneezy" };
for (String key : keys) {
Assert.assertNull(String.format("Should have returned null for key '%s'", key), this.testDictionary.get(key));
}
}
#Test
public void testSize() {
int expected = 4;
Assert.assertEquals(expected, this.testDictionary.size());
}
}

Removing from a MultiSet

So I've been tasked to create a method to remove an element from a MultiSet. I've been trying for a while, but sadly in vain. My code is as follows:
import java.util.*;
public class MultiSet<E> extends AbstractCollection<E> {
private HashMap<E, Integer> elements;
private int noOfElems;
public MultiSet() {
elements = new HashMap<E, Integer>();
noOfElems= 0;
}
public MultiSet(Collection<E> c) {
this();
addAll(c);
}
public int size() {
return noOfElems;
}
public Iterator<E> iterator() {
return new Iterator<E>() {
Iterator<E> iterator = elements.keySet().iterator();
int elemsLeft = 0;
E thisElem = null;
public boolean hasNext() {
return iterator.hasNext();
}
public E next() {
if (elemsLeft == 0) {
thisElem = iterator.next();
elemsLeft = elements.get(thisElem);
}
elemsLeft -= elemsLeft;
return null;
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
public boolean add(E e) {
Integer i = elements.get(e);
if(i == null) {
i = 1;
} else {
i += 1;
}
elements.put(e, i);
noOfElems++;
return true;
}
public String toString() {
return elements.toString();
}
public int hashCode() {
return elements.hashCode();
}
public boolean equals(MultiSet<E> other) {
if (this == other) {
return true;
}
if (other == null) {
return false;
}
if (this.getClass() != other.getClass()) {
return false;
}
MultiSet<?> obj = (MultiSet<?>) other;
return obj.elements.equals(elements);
}
public boolean remove(Object o) {
}
}
And I want to implement the remove method. Anything that will help me, even a few pointers on where to start, will be greatly appreciated. Thanks! (also, comments on the rest of my code will also be appreciated)

This multiset just stores the elements as hash keys mapped to a count of the number of occurrences. To remove all instances of an element, just delete the key:
public void remove_all(E e) {
elements.remove(e);
}
If you need to remove only one instance, then decrement the count unless it's already a 1. In that case, remove the key.
public void remove(E e) {
Integer i = elements.get(e);
if (i != null) {
if (i == 1) {
elements.remove(e);
} else {
elements.put(e, i - 1);
}
}
}
BTW it's a bit hard to believe this is your code. If you understand enough to write the methods you've already written, how could you not know even where to start on remove?

