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How to fix reading/writing value from .dat file

Please find attached Java code. I am unable to fix and execute the code. If I execute Locations.java, no error message is coming,
at main(MainAdventure.java:30)
at Locations.getLocation(Locations.java:96)
If I execute the MainAdventure.java, getting null pointer exception.
I tried to debug but no help.Can some one please guide me?
MainAdventure.java
public class MainAdventure {
private static Locations locations = new Locations();
public static void main(String[] args) throws IOException {
/*
* Change the program to allow players to type full words, or phrases, then move
* to the correct location based upon their input. The player should be able to
* type commands such as "Go West", "run South", or just "East" and the program
* will move to the appropriate location if there is one. As at present, an
* attempt to move in an invalid direction should print a message and remain in
* the same place.
*
* Single letter commands (N, W, S, E, Q) should still be available.
*/
Scanner scanner = new Scanner(System.in);
Map<String, String> vocabulary = new HashMap<String, String>();
vocabulary.put("QUIT", "Q");
vocabulary.put("NORTH", "N");
vocabulary.put("SOUTH", "S");
vocabulary.put("WEST", "W");
vocabulary.put("EAST", "E");
Location currentLocation = locations.getLocation(1);
while (true) {
System.out.println(currentLocation.getDescription());
if (currentLocation.getLocationID() == 0) {
break;
}
Map<String, Integer> exits = currentLocation.getExits();
System.out.print("Available exits are ");
for (String exit : exits.keySet()) {
System.out.print(exit + ", ");
}
System.out.println();
String direction = scanner.nextLine().toUpperCase();
if (direction.length() > 1) {
String[] words = direction.split(" ");
for (String word : words) {
if (vocabulary.containsKey(word)) {
direction = vocabulary.get(word);
break;
}
}
}
if (exits.containsKey(direction)) {
currentLocation = locations.getLocation(currentLocation.getExits().get(direction));
} else {
System.out.println("You cannot go in that direction");
}
}
locations.close();
}
}
Locations.java
public class Locations implements Map<Integer, Location> {
private static Map<Integer, Location> locations = new LinkedHashMap<Integer, Location>();
private static Map<Integer, IndexRecord> index = new LinkedHashMap<>();
private static RandomAccessFile ra;
public static void main(String[] args) throws IOException {
try (RandomAccessFile rao = new RandomAccessFile("locations_rand.dat", "rwd")) {
rao.writeInt(locations.size());
int indexSize = locations.size() * 3 * Integer.BYTES;
int locationStart = (int) (indexSize + rao.getFilePointer() + Integer.BYTES);
rao.writeInt(locationStart);
long indexStart = rao.getFilePointer();
int startPointer = locationStart;
rao.seek(startPointer);
for (Location location : locations.values()) {
rao.writeInt(location.getLocationID());
rao.writeUTF(location.getDescription());
StringBuilder builder = new StringBuilder();
for (String direction : location.getExits().keySet()) {
if (!direction.equalsIgnoreCase("Q")) {
builder.append(direction);
builder.append(",");
builder.append(location.getExits().get(direction));
builder.append(",");
}
}
rao.writeUTF(builder.toString());
IndexRecord record = new IndexRecord(startPointer, (int) (rao.getFilePointer() - startPointer));
index.put(location.getLocationID(), record);
startPointer = (int) rao.getFilePointer();
}
rao.seek(indexStart);
for (Integer locationID : index.keySet()) {
rao.writeInt(locationID);
rao.writeInt(index.get(locationID).getStartByte());
