I am having an Arraylist of Objects. Those object have an attribute or datatype - 'String'.
I need to sort the Arraylist by that string. How to achieve this?
You need to write a Comparator<MyObject> and use Collections.sort(List<T>, Comparator<? super T> to sort your List.
Or else, your MyObject can also implements Comparable<MyObject>, defining a natural ordering that compares on your specific attribute, and then use Collections.sort(List<T> instead.
See also
Java Tutorials/Object Ordering
Related questions
On sorting List on various criteria:
Sorting an ArrayList of Contacts
On Comparator and Comparable
When to use Comparable vs Comparator
difference between compare() and compareTo()
Comparable and Comparator contract with regards to null
Why does the Java Collections Framework offer two different ways to sort?
Another good way of doing this that is a bit more flexible if there is more than one property of an object that you may wish to sort by is to use Guava's Ordering class with its onResultOf(Function) option. This is ideally suited for sorting by properties since a Function can be used to retrieve and return a specific property of an object.
For a simple example, imagine a class Person with String getFirstName() and String getLastName() methods.
List<Person> people = ...;
Collections.sort(people, Ordering.natural().onResultOf(
new Function<Person, String>() {
public String apply(Person from) {
return from.getFirstName();
}
}));
The above will sort the list by first name.
To make it read nicer, you may want to define the functions you might want to use as public static final fields on the Person class. Then you could sort by last name like this:
Collections.sort(people, Ordering.natural().onResultOf(Person.GET_LAST_NAME));
As a fun aside note, this will all be a lot easier in Java 8 with lambda expressions and method references. You'll be able to write something like this without having to define any clumsy anonymous inner classes or static final fields:
import static java.util.Comparator.comparing;
...
people.sort(comparing(Person::getLastName));
Related
I want to declare an ArrayList with multiple types, to be more clear to do as the following :
public ArrayList<Integer, String, Boolean> a = new ArrayList<Integer, String, Boolean>();
I know that example will not work because ArrayList can be parameterized with only one type argument.
The HashMap can allow me to use two arguments, but that's not enough in my case.
I know that I can declare a class with multiple attributes and use it as a parameter in my ArrayList, but I don't want this method.
Isn't there any method to declare a List with multiple types, or to use some internal class to do that?
You can create it like this
public class Tuple<A, B, C> {
public A First;
public B Second;
public C Third;
}
and you declare it like this
List< Tuple <Integer, String, Bool> listOfTuple;
or check this out javatuples http://www.javatuples.org for Tuples from 1-10 Params
The Table structure in Guava seems to meet all of your needs nicely.
You can use polymorphism, create an interface lets say ListElement. All the objects you want to add to this list must implement this interface. Then just use an ArrayList of ListElement
The other possibility (since you are using classes like Integer) you can use a ArrayList of Object, since all objects extend the class Object.
I have a variant-style object foo that is capable of behaving as a java.util.Map and a java.util.List as well as other plain-old-data types. This object is written in C++ (modelled on the composite pattern) and I'm building a JNI so that I can use it in Java.
In Java, I'm motivated to write
public class foo implements
Streamable,
java.util.Map<String, foo>,
java.util.List<foo>
Then I encounter trouble. For example, I need to implement 3 flavors of remove:
public foo remove(int index)
public boolean remove(Object key)
public foo remove(Object key)
The first two are for java.util.list, the final one for java.util.map. This, of course, is a problem since you cannot have two functions with the same name and parameters but different return types.
Is there a way round this?
An adapter would work. Have one class implement Map and Stream, and another class implement List and Stream. All operations required by these adapter's respective interfaces would draw from a common underlying foo instance.
You could use a LinkedHashMap.
Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries.
Please refer to this question as it discusses the same problem you're having.