Java implementing comparator on ArrayList

I'm quite new to Java so this is probably pretty straight forward question.
I want to sort an ArrayList in the class MediaLib based on the natural order of a specified key.
I can't work out how to use my comparator (compareTo(MediaInterface, key)) which is in the Media class. Whats the best way to go about this?
package assign1;
import java.util.*;
public class Media implements MediaInterface {
private Map<String, Object> fields;
private static int compare;
public Media(String title, String format) {
fields = new TreeMap<String, Object>();
fields.put("title", title);
fields.put("format", format);
}
public Object get(String key) {
return fields.get(key);
}
public void put(String key, Object value) {
fields.put(key, value);
}
public boolean hasKeywords(String[] words, boolean combineWithAND) {
Collection<Object> values = (Collection<Object>) fields.values();
int count = 0;
int size = 0;
for (String s: words) {
for (Object o: values) {
String t = o.toString();
if (t.indexOf(s) >= 0) {
count++;
break;
}
}
size++;
}
if ((count == 0 && !combineWithAND) || (combineWithAND && (count != size))) {
return false;
}
return true;
}
public int compareTo(MediaInterface mi, String key) { //<<<<<<<------calling this!!
if (mi == null)
throw new NullPointerException();
Media m = (Media) mi;
Comparable mValue = (Comparable) m.get(key);
Comparable lValue = (Comparable) fields.get(key);
if ((mValue == null) && (lValue == null)){
return 0;
}
if ((lValue == null)){
return 1;
}
if ((mValue == null)){
return -1;
}
return (lValue).compareTo(mValue);
}
#Override
public int compareTo(MediaInterface mi) {
if (mi == null)
throw new NullPointerException();
Media m = (Media) mi;
Set<String> lSet = fields.keySet();
if (compareTo(m, "title") != 0) {
return compareTo(m, "title");
}
if (compareTo(m, "year") != 0) {
return compareTo(m, "year");
}
for (String s: lSet) {
if (compareTo(m, s) != 0) {
return compareTo(m, s);
}
}
return 0;
}
public boolean equals(Object object) {
if (object == null)
return false;
if (!(object instanceof Media))
return false;
Media m = (Media) object;
if (compareTo(m) != 0) {
return false;
}
return true;
}
}
package assign1;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
public class MediaLib implements Searchable {
private ArrayList<MediaInterface> media;
public MediaLib() {
media = new ArrayList<MediaInterface>();
}
#Override
public void add(MediaInterface mi) {
if (media.isEmpty()) {
media.add(mi);
}
else {
for (MediaInterface m: media) {
if (mi.equals(m)) {
return;
}
}
media.add(mi);
}
}
#Override
public boolean contains(MediaInterface mi) {
for (MediaInterface m: media) {
if (mi.equals(m)) {
return true;
}
}
return false;
}
#Override
public Collection<MediaInterface> findByKeyword(String[] words, boolean combineWithAND) {
Collection<MediaInterface> foundList = new ArrayList<MediaInterface>();
for (MediaInterface mi: media) {
if (mi.hasKeywords(words, combineWithAND)) {
foundList.add(mi);
}
}
return foundList;
}
#Override
public Collection<MediaInterface> findByTitle(String str) {
Collection<MediaInterface> foundList = new ArrayList<MediaInterface>();
for (MediaInterface mi: media) {
if ((mi.get("title")).equals(str)) {
foundList.add(mi);
}
}
return foundList;
}
#Override
public Collection<MediaInterface> getAllWithFormat(String formatName) {
Collection<MediaInterface> foundList = new ArrayList<MediaInterface>();
for (MediaInterface mi: media) {
if ((mi.get("format")).equals(formatName)) {
foundList.add(mi);
}
}
return foundList;
}
public Collection<MediaInterface> getAll() {
Collection<MediaInterface> fullList = new ArrayList<MediaInterface>();
for (MediaInterface mi: media) {
fullList.add(mi);
}
return fullList;
}
#Override
public void removeAllWithKeyword(String[] words, boolean combineWithAND) {
Collection<MediaInterface> foundList = findByKeyword(words, combineWithAND);
for (MediaInterface mi: foundList) {
media.remove(mi);
}
}
#Override
public void removeAllWithFormat(String format) {
Collection<MediaInterface> foundList = getAllWithFormat(format);
for (MediaInterface mi: foundList) {
media.remove(mi);
}
}
#Override
public void sort() {
Collections.sort(media);
}
#Override
public void sort(final String fieldName) {
Collections.sort(media, new Media.compareTo(MediaInterface, fieldName)) //<<<<<--------Trying to call compareTo()
}
}
public void parse(java.io.BufferedReader br) throws java.io.IOException {
while(br.readLine()!= null) {
Media mi = new Media(/n br.readLine(), br.readLine());
while
}
}
}

You already implement the Comparable interface in your MediaInterface class, this is a generic interface, so you then implement Comparable<MediaInterface> which will then require you to implement a method with the signature
public int compareTo(final MediaInterface other)
This is why your call to Collections.sort(media); compiles
In order to sort by a specific field name, you need to provide an instance of a Comparator, the easiest way to do this will be to create an inner class in your Media class which you can then pass into Collections.sort. For example
public class Media implements MediaInterface {
public static final class FieldComparator implements Comparator<Media> {
private final String field;
public FieldComparator(final String field) {
this.field = field;
}
public int compare(final Media a, final Media b) {
// implementation to compare a.field to b.field
}
}
}
You can then rewrite your second sort method as
#Override
public void sort(final String fieldName) {
Collections.sort(media, new Media.FieldComparator(fieldName));
}