rao.writeInt(index.get(locationID).getLength());
}
}
}
// 1. This first four bytes will contain the number of locations (bytes 0-3)
// 2. The next four bytes will contain the start offset of the locations section
// (bytes 4-7)
// 3. The next section of the file will contain the index (the index is 1692
// bytes long. It will start at byte 8 and end at byte 1699
// 4. The final section of the file will contain the location records (the
// data). It will start at byte 1700
static {
try {
ra = new RandomAccessFile("locations_rand.dat", "rwd");
int numLocations = ra.readInt();
long locationStartPoint = ra.readInt();
while (ra.getFilePointer() < locationStartPoint) {
int locationId = ra.readInt();
int locationStart = ra.readInt();
int locationLength = ra.readInt();
IndexRecord record = new IndexRecord(locationStart, locationLength);
index.put(locationId, record);
}
} catch (IOException e) {
System.out.println("IOException in static initializer: " + e.getMessage());
}
}
public Location getLocation(int locationId) throws IOException {
IndexRecord record = index.get(locationId);
ra.seek(record.getStartByte());
int id = ra.readInt();
String description = ra.readUTF();
String exits = ra.readUTF();
String[] exitPart = exits.split(",");
Location location = new Location(locationId, description, null);
if (locationId != 0) {
for (int i = 0; i < exitPart.length; i++) {
System.out.println("exitPart = " + exitPart[i]);
System.out.println("exitPart[+1] = " + exitPart[i + 1]);
String direction = exitPart[i];
int destination = Integer.parseInt(exitPart[++i]);
location.addExit(direction, destination);
}
}
return location;
}
#Override
public int size() {
return locations.size();
}
#Override
public boolean isEmpty() {
return locations.isEmpty();
}
#Override
public boolean containsKey(Object key) {
return locations.containsKey(key);
}
#Override
public boolean containsValue(Object value) {
return locations.containsValue(value);
}
#Override
public Location get(Object key) {
return locations.get(key);
}
#Override
public Location put(Integer key, Location value) {
return locations.put(key, value);
}
#Override
public Location remove(Object key) {
return locations.remove(key);
}
#Override
public void putAll(Map<? extends Integer, ? extends Location> m) {
}
#Override
public void clear() {
locations.clear();
}
#Override
public Set<Integer> keySet() {
return locations.keySet();
}
#Override
public Collection<Location> values() {
return locations.values();
}
#Override
public Set<Entry<Integer, Location>> entrySet() {
return locations.entrySet();
}
public void close() throws IOException {
ra.close();
}
}
Location.java
public class Location implements Serializable {
private final int locationID;
private final String description;
private final Map<String, Integer> exits;
private long serialVersionUID = 1L;
public Location(int locationID, String description, Map<String, Integer> exits) {
this.locationID = locationID;
this.description = description;
if (exits != null) {
this.exits = new LinkedHashMap<String, Integer>(exits);
} else {
this.exits = new LinkedHashMap<String, Integer>();
}
this.exits.put("Q", 0);
}
public int getLocationID() {
return locationID;
}
public String getDescription() {
return description;
}
public Map<String, Integer> getExits() {
return new LinkedHashMap<String, Integer>(exits);
}
protected void addExit(String direction, int location) {
exits.put(direction, location);
}
}
It looks like that the error I get, is caused by ra.seek(record.getStartByte()); inside Locations class, at the method getLocation(int locationId);.
Here is the .dat file.