Going through docjar for String, I happened to see the following piece of code:
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
private static class CaseInsensitiveComparator
implements Comparator<String>, java.io.Serializable {
// use serialVersionUID from JDK 1.2.2 for interoperability
private static final long serialVersionUID = 8575799808933029326L;
public int compare(String s1, String s2) {
// ...
}
}
My question is, why can't we just implement Comparator, like Comparable, and use a private method rather than a nested class?
Also on a side note, why doesn't Comparator have a method with a single parameter similar to compareTo in Comparable?
Because a String is not a Comparator. Sure, they are indeed Comparable, but they are not "comparison functions"1 themselves: it makes no sense to have String implement Comparator.
On the other hand, CaseInsensitiveComparator is a specific comparison function, only pertaining to strings. So, it is declared as a static nested class.
1 See Comparator
My question is why cannot we just implement comparator, just like comparable and use a private function rather than inner class ?
Well (pretty much) the whole point of the Comparator interface is that an implementation of the interface is a separate class to the class of the objects being compared.
In theory you could do what you are suggesting, but the net result is counter-intuitive
public class MyKey implements Comparator<MyKey> {
private String field;
public boolean compare(MyKey m1, MyKey m2) {
// We must ignore this.field! We are comparing m1 and m2 ...
return m1.field.compareTo(m2.field);
}
}
MyKey[] keys = ...
Arrays.sort(keys, new MyKey()); // Note we have to pass an instance
// to provide the Comparator.
In addition to being a bit counter-intuitive, your idea is limited in the sense that MyKey can only "provide" one comparator this way.
Frankly, if you are going to do this, it makes more sense to have MyKey implement Comparable<MyKey>.
Suppose that they had implemented String the way you proposed. Then this ...
String[] strings = new String[]{"a", "c", "B"};
Arrays.sort(strings);
... means sort case sensitive, but ...
String[] strings = new String[]{"a", "c", "B"};
Arrays.sort(strings, "weasel");
... would mean sort case insensitive. Does that really strike you as a good idea? Really?
You can only implement one interface of the same type. String already implements Comparable for lexicographical comparison:
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence
{
But it needs more compareTo method (eg: to perform case insensitive comparison). Following would give you compiler error:
public final class String
implements java.io.Serializable, Comparable<String>, Comparable<String>, CharSequence
{
Hence this is probably one reason it has additional comparator.
The idea of comparator is an object that provide comparison service between two items, not a contract where one is comparable to another (comparable interface)
Implementing Comparator on a String will compile, but this will be semantically wrong
public final class String
implements java.io.Serializable, Comparable<String>, Comparator<String>, CharSequence
{
Having this be a class rather than a function allows us to polymorphically apply different comparison strategies to Collections (or other classes which may be performing comparisons). This way the method using the comparator doesn't need to know whether it's a case-sensitive comparator or a case-insensitive comparator or something else; it just performs the comparison. If this were a function however, such polymorphism would not apply. The method performing the comparison would either have to use "the" comparison method, or would have to know what kind of comparison was being performed, in order to select the correct method.
Having two parameters rather than tying the first parameter to one particular String allows us to use the same comparator on any number of Strings. Otherwise we'd have to keep track of a lot of comparators when dealing with large collections. It also allows usage of this comparator with either a subclass on either the left side or the right side (or both); with just one parameter there would not be that flexibility.
The primary reason is to expose the Comparator object for things like TreeMap, where Strings can be used as keys, or other objects where ordering or a different kind of equality is useful. It can't be a static method since the Comparator interface can't specify a static method and the Comparator has to be an object that implements the interface to be used in classes like TreeMap. We need a separate one that's case insensitive because the default comparison (implemented by Comparable's methods) is already taken by the case sensitive comparison.
why cannot we just implement comparator, just like comparable and use
a private function rather than inner class
String class implements Comparable<String> interface, which is used to compare two strings lexicographically, which is a pretty common way to compare two strings.