How to use Comparator in Java to sort

I learned how to use the comparable but I'm having difficulty with the Comparator. I am having a error in my code:
Exception in thread "main" java.lang.ClassCastException: New.People cannot be cast to java.lang.Comparable
at java.util.Arrays.mergeSort(Unknown Source)
at java.util.Arrays.sort(Unknown Source)
at java.util.Collections.sort(Unknown Source)
at New.TestPeople.main(TestPeople.java:18)
Here is my code:
import java.util.Comparator;
public class People implements Comparator {
private int id;
private String info;
private double price;
public People(int newid, String newinfo, double newprice) {
setid(newid);
setinfo(newinfo);
setprice(newprice);
}
public int getid() {
return id;
}
public void setid(int id) {
this.id = id;
}
public String getinfo() {
return info;
}
public void setinfo(String info) {
this.info = info;
}
public double getprice() {
return price;
}
public void setprice(double price) {
this.price = price;
}
public int compare(Object obj1, Object obj2) {
Integer p1 = ((People) obj1).getid();
Integer p2 = ((People) obj2).getid();
if (p1 > p2) {
return 1;
} else if (p1 < p2){
return -1;
} else {
return 0;
}
}
}
import java.util.ArrayList;
import java.util.Collections;
public class TestPeople {
public static void main(String[] args) {
ArrayList peps = new ArrayList();
peps.add(new People(123, "M", 14.25));
peps.add(new People(234, "M", 6.21));
peps.add(new People(362, "F", 9.23));
peps.add(new People(111, "M", 65.99));
peps.add(new People(535, "F", 9.23));
Collections.sort(peps);
for (int i = 0; i < peps.size(); i++){
System.out.println(peps.get(i));
}
}
}
I believe it has to do something with the casting in the compare method but I was playing around with it and still could not find the solution

There are a couple of awkward things with your example class:
it's called People while it has a price and info (more something for objects, not people);
when naming a class as a plural of something, it suggests it is an abstraction of more than one thing.
Anyway, here's a demo of how to use a Comparator<T>:
public class ComparatorDemo {
public static void main(String[] args) {
List<Person> people = Arrays.asList(
new Person("Joe", 24),
new Person("Pete", 18),
new Person("Chris", 21)
);
Collections.sort(people, new LexicographicComparator());
System.out.println(people);
Collections.sort(people, new AgeComparator());
System.out.println(people);
}
}
class LexicographicComparator implements Comparator<Person> {
#Override
public int compare(Person a, Person b) {
return a.name.compareToIgnoreCase(b.name);
}
}
class AgeComparator implements Comparator<Person> {
#Override
public int compare(Person a, Person b) {
return a.age < b.age ? -1 : a.age == b.age ? 0 : 1;
}
}
class Person {
String name;
int age;
Person(String n, int a) {
name = n;
age = a;
}
#Override
public String toString() {
return String.format("{name=%s, age=%d}", name, age);
}
}
EDIT
And an equivalent Java 8 demo would look like this:
public class ComparatorDemo {
public static void main(String[] args) {
List<Person> people = Arrays.asList(
new Person("Joe", 24),
new Person("Pete", 18),
new Person("Chris", 21)
);
Collections.sort(people, (a, b) -> a.name.compareToIgnoreCase(b.name));
System.out.println(people);
Collections.sort(people, (a, b) -> a.age < b.age ? -1 : a.age == b.age ? 0 : 1);
System.out.println(people);
}
}

Here's a super short template to do the sorting right away :
Collections.sort(people, new Comparator<Person>() {
#Override
public int compare(final Person lhs, Person rhs) {
// TODO return 1 if rhs should be before lhs
// return -1 if lhs should be before rhs
// return 0 otherwise (meaning the order stays the same)
}
});
If it's hard to remember, try to just remember that it's similar (in terms of the sign of the number) to:
lhs-rhs
That's in case you want to sort in ascending order : from smallest number to largest number.

