关于比较器:Comparable与挽救的比较器:Comparator
比较器：Comparable
数组实际上会分为普通数组与对象数组两类使用情况，如果是普通数组则可以直接根据数据的大小关系进行排序（调用Arrays.sort()排序）。而对象数组由于其本身存放的都是地址数据，是不可能依据大小关系来实现排序的，但是在Arrays类中依然重载了一个sort()方法（对象数组排序：public static void sort(Object[] a)），此方法可以直接针对于对象数组实现排序。但是如果要想使用此方法进行排序，那么必须有一个前提：对象所在的类一定要实现Comparable接口，否则代码执行时会出现ClassCastException异常。而Comparable接口就属于比较器的一种，此接口定义如下：
public interface Comparable {
public int compareTo(T o);
} 实现对象数组排序:
import java.util.Arrays;
class Book implements Comparable {// 实现比较器
private String title ;
private double price ;
public Book(String title,double price) {
this.title = title ;
this.price = price ;
}
@Override
public String toString() {
return "书名：" + this.title + "，价格：" + this.price + "\n" ;
}
@Override
public int compareTo(Book o) { // Arrays.sort()会自动调用此方法比较
if (this.price > o.price) {
return 1 ;
} else if (this.price < o.price) {
return -1 ;
} else {
return 0;
}
}
}
public class TestDemo {
public static void main(String[] args) throws Exception {
Book books [] = new Book [] {
new Book("经典1",79.8) ,
new Book("经典2",69.8) ,
new Book("经典3",99.8) ,
new Book("经典4",89.8)
} ;
Arrays.sort(books); // 对象数组排序
System.out.println(Arrays.toString(books));
}
}
挽救的比较器：Comparator
利用Comparable接口实现的比较器是属于常见的用法，但是从另外一个角度来讲，如果要使用Comparable比较器，那么就意味着在类定义时就必须考虑好排序的需求。但是如果说现在某一个类定义的时候并没有实现Comparable接口，可是在不能修改类定义时又需要进行对象数组排序该怎么办呢？为此，在Java中又提供了另外一种比较器：Comparator接口（挽救的比较器），此接口定义如下：
@FunctionalInterface
public interface Comparator {
public int compare(T o1, T o2);
public boolean equals(Object obj);
} 通过定义可以发现,在Comparator接口上使用了 "@FunctionalInterface" 注解声明,所以此接口为一个函数式接口.该接口提供了一个compare()方法,此方法的返回结果为: 1(>0),-1(<0),0(=0).
函数式接口不是应该只有一个方法吗?这个接口为什么定义了两个?
因为:Object类中的方法不属于限定范围.虽然Comparator接口中定义了两个抽象方法,但是子类真正在覆写时只需要覆写compare()方法即可,而equals()这样的方法在Object类中已经有默认实现(地址除外). 另外Comparator接口定义之初只定义了compare()与equals()两个抽象方法,而在JDK1.8之后,Comparator接口中处理抽象方法外也定义了default和static的普通方法.
jdk1.8中的Cmparator接口:
package java.util;
import java.io.Serializable;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.function.ToDoubleFunction;
import java.util.Comparators;
/**
* A comparison function, which imposes a total ordering on some
* collection of objects. Comparators can be passed to a sort method (such
* as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
* Arrays#sort(Object[],Comparator) 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 {@link SortedSet sorted sets} or {@link
* SortedMap sorted maps}), or to provide an ordering for collections of
* objects that don't have a {@link Comparable 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 {@code a} and {@code b} such that
* {@code (a.equals(b) && c.compare(a, b) != 0)}
* to an empty {@code TreeSet} with comparator {@code c}.
* The second {@code add} operation will return
* true (and the size of the tree set will increase) because {@code a} and
* {@code b} are not equivalent from the tree set's perspective, even though
* this is contrary to the specification of the
* {@link Set#add 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 {@link TreeSet}, {@link 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:
* {(x, y) such that c.compare(x, y) <= 0}.
* The quotient for this total order is:
* {(x, y) such that c.compare(x, y) == 0}.
*
*
* 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' {@link Object#equals(Object)
* equals(Object)} method(s):
* {(x, y) such that x.equals(y)}.
*
* Unlike {@code 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.
*
* @param the type of objects that may be compared by this comparator
*
* @author Josh Bloch
* @author Neal Gafter
* @see Comparable
* @see java.io.Serializable
* @since 1.2
*/
@FunctionalInterface
public interface Comparator {
/**
* 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.
*
* 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.
*
* 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."
*
* @param o1 the first object to be compared.
* @param o2 the second object to be compared.
* @return a negative integer, zero, or a positive integer as the
* first argument is less than, equal to, or greater than the
* second.
* @throws NullPointerException if an argument is null and this
* comparator does not permit null arguments
* @throws ClassCastException if the arguments' types prevent them from
* being compared by this comparator.