However, sometimes you need to compare two strings in different way, for example comparing them while ignoring case sensitivity. Comparator interface provides a way to compare two objects differently. In this case, CaseInsensitiveComparator is implemented to provide this capability and is used in String.compareToIgnoreCase() method.
For anyone who might have a question like this, you probably need "Collections.sort", not "Collection.sort", the mistake I made below.
I have defined a class defined as
public class Store implements Serializable, Comparable<Store> { ... }
I have a field in another class defined as:
ArrayList<Store> fStores = new ArrayList<Store>();
I want to sort this collection, so in a method I call:
Collection.sort(fStores);
However, I get the following compilation error:
The method sort(ArrayList<Store>) is undefined for the type Collection
ArrayList implements List, and from the documentation:
public static <T extends Comparable<? super T>> void sort(List<T> list)
So, why do I get the error? I have also tried creating my own descendant of Comparator and passing that to the sort method with no luck.
I'm guessing there's something about "< T extends Comparable< ? super T > >" I'm not understanding... ?
There are basically 2 things that you need to look at :
Collections
From the Collections
This class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, "wrappers", which return a new collection backed by a specified collection, and a few other odds and ends
So basically if you have to sort or do any such kind of algorithms use this.
Next is :->
Collection
This is an interface that provides the basis of Java's collection framework. It does not include Map and Sorted Map. It represents a group of objects known as its elements and has implementations for concrete implementations. You need to think of this when you want to work with ArrayLists and Maps.
So, bottom line, you have a static algorithm to run which is present in Collections. So, use Collections.sort
You need to write Collections instead of Collection. They're related, but different. :-)
It's Collection**s**, not Collection:
http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html
vs.
http://docs.oracle.com/javase/6/docs/api/java/util/Collection.html
Did you mean Collections.sort() (collections plural)?
It's Collections.sort() with an s.
import java.util.Collections and you should be fine.
Your issue is that you are calling Collection.sort(), not Collections.sort()
These are two separate classes:
Collections
Collection
Collection is an interface while Collections is a utility class.
Also, if you are using eclipse, double check that the import statement for Collections is the right one!
In case you got here due to a similar error:
The method sort(Comparator<...>) is undefined for the type List<...>
Then you're probably using JDK lower then 1.8. Change it in Java Build Path:
What is the difference between Java's compare() and compareTo() methods? Do those methods give same answer?
From JavaNotes:
a.compareTo(b):
Comparable interface : Compares values and returns an int which tells if the values compare less than, equal, or greater than.
If your class objects have a natural order, implement the Comparable<T> interface and define this method. All Java classes that have a natural ordering implement Comparable<T> - Example: String, wrapper classes, BigInteger
compare(a, b):
Comparator interface : Compares values of two objects. This is implemented as part of the Comparator<T> interface, and the typical use is to define one or more small utility classes that implement this, to pass to methods such as sort() or for use by sorting data structures such as TreeMap and TreeSet. You might want to create a Comparator object for the following:
Multiple comparisons. To provide several different ways to sort something. For example, you might want to sort a Person class by name, ID, age, height, ... You would define a Comparator for each of these to pass to the sort() method.
System class To provide comparison methods for classes that you have no control over. For example, you could define a Comparator for Strings that compared them by length.
Strategy pattern To implement a Strategy pattern, which is a situation where you want to represent an algorithm as an object that you can pass as a parameter, save in a data structure, etc.
If your class objects have one natural sorting order, you may not need compare().
Summary from http://www.digizol.com/2008/07/java-sorting-comparator-vs-comparable.html
Comparable
A comparable object is capable of comparing itself with another object.
Comparator
A comparator object is capable of comparing two different objects. The class is not comparing its instances, but some other class’s instances.
Use case contexts:
Comparable interface
The equals method and == and != operators test for equality/inequality, but do not provide a way to test for relative values.
Some classes (eg, String and other classes with a natural ordering) implement the Comparable<T> interface, which defines a compareTo() method.