Use People implements Comparable<People> instead; this defines the natural ordering for People.
A Comparator<People> can also be defined in addition, but People implements Comparator<People> is not the right way of doing things.
The two overloads for Collections.sort are different:
<T extends Comparable<? super T>> void sort(List<T> list)
Sorts Comparable objects using their natural ordering
<T> void sort(List<T> list, Comparator<? super T> c)
Sorts whatever using a compatible Comparator
You're confusing the two by trying to sort a Comparator (which is again why it doesn't make sense that Person implements Comparator<Person>). Again, to use Collections.sort, you need one of these to be true:
The type must be Comparable (use the 1-arg sort)
A Comparator for the type must be provided (use the 2-args sort)
Related questions
When to use Comparable vs Comparator
Sorting an ArrayList of Contacts
Also, do not use raw types in new code. Raw types are unsafe, and it's provided only for compatibility.
That is, instead of this:
ArrayList peps = new ArrayList(); // BAD!!! No generic safety!
you should've used the typesafe generic declaration like this:
List<People> peps = new ArrayList<People>(); // GOOD!!!
You will then find that your code doesn't even compile!! That would be a good thing, because there IS something wrong with the code (Person does not implements Comparable<Person>), but because you used raw type, the compiler didn't check for this, and instead you get a ClassCastException at run-time!!!
This should convince you to always use typesafe generic types in new code. Always.
See also
What is a raw type and why shouldn't we use it?

For the sake of completeness, here's a simple one-liner compare method:
Collections.sort(people, new Comparator<Person>() {
#Override
public int compare(Person lhs, Person rhs) {
return Integer.signum(lhs.getId() - rhs.getId());
}
});

Java 8 added a new way of making Comparators that reduces the amount of code you have to write, Comparator.comparing. Also check out Comparator.reversed
Here's a sample
import org.junit.Test;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import static org.junit.Assert.assertTrue;
public class ComparatorTest {
#Test
public void test() {
List<Person> peopleList = new ArrayList<>();
peopleList.add(new Person("A", 1000));
peopleList.add(new Person("B", 1));
peopleList.add(new Person("C", 50));
peopleList.add(new Person("Z", 500));
//sort by name, ascending
peopleList.sort(Comparator.comparing(Person::getName));
assertTrue(peopleList.get(0).getName().equals("A"));
assertTrue(peopleList.get(peopleList.size() - 1).getName().equals("Z"));
//sort by name, descending
peopleList.sort(Comparator.comparing(Person::getName).reversed());
assertTrue(peopleList.get(0).getName().equals("Z"));
assertTrue(peopleList.get(peopleList.size() - 1).getName().equals("A"));
//sort by age, ascending
peopleList.sort(Comparator.comparing(Person::getAge));
assertTrue(peopleList.get(0).getAge() == 1);
assertTrue(peopleList.get(peopleList.size() - 1).getAge() == 1000);
//sort by age, descending
peopleList.sort(Comparator.comparing(Person::getAge).reversed());
assertTrue(peopleList.get(0).getAge() == 1000);
assertTrue(peopleList.get(peopleList.size() - 1).getAge() == 1);
}
class Person {
String name;
int age;
Person(String n, int a) {
name = n;
age = a;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
public void setName(String name) {
this.name = name;
}
public void setAge(int age) {
this.age = age;
}
}
}

For the sake of completeness.
Using Java8
people.sort(Comparator.comparingInt(People::getId));
if you want in descending order
people.sort(Comparator.comparingInt(People::getId).reversed());

You want to implement Comparable, not Comparator. You need to implement the compareTo method. You're close though. Comparator is a "3rd party" comparison routine. Comparable is that this object can be compared with another.
public int compareTo(Object obj1) {
People that = (People)obj1;
Integer p1 = this.getId();
Integer p2 = that.getid();
if (p1 > p2 ){
return 1;
}
else if (p1 < p2){
return -1;
}
else
return 0;
}
Note, you may want to check for nulls in here for getId..just in case.