Make an object of a class from a string in java

I have different classes, each receives strings and convert it to different html formats.
Then I have a chooser class where i have switch cases to divert the strings e.g.
i send the strings to chooser and stylename like this
Chooser chooser = new Chooser();
String text = chooser.format(sometext, stylename);
the Chooser class is like this:
public String format(String sometext, String stylename) {
switch (stylename) {
case "NewStyle":
NewStyle ns = new NewStyle();
str = ns.refprocess(sometext);
break;
case "Anotherstyle":
Anotherstyle as = new Anotherstyle();
str = as.refprocess(sometext);
break;
case "Tet_Letters":
Turk_J_Chem tet_letters = new Turk_J_Chem();
str = tet_letters.refprocess(sometext);
break;
}
}
it there any short way? so that when i send the stylename as String, it converts the to Class, make its object and then send the sometext to that class only?

I assume that you have an interface:
static interface Style {
String refprocess(String text);
}
and 3 implemntations:
static class NewStyle implements Style {
#Override
public String refprocess(final String text) {
return "new style of " + text;
}
}
static class Anotherstyle implements Style {
#Override
public String refprocess(final String text) {
return "Another style of " + text;
}
}
static class Turk_J_Chem implements Style {
#Override
public String refprocess(final String text) {
return "Turk_J_Chem's style of " + text;
}
}
Then you can create an enum of classes, and format using the enum:
enum Styles1 {
NEW(NewStyle.class, "NewStyle"),
ANOTHER(Anotherstyle.class, "Anotherstyle"),
TET_LETTERS(Turk_J_Chem.class, "Tet_Letters");
String str;
Class<? extends Style> clazz;
Styles1(final Class<? extends Style> clazz, final String str) {
this.clazz = clazz;
this.str = str;
}
}
static String formatUsingEnum1(final String sometext, final String stylename) {
for (final Styles1 style : Styles1.values()) {
if (style.str.equals(stylename)) {
try {
return style.clazz.newInstance().refprocess(sometext);
} catch (final Exception e) {
break;
}
}
}
throw new NotImplementedException(stylename);
}
Or you can populate the enum with real instances:
enum Styles2 {
NEW(new NewStyle(), "NewStyle"),
ANOTHER(new Anotherstyle(), "Anotherstyle"),
TET_LETTERS(new Turk_J_Chem(), "Tet_Letters");
String str;
Style style;
Styles2(final Style instance, final String str) {
style = instance;
this.str = str;
}
}
static String formatUsingEnum2(final String sometext, final String stylename) {
for (final Styles2 style : Styles2.values()) {
if (style.str.equals(stylename)) {
try {
return style.style.refprocess(sometext);
} catch (final Exception e) {
break;
}
}
}
throw new NotImplementedException(stylename);
}
But you can insert an abstract function into the enum, with inline implementation for all instances:
enum Styles3 {
NewStyle() {
#Override
String refprocess(final String text) {
return "new style of " + text;
}
},
Anotherstyle {
#Override
String refprocess(final String text) {
return "Another style of " + text;
}
},
TET_LETTERS {
#Override
String refprocess(final String text) {
return "Turk_J_Chem's style of " + text;
}
};
abstract String refprocess(String text);
}
static String formatUsingEnum3(final String sometext, final String stylename) {
for (final Styles3 style : Styles3.values()) {
if (style.name().equalsIgnoreCase(stylename)) {
try {
return style.refprocess(sometext);
} catch (final Exception e) {
break;
}
}
}
throw new NotImplementedException(stylename);
}
Enjoy...

Create map of maps from eclipse toString()

Eclipse can auto-generate a toString() method from a object's fields. If those fields are objects then they too may have similarly auto-generated toString() methods.
e.g. a President object might look like this:
President [country=USA, name=Name [title=Mr, forename=Barack, surname=Obama], address=Address [houseNumber=1600, street=Pennsylvania Avenue, town=Washington]]
which is easier to read if I format it:
President [
country=USA,
name=Name [
title=Mr,
forename=Barack,
surname=Obama],
address=Address [
houseNumber=1600,
street=Pennsylvania Avenue,
town=Washington]]
What is the best way to parse this String to create a map of maps?

I've got a solution, but it's not pretty. I was hoping to be able to avoid the low level String manipulation somehow, but here it is:
import java.util.LinkedHashMap;
import java.util.Map;
public class MappedObject {
public String className;
public Map<String, String> leafFields = new LinkedHashMap<>();
public Map<String, MappedObject> treeFields = new LinkedHashMap<>();
#Override
public String toString() {
return "[className=" + className
+ (leafFields.isEmpty() ? "" : ", leafFields=" + leafFields)
+ (treeFields.isEmpty() ? "" : ", treeFields=" + treeFields)
+ "]";
}
public static MappedObject createFromString(String s) {
MappedObject mo = new MappedObject();
new Mapper(s).mapObject(mo);
return mo;
}
private static class Mapper {
private String s;
public Mapper(String s) {
this.s = s;
}
private String mapObject(MappedObject mo) {
mo.className = removeFirstNCharacters(s.indexOf(' '));
while (s.contains("=")) {
removeLeadingNonLetters();
String key = removeFirstNCharacters(s.indexOf('='));
removeFirstNCharacters(1); // remove the =
String leafValue = getLeafValue();
if (leafValue != null) {
mo.leafFields.put(key, leafValue);
if (s.startsWith("]")) { // that was the last field in the tree
return s;
}
} else {
MappedObject treeField = new MappedObject();
mo.treeFields.put(key, treeField);
s = new Mapper(s).mapObject(treeField);
}
}
return s; // s contains only close brackets - ]
}
private void removeLeadingNonLetters() {
int i = 0;
while (!Character.isLetter(s.charAt(i))) {
i++;
}
removeFirstNCharacters(i);
}
private String removeFirstNCharacters(int n) {
String value = s.substring(0, n);
s = s.substring(value.length());
return value;
}
private String getLeafValue() {
int endIndex = getEndIndex();
if (!s.contains("[") || s.indexOf('[') > endIndex) {
return removeFirstNCharacters(endIndex);
}
return null;
}
/** The end of the value, if it's a leaf field. */
private int getEndIndex() {
if(s.contains(",")) {
return Math.min(s.indexOf(','), s.indexOf(']'));
}
return s.indexOf(']');
}
}
}