You will want to implement Comparable<T> in your class if you want to use it with Collections.sort() or Arrays.sort() methods.
Defining a Comparator object
You can create Comparators to sort any arbitrary way for any class.
For example, the String class defines the CASE_INSENSITIVE_ORDER comparator.
The difference between the two approaches can be linked to the notion of:
Ordered Collection:
When a Collection is ordered, it means you can iterate in the collection in a specific (not-random) order (a Hashtable is not ordered).
A Collection with a natural order is not just ordered, but sorted. Defining a natural order can be difficult! (as in natural String order).
Another difference, pointed out by HaveAGuess in the comments:
Comparable is in the implementation and not visible from the interface, so when you sort you don't really know what is going to happen.
Comparator gives you reassurance that the ordering will be well defined.
compareTo() is from the Comparable interface.
compare() is from the Comparator interface.
Both methods do the same thing, but each interface is used in a slightly different context.
The Comparable interface is used to impose a natural ordering on the objects of the implementing class. The compareTo() method is called the natural comparison method. The Comparator interface is used to impose a total ordering on the objects of the implementing class. For more information, see the links for exactly when to use each interface.
Similarities:
Both are custom ways to compare two objects.
Both return an int describing the relationship between two objects.
Differences:
The method compare() is a method that you are obligated to implement if you implement the Comparator interface. It allows you to pass two objects into the method and it returns an int describing their relationship.
Comparator comp = new MyComparator();
int result = comp.compare(object1, object2);
The method compareTo() is a method that you are obligated to implement if you implement the Comparable interface. It allows an object to be compared to objects of similar type.
String s = "hi";
int result = s.compareTo("bye");
Summary:
Basically they are two different ways to compare things.
The methods do not have to give the same answers. That depends on which objects/classes you call them.
If you are implementing your own classes which you know you want to compare at some stage, you may have them implement the Comparable interface and implement the compareTo() method accordingly.
If you are using some classes from an API which do not implement the Comparable interface, but you still want to compare them. I.e. for sorting. You may create your own class which implements the Comparator interface and in its compare() method you implement the logic.
Using Comparator, we can have n number of comparison logic written for a class.
E.g.
For a Car Class
We can have a Comparator class to compare based on car model number. We can also have a Comparator class to compare based on car model year.
Car Class
public class Car {
int modelNo;
int modelYear;
public int getModelNo() {
return modelNo;
}
public void setModelNo(int modelNo) {
this.modelNo = modelNo;
}
public int getModelYear() {
return modelYear;
}
public void setModelYear(int modelYear) {
this.modelYear = modelYear;
}
}
Comparator #1 based on Model No
public class CarModelNoCompartor implements Comparator<Car>{
public int compare(Car o1, Car o2) {
return o1.getModelNo() - o2.getModelNo();
}
}
Comparator #2 based on Model Year
public class CarModelYearComparator implements Comparator<Car> {
public int compare(Car o1, Car o2) {
return o1.getModelYear() - o2.getModelYear();
}
}
But this is not possible with the case of Comparable interface.
In case of Comparable interface, we can have only one logic in compareTo() method.
Comparable interface contains a method called compareTo(obj) which takes only one argument and it compares itself with another instance or objects of the same class.
Comparator interface contains a method called compare(obj1,obj2) which takes two arguments and it compares the value of two objects from the same or different classes.
compareTo(T object)
comes from the java.lang.Comparable interface, implemented to compare this object with another to give a negative int value for this object being less than, 0 for equals, or positive value for greater than the other. This is the more convenient compare method, but must be implemented in every class you want to compare.
compare(T obj1, T obj2)
comes from the java.util.Comparator interface, implemented in a separate class that compares another class's objects to give a negative int value for the first object being less than, 0 for equals, or positive value for greater than the second object. It is needed when you cannot make a class implement compareTo() because it is not modifiable. It is also used when you want different ways to compare objects, not just one (such as by name or age).