Two corrections:
You have to make an ArrayList of People objects:
ArrayList<People> preps = new ArrayList<People>();
After adding the objects to the preps, use:
Collections.sort(preps, new CompareId());
Also, add a CompareId class as:
class CompareId implements Comparator {
public int compare(Object obj1, Object obj2) {
People t1 = (People)obj1;
People t2 = (People)obj2;
if (t1.marks > t2.marks)
return 1;
else
return -1;
}
}

Here's an example of a Comparator that will work for any zero arg method that returns a Comparable. Does something like this exist in a jdk or library?
import java.lang.reflect.Method;
import java.util.Comparator;
public class NamedMethodComparator implements Comparator<Object> {
//
// instance variables
//
private String methodName;
private boolean isAsc;
//
// constructor
//
public NamedMethodComparator(String methodName, boolean isAsc) {
this.methodName = methodName;
this.isAsc = isAsc;
}
/**
* Method to compare two objects using the method named in the constructor.
*/
#Override
public int compare(Object obj1, Object obj2) {
Comparable comp1 = getValue(obj1, methodName);
Comparable comp2 = getValue(obj2, methodName);
if (isAsc) {
return comp1.compareTo(comp2);
} else {
return comp2.compareTo(comp1);
}
}
//
// implementation
//
private Comparable getValue(Object obj, String methodName) {
Method method = getMethod(obj, methodName);
Comparable comp = getValue(obj, method);
return comp;
}
private Method getMethod(Object obj, String methodName) {
try {
Class[] signature = {};
Method method = obj.getClass().getMethod(methodName, signature);
return method;
} catch (Exception exp) {
throw new RuntimeException(exp);
}
}
private Comparable getValue(Object obj, Method method) {
Object[] args = {};
try {
Object rtn = method.invoke(obj, args);
Comparable comp = (Comparable) rtn;
return comp;
} catch (Exception exp) {
throw new RuntimeException(exp);
}
}
}

public static Comparator<JobSet> JobEndTimeComparator = new Comparator<JobSet>() {
public int compare(JobSet j1, JobSet j2) {
int cost1 = j1.cost;
int cost2 = j2.cost;
return cost1-cost2;
}
};

The solution can be optimized in following way:
Firstly, use a private inner class as the scope for the fields is to be the enclosing class TestPeople so as the implementation of class People won't get exposed to outer world. This can be understood in terms of creating an APIthat expects a sorted list of people
Secondly, using the Lamba expression(java 8) which reduces the code, hence development effort
Hence code would be as below:
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
public class TestPeople {
public static void main(String[] args) {
ArrayList<People> peps = new ArrayList<>();// Be specific, to avoid
// classCast Exception
TestPeople test = new TestPeople();
peps.add(test.new People(123, "M", 14.25));
peps.add(test.new People(234, "M", 6.21));
peps.add(test.new People(362, "F", 9.23));
peps.add(test.new People(111, "M", 65.99));
peps.add(test.new People(535, "F", 9.23));
/*
* Collections.sort(peps);
*
* for (int i = 0; i < peps.size(); i++){
* System.out.println(peps.get(i)); }
*/
// The above code can be replaced by followin:
peps.sort((People p1, People p2) -> p1.getid() - p2.getid());
peps.forEach((p) -> System.out.println(" " + p.toString()));
}
private class People {
private int id;
#Override
public String toString() {
return "People [id=" + id + ", info=" + info + ", price=" + price + "]";
}
private String info;
private double price;
public People(int newid, String newinfo, double newprice) {
setid(newid);
setinfo(newinfo);
setprice(newprice);
}
public int getid() {
return id;
}
public void setid(int id) {
this.id = id;
}
public String getinfo() {
return info;
}
public void setinfo(String info) {
this.info = info;
}
public double getprice() {
return price;
}
public void setprice(double price) {
this.price = price;
}
}
}

Here is a lambda version of comparator. This will sort a string list according to length.
Collections.sort(str, (str1, str2) -> {
if(str1.length() < str2.length())
return 1;
else if(str2.length() < str1.length())
return -1;
else
return 0;
});

You should use the overloaded sort(peps, new People()) method
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Test
{
public static void main(String[] args)
{
List<People> peps = new ArrayList<>();
peps.add(new People(123, "M", 14.25));
peps.add(new People(234, "M", 6.21));
peps.add(new People(362, "F", 9.23));
peps.add(new People(111, "M", 65.99));
peps.add(new People(535, "F", 9.23));
Collections.sort(peps, new People().new ComparatorId());
for (int i = 0; i < peps.size(); i++)
{
System.out.println(peps.get(i));
}
}
}
class People
{
private int id;
private String info;
private double price;
public People()
{
}
public People(int newid, String newinfo, double newprice) {
setid(newid);
setinfo(newinfo);
setprice(newprice);
}
public int getid() {
return id;
}
public void setid(int id) {
this.id = id;
}
public String getinfo() {
return info;
}
public void setinfo(String info) {
this.info = info;
}
public double getprice() {
return price;
}
public void setprice(double price) {
this.price = price;
}
class ComparatorId implements Comparator<People>
{
#Override
public int compare(People obj1, People obj2) {
Integer p1 = obj1.getid();
Integer p2 = obj2.getid();
if (p1 > p2) {
return 1;
} else if (p1 < p2){
return -1;
} else {
return 0;
}
}
}
}

