How to make two objects in Java comparable using "<" or ">"
e.g.
MyObject<String> obj1= new MyObject<String>(“blablabla”, 25);
MyObject<String> obj2= new MyObject<String>(“nannaanana”, 17);
if (obj1 > obj2)
do something.
I've made MyObject class header as
public class MyObject<T extends Comparable<T>> implements Comparable<MyObject<T>>
and created method Comp but all the gain I got is now I can use "sort" on the list of objects, but how can I compare two objects to each other directly? Is
if(obj1.compareTo(obj2) > 0)
do something
the only way?
You cannot do operator overloading in Java. This means you are not able to define custom behaviors for operators such as +, >, <, ==, etc. in your own classes.
As you already noted, implementing Comparable and using the compareTo() method is probably the way to go in this case.
Another option is to create a Comparator (see the docs), specially if it doesn't make sense for the class to implement Comparable or if you need to compare objects from the same class in different ways.
To improve the code readability you could use compareTo() together with custom methods that may look more natural. For example:
boolean isGreaterThan(MyObject<T> that) {
return this.compareTo(that) > 0;
}
boolean isLessThan(MyObject<T> that) {
return this.compareTo(that) < 0;
}
Then you could use them like this:
if (obj1.isGreaterThan(obj2)) {
// do something
}
Using Comparable.compareTo(T) is the only option (or Comparator). The interface only defines that one method (while Comparator adds equals), and it compares this object with the specified object for order. Further, Java does not permit operator overloading (so you won't be able to directly change the operand used for invoking that method; or in fact modify the interface).
It is not the only way. You can implement a Comparator as well. Comparator uses compare() method as oppose to Comparable which uses compareTo() method.
The reason you can't use > or < to compare objects directly is because Java won't know which variable you want to use for the comparison (as there might exist more than one variable in the object).
In order to compare objects, those objects must be comparable. You need to define and tell Java how you want to compare them.
Java collection provides a sort method. However some school does give assignment of asking you to write you own sort methods which ultimately still uses the compareTo() for comparison.
You can take a look on the subtle differences between Comparable vs Comparator here: What is the difference between compare() and compareTo()?
I think it is also worth mentioning that, by default Java compares String (objects) in a lexicographical order if you did not override the compareTo() method.
I would advocated that readability must be a primer for us as developers.
Apache Commons Lang (commons-lang) provides a simple fluent utility which reads a lot clearer:
if (is(obj1).greaterThan(obj2)) {
// do something
}
Note: is is shorthand for ComparableUtils.is which can be imported the following this static import statement:
import static org.apache.commons.lang3.compare.ComparableUtils.is;
I want to use a HashSet to store some objects:
public class StoredObject{
Type type; //Type is an enum
//other fields
Type getType(){return type;}
}
Now, I want to store only one StoredObject of the same Type, so I override contains() in a subclass of HashSet:
public MySet<E extends StoredObject> extends java.util.HashSet<E>{
#Override
public boolean contains(Object o) {
if(StoredObject.class.isAssignableFrom(o.getClass())) {//if o implements StoredObject
for(StoredObject s : this) {
if(s.getType() == ((StoredObject) o).getType()) return true;
}
}
return false
}
}
Before this I wanted to use HashSet and modify the equals() of StoredObject. However, the way above seems like a shorter and safer way, especially as in my case the stored objects all implement an interface and don't extend the same class.
Now my question: Is this implementation safe? I tried to search for things it could break, but did not find any. I read that overriding equals() can break Collections.
Also, does this subclass defeats the purpose of an HashSet, since it does not use the HashMap for contains()?
HashMap<Type,StoredObject> is the appropriate collection for this.
If you override equals(Object) then you must also override hashCode (it's also not a bad idea to make it implement Comparable and perhaps override toString). Use the #Override annotation to ensure you have the right parameter types and spelling - very easy to get wrong and confusing to debug.
What can go wrong?
There's a lot of methods in HashSet to override, so that's a lot of work.
More methods may be added to HashSet in future versions of Java - how are you going to look out for this?
contains should be an O(1) operation (assuming a good distribution of hash codes), but the OP implementation is O(n).
Set.equals on another Set will report incorrect results.
Note also that StoredObject.class.isAssignableFrom(o.getClass()) is better written as o instanceof StoredObject (assuming you've got isAssignableFrom the right way around).
Is this implementation safe?