The relationship of the object having this method and its collaborators is different.
compareTo() is a method of the interface Comparable, so it is used to compare THIS instance to another one.
compare() is a method of the interface Comparator, so it is used to compare two different instances of another class with each other.
If you will, implementing Comparable means that instances of the class can be easily compared.
Implementing Comparator means, that instances are suited to compare different objects (of other classes).
The main difference is in the use of the interfaces:
Comparable (which has compareTo()) requires the objects to be compared (in order to use a TreeMap, or to sort a list) to implement that interface. But what if the class does not implement Comparable and you can't change it because it's part of a 3rd party library? Then you have to implement a Comparator, which is a bit less convenient to use.
compareTo() is called on one object, to compare it to another object.
compare() is called on some object to compare two other objects.
The difference is where the logic that does actual comparison is defined.
One more point:
Comparable is used to define a default ordering for objects within a class
Comparator is used to define a custom ordering to be passed to a method.
comparator-vs-comparable
When you want to sort a List which include the Object Foo, the Foo class has to implement the Comparable interface, because the sort methode of the List is using this methode.
When you want to write a Util class which compares two other classes you can implement the Comparator class.
Employee Table
Name, DoB, Salary
Tomas , 2/10/1982, 300
Daniel , 3/11/1990, 400
Kwame , 2/10/1998, 520
The Comparable interface allows you to sort a
list of objects eg Employees with reference to one primary field – for
instance, you could sort by name or by salary with the CompareTo() method
emp1.getName().compareTo(emp2.getName())
A more flexible interface for such requirements is provided by
the Comparator interface, whose only method is compare()
public interface Comparator<Employee> {
int compare(Employee obj1, Employee obj2);
}
Sample code
public class NameComparator implements Comparator<Employee> {
public int compare(Employee e1, Employee e2) {
// some conditions here
return e1.getName().compareTo(e2.getName()); // returns 1 since (T)omas > (D)an
return e1.getSalary().compareTo(e2.getSalary()); // returns -1 since 400 > 300
}
}
There is a technical aspect that should be emphasized, too. Say you need comparison behavior parameterization from a client class, and you are wondering whether to use Comparable or Comparator for a method like this:
class Pokemon {
int healthPoints;
int attackDamage;
public void battle (Comparable<Pokemon> comparable, Pokemon opponent) {
if (comparable.compareTo(opponent) > 0) { //comparable needs to, but cannot, access this.healthPoints for example
System.out.println("battle won");
} else {
System.out.println("battle lost");
}
}
}
comparable would a lambda or an object, and there is no way for comparable to access the fields of this Pokemon. (In a lambda, this refers to the outer class instance in the lambda's scope, as defined in the program text.) So this doesn't fly, and we have to use a Comparator with two arguments.
Use Comparable interface for sorting on the basis of more than one value like age,name,dept_name...
For one value use Comparator interface
Important Answar
String name;
int roll;
public int compare(Object obj1,Object obj2) { // For Comparator interface
return obj1.compareTo(obj1);
}
public int compareTo(Object obj1) { // For Comparable Interface
return obj1.compareTo(obj);
}
Here in return obj1.compareTo(obj1) or return obj1.compareTo(obj) statement
only take Object; primitive is not allowed.
For Example
name.compareTo(obj1.getName()) // Correct Statement.
But
roll.compareTo(obj1.getRoll())
// Wrong Statement Compile Time Error Because roll
// is not an Object Type, it is primitive type.
name is String Object so it worked.
If you want to sort roll number of student than use below code.
public int compareTo(Object obj1) { // For Comparable Interface
Student s = (Student) obj1;
return rollno - s.getRollno();
}
or
public int compare(Object obj1,Object obj2) { // For Comparator interface
Student s1 = (Student) obj1;
Student s2 = (Student) obj2;
return s1.getRollno() - s2.getRollno();
}