Here is my answer for a simple comparator tool
public class Comparator {
public boolean isComparatorRunning = false;
public void compareTableColumns(List<String> tableNames) {
if(!isComparatorRunning) {
isComparatorRunning = true;
try {
for (String schTableName : tableNames) {
Map<String, String> schemaTableMap = ComparatorUtil.getSchemaTableMap(schTableName);
Map<String, ColumnInfo> primaryColMap = ComparatorUtil.getColumnMetadataMap(DbConnectionRepository.getConnectionOne(), schemaTableMap);
Map<String, ColumnInfo> secondaryColMap = ComparatorUtil.getColumnMetadataMap(DbConnectionRepository.getConnectionTwo(), schemaTableMap);
ComparatorUtil.publishColumnInfoOutput("Comparing table : "+ schemaTableMap.get(CompConstants.TABLE_NAME));
compareColumns(primaryColMap, secondaryColMap);
}
} catch (Exception e) {
ComparatorUtil.publishColumnInfoOutput("ERROR"+e.getMessage());
}
isComparatorRunning = false;
}
}
public void compareColumns(Map<String, ColumnInfo> primaryColMap, Map<String, ColumnInfo> secondaryColMap) {
try {
boolean isEqual = true;
for(Map.Entry<String, ColumnInfo> entry : primaryColMap.entrySet()) {
String columnName = entry.getKey();
ColumnInfo primaryColInfo = entry.getValue();
ColumnInfo secondaryColInfo = secondaryColMap.remove(columnName);
if(secondaryColInfo == null) {
// column is not present in Secondary Environment
ComparatorUtil.publishColumnInfoOutput("ALTER", primaryColInfo);
isEqual = false;
continue;
}
if(!primaryColInfo.equals(secondaryColInfo)) {
isEqual = false;
// Column not equal in secondary env
ComparatorUtil.publishColumnInfoOutput("MODIFY", primaryColInfo);
}
}
if(!secondaryColMap.isEmpty()) {
isEqual = false;
for(Map.Entry<String, ColumnInfo> entry : secondaryColMap.entrySet()) {
// column is not present in Primary Environment
ComparatorUtil.publishColumnInfoOutput("DROP", entry.getValue());
}
}
if(isEqual) {
ComparatorUtil.publishColumnInfoOutput("--Exact Match");
}
} catch (Exception e) {
ComparatorUtil.publishColumnInfoOutput("ERROR"+e.getMessage());
}
}
public void compareTableColumnsValues(String primaryTableName, String primaryColumnNames, String primaryCondition, String primaryKeyColumn,
String secTableName, String secColumnNames, String secCondition, String secKeyColumn) {
if(!isComparatorRunning) {
isComparatorRunning = true;
Connection conn1 = DbConnectionRepository.getConnectionOne();
Connection conn2 = DbConnectionRepository.getConnectionTwo();
String query1 = buildQuery(primaryTableName, primaryColumnNames, primaryCondition, primaryKeyColumn);
String query2 = buildQuery(secTableName, secColumnNames, secCondition, secKeyColumn);
try {
Map<String,Map<String, Object>> query1Data = executeAndRefactorData(conn1, query1, primaryKeyColumn);
Map<String,Map<String, Object>> query2Data = executeAndRefactorData(conn2, query2, secKeyColumn);
for(Map.Entry<String,Map<String, Object>> entry : query1Data.entrySet()) {
String key = entry.getKey();
Map<String, Object> value = entry.getValue();
Map<String, Object> secondaryValue = query2Data.remove(key);
if(secondaryValue == null) {
ComparatorUtil.publishColumnValuesInfoOutput("NO SUCH VALUE AVAILABLE IN SECONDARY DB "+ value.toString());
continue;
}
compareMap(value, secondaryValue, key);
}
if(!query2Data.isEmpty()) {
ComparatorUtil.publishColumnValuesInfoOutput("Extra Values in Secondary table "+ ((Map)query2Data.values()).values().toString());
}
} catch (Exception e) {
ComparatorUtil.publishColumnValuesInfoOutput("ERROR"+e.getMessage());
}
isComparatorRunning = false;
}
}
private void compareMap(Map<String, Object> primaryValues, Map<String, Object> secondaryValues, String columnIdentification) {
