Comparator function in java

Comparator function in java

A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort ) to allow precise control over the sort order. Comparators can also be used to control the order of certain data structures (such as sorted sets or sorted maps ), or to provide an ordering for collections of objects that don’t have a natural ordering . The ordering imposed by a comparator c on a set of elements S is said to be consistent with equals if and only if c.compare(e1, e2)==0 has the same boolean value as e1.equals(e2) for every e1 and e2 in S . Caution should be exercised when using a comparator capable of imposing an ordering inconsistent with equals to order a sorted set (or sorted map). Suppose a sorted set (or sorted map) with an explicit comparator c is used with elements (or keys) drawn from a set S . If the ordering imposed by c on S is inconsistent with equals, the sorted set (or sorted map) will behave «strangely.» In particular the sorted set (or sorted map) will violate the general contract for set (or map), which is defined in terms of equals . For example, suppose one adds two elements a and b such that (a.equals(b) && c.compare(a, b) != 0) to an empty TreeSet with comparator c . The second add operation will return true (and the size of the tree set will increase) because a and b are not equivalent from the tree set’s perspective, even though this is contrary to the specification of the Set.add method. Note: It is generally a good idea for comparators to also implement java.io.Serializable , as they may be used as ordering methods in serializable data structures (like TreeSet , TreeMap ). In order for the data structure to serialize successfully, the comparator (if provided) must implement Serializable . For the mathematically inclined, the relation that defines the imposed ordering that a given comparator c imposes on a given set of objects S is:

It follows immediately from the contract for compare that the quotient is an equivalence relation on S , and that the imposed ordering is a total order on S . When we say that the ordering imposed by c on S is consistent with equals, we mean that the quotient for the ordering is the equivalence relation defined by the objects’ equals(Object) method(s):

Unlike Comparable , a comparator may optionally permit comparison of null arguments, while maintaining the requirements for an equivalence relation. This interface is a member of the Java Collections Framework.

Method Summary

Accepts a function that extracts a Comparable sort key from a type T , and returns a Comparator that compares by that sort key.

Accepts a function that extracts a sort key from a type T , and returns a Comparator that compares by that sort key using the specified Comparator .

Accepts a function that extracts a double sort key from a type T , and returns a Comparator that compares by that sort key.

Accepts a function that extracts an int sort key from a type T , and returns a Comparator that compares by that sort key.

Accepts a function that extracts a long sort key from a type T , and returns a Comparator that compares by that sort key.

Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator .

Method Detail

compare

Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x)) for all x and y . (This implies that compare(x, y) must throw an exception if and only if compare(y, x) throws an exception.) The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0 . Finally, the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z . It is generally the case, but not strictly required that (compare(x, y)==0) == (x.equals(y)) . Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is «Note: this comparator imposes orderings that are inconsistent with equals.» In the foregoing description, the notation sgn( expression ) designates the mathematical signum function, which is defined to return one of -1 , 0 , or 1 according to whether the value of expression is negative, zero, or positive, respectively.

equals

Indicates whether some other object is «equal to» this comparator. This method must obey the general contract of Object.equals(Object) . Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. Thus, comp1.equals(comp2) implies that sgn(comp1.compare(o1, o2))==sgn(comp2.compare(o1, o2)) for every object reference o1 and o2 . Note that it is always safe not to override Object.equals(Object) . However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct comparators impose the same order.

reversed

thenComparing

default ComparatorT> thenComparing​(ComparatorT> other)

Returns a lexicographic-order comparator with another comparator. If this Comparator considers two elements equal, i.e. compare(a, b) == 0 , other is used to determine the order. The returned comparator is serializable if the specified comparator is also serializable.

API Note: For example, to sort a collection of String based on the length and then case-insensitive natural ordering, the comparator can be composed using following code,

 Comparator cmp = Comparator.comparingInt(String::length) .thenComparing(String.CASE_INSENSITIVE_ORDER); 

thenComparing

default ComparatorT> thenComparing​(FunctionT,​? extends U> keyExtractor, Comparator keyComparator)

Returns a lexicographic-order comparator with a function that extracts a key to be compared with the given Comparator .

thenComparing

default Comparable> ComparatorT> thenComparing​(FunctionT,​? extends U> keyExtractor)

thenComparingInt

default ComparatorT> thenComparingInt​(ToIntFunctionT> keyExtractor)

thenComparingLong

default ComparatorT> thenComparingLong​(ToLongFunctionT> keyExtractor)

thenComparingDouble

default ComparatorT> thenComparingDouble​(ToDoubleFunctionT> keyExtractor)

reverseOrder

Returns a comparator that imposes the reverse of the natural ordering. The returned comparator is serializable and throws NullPointerException when comparing null .

naturalOrder

Returns a comparator that compares Comparable objects in natural order. The returned comparator is serializable and throws NullPointerException when comparing null .

nullsFirst

static Comparator nullsFirst​(Comparator comparator)

Returns a null-friendly comparator that considers null to be less than non-null. When both are null , they are considered equal. If both are non-null, the specified Comparator is used to determine the order. If the specified comparator is null , then the returned comparator considers all non-null values to be equal. The returned comparator is serializable if the specified comparator is serializable.

nullsLast

static Comparator nullsLast​(Comparator comparator)

Returns a null-friendly comparator that considers null to be greater than non-null. When both are null , they are considered equal. If both are non-null, the specified Comparator is used to determine the order. If the specified comparator is null , then the returned comparator considers all non-null values to be equal. The returned comparator is serializable if the specified comparator is serializable.

comparing

static Comparator comparing​(Function keyExtractor, Comparator keyComparator)

Accepts a function that extracts a sort key from a type T , and returns a Comparator that compares by that sort key using the specified Comparator . The returned comparator is serializable if the specified function and comparator are both serializable.

API Note: For example, to obtain a Comparator that compares Person objects by their last name ignoring case differences,

 Comparator cmp = Comparator.comparing( Person::getLastName, String.CASE_INSENSITIVE_ORDER); 

comparing

static Comparable> Comparator comparing​(Function keyExtractor)

Accepts a function that extracts a Comparable sort key from a type T , and returns a Comparator that compares by that sort key. The returned comparator is serializable if the specified function is also serializable.

 Comparator byLastName = Comparator.comparing(Person::getLastName); 

comparingInt

static Comparator comparingInt​(ToIntFunction keyExtractor)

Accepts a function that extracts an int sort key from a type T , and returns a Comparator that compares by that sort key. The returned comparator is serializable if the specified function is also serializable.

comparingLong

static Comparator comparingLong​(ToLongFunction keyExtractor)

Accepts a function that extracts a long sort key from a type T , and returns a Comparator that compares by that sort key. The returned comparator is serializable if the specified function is also serializable.

comparingDouble

static Comparator comparingDouble​(ToDoubleFunction keyExtractor)

Accepts a function that extracts a double sort key from a type T , and returns a Comparator that compares by that sort key. The returned comparator is serializable if the specified function is also serializable.

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