Absolutely not. There are other methods on HashSet that wouldn't work correctly, e.g. add(), leaving the size of the set incorrect.
Besides, that implementation would totally ruin the performance of the contains method, making it run in O(n) instead of O(1).
If you need a Set with a definition of equality that differs from the objects natural definition as implemented by equals() and hashCode(), use a TreeSet and supply a custom Comparator.
class MySet<E extends StoredObject> extends java.util.TreeSet<E> {
public MySet() {
super(Comparator.comparing(StoredObject::getType));
}
}
I do agree with Tom Hawtin - tackline, that HashMap<Type, StoredObject> is a better option, because it allows you to get the StoredObject for a given Type, which is otherwise very difficult to do with a Set. It also allows you to check for existence given just a Type, without having to create a dummy StoredObject object for the check.
Wondering what needs to be done for listed method
public final int compareTo(final FieldDTO o) {
return o.available.compareTo(this.available);
its throwing exception on line 2 stating
Bad practice - Class defines compareTo(...) and uses Object.equals() 16 days
field defines compareTo(FieldDTO) and uses Object.equals()
Not sure how should i handle this.
Thanks in advance.
If you define compareTo you should at least define equals
boolean equals(it) {
return compareTo(it) == 0;
}
otherwise you will get strange problems when you put your object in Maps and Sets. It is generally good practice to also define hashCode.
you need to override Object class equals() and hashCode() methods.
Use IDE generated code for these, it will pull all the Object attributes and creates method for you.
On eclipse IDE:
Right click on the class
Select Source
Generate hashCode() and equals()...
This is the documentation from FindBugs:
Eq: Class defines compareTo(...) and uses Object.equals()
(EQ_COMPARETO_USE_OBJECT_EQUALS)
This class defines a compareTo(...) method but inherits its equals()
method from java.lang.Object. Generally, the value of compareTo should
return zero if and only if equals returns true. If this is violated,
weird and unpredictable failures will occur in classes such as
PriorityQueue. In Java 5 the PriorityQueue.remove method uses the
compareTo method, while in Java 6 it uses the equals method.
From the JavaDoc for the compareTo method in the Comparable interface:
It is strongly recommended, but not strictly required that
(x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any class
that implements the Comparable interface and violates this condition
should clearly indicate this fact. The recommended language is "Note:
this class has a natural ordering that is inconsistent with equals."
So it seems you need to implement the equals method thus overriding the default implementation from Object.
Say I have my own class
public class MyObj { /* ... */ }
It has some attributes and methods. It DOES NOT implement equals, DOES NOT implement hashCode.
Once we call equals and hashCode, what are the default implementations? From Object class? And what are they? How the default equals will work? How the default hashCode will work and what will return? == will just check if they reference to the same object, so it's easy, but what about equals() and hashCode() methods?
Yes, the default implementation is Object's (generally speaking; if you inherit from a class that redefined equals and/or hashCode, then you'll use that implementation instead).
From the documentation:
equals
The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns
true if and only if x and y refer to the same object (x == y has the value true).
hashCode
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)
From Object in one of the JVM implementations:
public boolean equals(Object object) {
return this == object;
}
public int hashCode() {
return VMMemoryManager.getIdentityHashCode(this);
}
In both cases it's just comparing the memory addresses of the objects in question.
There are default implementations of equals() and hashCode() in Object. If you don't provide your own implementation, those will be used. For equals(), this means an == comparison: the objects will only be equal if they are exactly the same object. For hashCode(), the Javadoc has a good explanation.
For more information, see Effective Java, Chapter 3 (pdf), item 8.
Yes, from Object class since your class extends Object implicitly. equals simply returns this == obj. hashCode implementation is native. Just a guess - it returns the pointer to the object.
If you do not provide your own implementation, one derived from Object would be used. It is OK, unless you plan to put your class instances into i.e. HashSet (any collection that actually use hashCode() ), or something that need to check object's equality (i.e. HashSet's contains() method). Otherwise it will work incorrectly, if that's what you are asking for.
It is quite easy to provide your own implementation of these methods thanks to HashCodeBuilder and EqualsBuilder from Apache Commons Lang.
IBM's developerworks says:
Under this default implementation, two
references are equal only if they
refer to the exact same object.
Similarly, the default implementation
of hashCode() provided by Object is
derived by mapping the memory address
of the object to an integer value.
However, to be sure of the exact implementation details for a particular vendor's Java version it's probably best to look as the source (if it's available)
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();
}