for(Map.Entry<String, Object> entry : primaryValues.entrySet()) {
String key = entry.getKey();
Object value = entry.getValue();
Object secValue = secondaryValues.get(key);
if(value!=null && secValue!=null && !String.valueOf(value).equalsIgnoreCase(String.valueOf(secValue))) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Secondary Table does not match value ("+ value +") for column ("+ key+")");
}
if(value==null && secValue!=null) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Values not available in primary table for column "+ key);
}
if(value!=null && secValue==null) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Values not available in Secondary table for column "+ key);
}
}
}
private String buildQuery(String tableName, String column, String condition, String keyCol) {
if(!"*".equalsIgnoreCase(column)) {
String[] keyColArr = keyCol.split(",");
for(String key: keyColArr) {
if(!column.contains(key.trim())) {
column+=","+key.trim();
}
}
}
StringBuilder queryBuilder = new StringBuilder();
queryBuilder.append("select "+column+" from "+ tableName);
if(!ComparatorUtil.isNullorEmpty(condition)) {
queryBuilder.append(" where 1=1 and "+condition);
}
return queryBuilder.toString();
}
private Map<String,Map<String, Object>> executeAndRefactorData(Connection connection, String query, String keyColumn) {
Map<String,Map<String, Object>> result = new HashMap<String, Map<String,Object>>();
try {
PreparedStatement preparedStatement = connection.prepareStatement(query);
ResultSet resultSet = preparedStatement.executeQuery();
resultSet.setFetchSize(1000);
if (resultSet != null && !resultSet.isClosed()) {
while (resultSet.next()) {
Map<String, Object> columnValueDetails = new HashMap<String, Object>();
int columnCount = resultSet.getMetaData().getColumnCount();
for (int i=1; i<=columnCount; i++) {
String columnName = String.valueOf(resultSet.getMetaData().getColumnName(i));
Object columnValue = resultSet.getObject(columnName);
columnValueDetails.put(columnName, columnValue);
}
String[] keys = keyColumn.split(",");
String newKey = "";
for(int j=0; j<keys.length; j++) {
newKey += String.valueOf(columnValueDetails.get(keys[j]));
}
result.put(newKey , columnValueDetails);
}
}
} catch (SQLException e) {
ComparatorUtil.publishColumnValuesInfoOutput("ERROR"+e.getMessage());
}
return result;
}
}
Utility Tool for the same
public class ComparatorUtil {
public static Map<String, String> getSchemaTableMap(String tableNameWithSchema) {
if(isNullorEmpty(tableNameWithSchema)) {
return null;
}
Map<String, String> result = new LinkedHashMap<>();
int index = tableNameWithSchema.indexOf(".");
String schemaName = tableNameWithSchema.substring(0, index);
String tableName = tableNameWithSchema.substring(index+1);
result.put(CompConstants.SCHEMA_NAME, schemaName);
result.put(CompConstants.TABLE_NAME, tableName);
return result;
}
public static Map<String, ColumnInfo> getColumnMetadataMap(Connection conn, Map<String, String> schemaTableMap) {
try {
String schemaName = schemaTableMap.get(CompConstants.SCHEMA_NAME);
String tableName = schemaTableMap.get(CompConstants.TABLE_NAME);
ResultSet resultSetConnOne = conn.getMetaData().getColumns(null, schemaName, tableName, null);
Map<String, ColumnInfo> resultSetTwoColInfo = getColumnInfo(schemaName, tableName, resultSetConnOne);
return resultSetTwoColInfo;
} catch (SQLException e) {
e.printStackTrace();
}
return null;
}
/* Number Type mapping
* 12-----VARCHAR
* 3-----DECIMAL
* 93-----TIMESTAMP
* 1111-----OTHER
*/
public static Map<String, ColumnInfo> getColumnInfo(String schemaName, String tableName, ResultSet columns) {