Java XML parsing error with Jackson

I am trying to convert this xml :
<resources>
<string name="string_one">one</string>
<string name="string_two">two</string>
<string name="test2">test2</string>
<plurals name="plurals_one">
<item quantity="one">one</item>
<item quantity="other">other</item>
</plurals>
<string name="test1">test1</string>
</resources>
to something like this:
AndroidTextResource{test1, test1}
AndroidTextResource{plurals_one#PLURAL#one, one}
AndroidTextResource{plurals_one#PLURAL#other, other}
AndroidTextResource{string_one, one}
AndroidTextResource{string_two, two}
The problem is it doesn't parse anything after the "plurals" tag.
That is what I get:
AndroidTextResource{test1, test1}
AndroidTextResource{plurals_one#PLURAL#one, one}
AndroidTextResource{plurals_one#PLURAL#other, other}
Here is my code:
#JacksonXmlRootElement(localName="resources")
public class AndroidTextResources {
#JsonPropertyOrder({"key", "value"})
public static class MyClass {
#JacksonXmlProperty(isAttribute=true, localName="name")
public String key;
#JacksonXmlText
public String value;
public AndroidTextResource() {
}
public AndroidTextResource(final String key, final String value) {
this.key = key;
this.value = value;
}
#Override
public int hashCode() {
return Objects.hashCode(
key,
value);
}
#Override
public boolean equals(final Object other) {
if (other == this) { return true; }
if (other == null) { return false; }
if (! (other instanceof AndroidTextResource)) { return false; }
final AndroidTextResource that = (AndroidTextResource)other;
return
equal(this.key, that.key) &&
equal(this.value, that.value)
;
}
#Override
public String toString() {
return Objects.toStringHelper(getClass().getSimpleName())
.addValue(key)
.addValue(value)
.toString();
}
}
public static class AndroidPlural {
public static class AndroidPluralItem {
#JacksonXmlProperty(isAttribute=true)
public String quantity;
#JacksonXmlText
public String value;
public AndroidPluralItem() {}
public AndroidPluralItem(final String quantity, final String value) {
this.quantity = quantity;
this.value = value;
}
}
#JacksonXmlProperty(isAttribute=true, localName="name")
public String key;
#JacksonXmlElementWrapper(useWrapping=false)
#JacksonXmlProperty(isAttribute=false, localName="item")
public List<AndroidPluralItem> items = new ArrayList<MyClass.AndroidPlural.AndroidPluralItem>();
public AndroidPlural() {}
public AndroidPlural(final String key) {
this.key = key;
}
public List<AndroidTextResource> toTextResources() {
final List<AndroidTextResource> result = new ArrayList<MyClass.AndroidTextResource>(items.size());
for (final AndroidPluralItem item : items) {
final AndroidTextResource res = new AndroidTextResource();
res.key = key + "#PLURAL#" + item.quantity;
res.value = item.value;
result.add(res);
}
return result;
}
}
#JacksonXmlElementWrapper(useWrapping=false)
#JacksonXmlProperty(isAttribute=false, localName="string")
public List<AndroidTextResource> textResources = new ArrayList<MyClass.AndroidTextResource>();
#JacksonXmlElementWrapper(useWrapping=false)
#JacksonXmlProperty(isAttribute=false, localName="plurals")
public List<AndroidPlural> plurals = new ArrayList<MyClass.AndroidPlural>();
}
public class AndroidResourceToCsv {
public static void main(final String[] args) throws Exception {
if (args.length != 2) {
System.out
.println("You need to specify exactly one input and one output file.");
System.exit(-1);
}
final File inputFile = new File(args[0]);
final File outputFile = new File(args[1]);
InputStream inputStream = null;
try {
inputStream = new FileInputStream(inputFile);
} finally { inputStream.close(); }
final List<AndroidTextResource> allResources = readCsv(inputStream);
outputFile.getParentFile().mkdirs();
final CsvMapper csvMapper = new CsvMapper();
final CsvSchema schema = csvMapper.schemaFor(AndroidTextResource.class)
.withColumnSeparator(';');
final ObjectWriter writer = csvMapper.writer().withSchema(schema);
writer.writeValue(outputFile, allResources);
// TODO: parse three times, for string, plural & string array, ignore
// other values
}
public static List<AndroidTextResource> readCsv(final InputStream in)
throws JsonParseException, JsonMappingException, IOException {
final ObjectMapper xmlMapper = new XmlMapper();
final MyClass = xmlMapper.readValue(in,
MyClass.class);
final List<AndroidTextResource> allResources = new ArrayList<MyClass.AndroidTextResource>();
allResources.addAll(resources.textResources);
for (final AndroidPlural plural : resources.plurals) {
allResources.addAll(plural.toTextResources());
}
return allResources;
}
}
Does anyone knows what is wrong??Thanks in advance!

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.