try {
Map<String, ColumnInfo> tableColumnInfo = new LinkedHashMap<String, ColumnInfo>();
while (columns.next()) {
ColumnInfo columnInfo = new ColumnInfo();
columnInfo.setSchemaName(schemaName);
columnInfo.setTableName(tableName);
columnInfo.setColumnName(columns.getString("COLUMN_NAME"));
columnInfo.setDatatype(columns.getString("DATA_TYPE"));
columnInfo.setColumnsize(columns.getString("COLUMN_SIZE"));
columnInfo.setDecimaldigits(columns.getString("DECIMAL_DIGITS"));
columnInfo.setIsNullable(columns.getString("IS_NULLABLE"));
tableColumnInfo.put(columnInfo.getColumnName(), columnInfo);
}
return tableColumnInfo;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
public static boolean isNullOrEmpty(Object obj) {
if (obj == null)
return true;
if (String.valueOf(obj).equalsIgnoreCase("NULL"))
return true;
if (obj.toString().trim().length() == 0)
return true;
return false;
}
public static boolean isNullorEmpty(String str) {
if(str == null)
return true;
if(str.trim().length() == 0)
return true;
return false;
}
public static void publishColumnInfoOutput(String type, ColumnInfo columnInfo) {
String str = "ALTER TABLE "+columnInfo.getSchemaName()+"."+columnInfo.getTableName();
switch(type.toUpperCase()) {
case "ALTER":
if("NUMBER".equalsIgnoreCase(columnInfo.getDatatype()) || "DATE".equalsIgnoreCase(columnInfo.getDatatype())) {
str += " ADD ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype()+");";
} else {
str += " ADD ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype() +"("+columnInfo.getColumnsize()+"));";
}
break;
case "DROP":
str += " DROP ("+columnInfo.getColumnName()+");";
break;
case "MODIFY":
if("NUMBER".equalsIgnoreCase(columnInfo.getDatatype()) || "DATE".equalsIgnoreCase(columnInfo.getDatatype())) {
str += " MODIFY ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype()+");";
} else {
str += " MODIFY ("+columnInfo.getColumnName()+" "+ columnInfo.getDatatype() +"("+columnInfo.getColumnsize()+"));";
}
break;
}
publishColumnInfoOutput(str);
}
public static Map<Integer, String> allJdbcTypeName = null;
public static Map<Integer, String> getAllJdbcTypeNames() {
Map<Integer, String> result = new HashMap<Integer, String>();
if(allJdbcTypeName != null)
return allJdbcTypeName;
try {
for (Field field : java.sql.Types.class.getFields()) {
result.put((Integer) field.get(null), field.getName());
}
} catch (Exception e) {
e.printStackTrace();
}
return allJdbcTypeName=result;
}
public static String getStringPlaces(String[] attribs) {
String params = "";
for(int i=0; i<attribs.length; i++) { params += "?,"; }
return params.substring(0, params.length()-1);
}
}
Column Info Class
public class ColumnInfo {
private String schemaName;
private String tableName;
private String columnName;
private String datatype;
private String columnsize;
private String decimaldigits;
private String isNullable;

If you are using Java 8 then it's better to use below code like this:
Comparator<People> comparator = Comparator.comparing(People::getName);
And then simply use:
Collections.sort(list, comparator);
If you are using Java 7 or below then you can use a comparator for customized sorting order by implementing compare method.
For example:
import java.util.Comparator;
public class PeopleNameComparator implements Comparator<People> {
#Override
public int compare(People people1, People people2) {
return people1.getName().compareTo(people2.getName());
}
}
And then simply use like this:
Collections.sort(list, new PeopleNameComparator);

Do not waste time implementing Sorting Algorithm by your own. Instead use Collections.sort() to sort data.