*/
int compare(T o1, T o2);
/**
* Indicates whether some other object is "equal to" this
* comparator. This method must obey the general contract of
* {@link 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.
*
* @param obj the reference object with which to compare.
* @return true only if the specified object is also
* a comparator and it imposes the same ordering as this
* comparator.
* @see Object#equals(Object)
* @see Object#hashCode()
*/
boolean equals(Object obj);
/**
* Returns a comparator that imposes the reverse ordering of this
* comparator.
*
* @return a comparator that imposes the reverse ordering of this
* comparator.
* @since 1.8
*/
default Comparator reversed() {
return Collections.reverseOrder(this);
}
/**
* Returns a lexicographic-order comparator with another comparator.
* If this {@code Comparator} considers two elements equal, i.e.
* {@code compare(a, b) == 0}, {@code other} is used to determine the order.
*
* The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @apiNote
* For example, to sort a collection of {@code String} based on the length
* and then case-insensitive natural ordering, the comparator can be
* composed using following code,
*
*
{@code
* Comparator cmp = Comparator.comparingInt(String::length)
* .thenComparing(String.CASE_INSENSITIVE_ORDER);
* }
*
* @param other the other comparator to be used when this comparator
* compares two objects that are equal.
* @return a lexicographic-order comparator composed of this and then the
* other comparator
* @throws NullPointerException if the argument is null.
* @since 1.8
*/
default Comparator thenComparing(Comparator other) {
Objects.requireNonNull(other);
return (Comparator & Serializable) (c1, c2) -> {
int res = compare(c1, c2);
return (res != 0) ? res : other.compare(c1, c2);
};
}
/**
* Returns a lexicographic-order comparator with a function that
* extracts a key to be compared with the given {@code Comparator}.
*
* @implSpec This default implementation behaves as if {@code
* thenComparing(comparing(keyExtractor, cmp))}.
*
* @param the type of the sort key
* @param keyExtractor the function used to extract the sort key
* @param keyComparator the {@code Comparator} used to compare the sort key
* @return a lexicographic-order comparator composed of this comparator
* and then comparing on the key extracted by the keyExtractor function
* @throws NullPointerException if either argument is null.
* @see #comparing(Function, Comparator)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default Comparator thenComparing(
Function keyExtractor,
Comparator keyComparator)
{
return thenComparing(comparing(keyExtractor, keyComparator));
}
/**
* Returns a lexicographic-order comparator with a function that
* extracts a {@code Comparable} sort key.
*
* @implSpec This default implementation behaves as if {@code
* thenComparing(comparing(keyExtractor))}.
*
* @param the type of the {@link Comparable} sort key
* @param keyExtractor the function used to extract the {@link
* Comparable} sort key
* @return a lexicographic-order comparator composed of this and then the
* {@link Comparable} sort key.
* @throws NullPointerException if the argument is null.
* @see #comparing(Function)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default > Comparator thenComparing(
Function keyExtractor)
{
return thenComparing(comparing(keyExtractor));
}
/**
* Returns a lexicographic-order comparator with a function that
* extracts a {@code int} sort key.
*
* @implSpec This default implementation behaves as if {@code
* thenComparing(comparingInt(keyExtractor))}.
*
* @param keyExtractor the function used to extract the integer sort key
* @return a lexicographic-order comparator composed of this and then the
* {@code int} sort key
* @throws NullPointerException if the argument is null.
* @see #comparingInt(ToIntFunction)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default Comparator thenComparingInt(ToIntFunction keyExtractor) {
return thenComparing(comparingInt(keyExtractor));
}
/**
* Returns a lexicographic-order comparator with a function that
* extracts a {@code long} sort key.
*
* @implSpec This default implementation behaves as if {@code
* thenComparing(comparingLong(keyExtractor))}.
*
* @param keyExtractor the function used to extract the long sort key
* @return a lexicographic-order comparator composed of this and then the
* {@code long} sort key
* @throws NullPointerException if the argument is null.
* @see #comparingLong(ToLongFunction)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default Comparator thenComparingLong(ToLongFunction keyExtractor) {
return thenComparing(comparingLong(keyExtractor));
}
/**
* Returns a lexicographic-order comparator with a function that
* extracts a {@code double} sort key.
*
* @implSpec This default implementation behaves as if {@code
* thenComparing(comparingDouble(keyExtractor))}.
*
* @param keyExtractor the function used to extract the double sort key
* @return a lexicographic-order comparator composed of this and then the
* {@code double} sort key
* @throws NullPointerException if the argument is null.
* @see #comparingDouble(ToDoubleFunction)
* @see #thenComparing(Comparator)
* @since 1.8
*/
default Comparator thenComparingDouble(ToDoubleFunction keyExtractor) {
return thenComparing(comparingDouble(keyExtractor));
}
/**
* Returns a comparator that imposes the reverse of the natural
* ordering.
*
* The returned comparator is serializable and throws {@link
* NullPointerException} when comparing {@code null}.
*
* @param the {@link Comparable} type of element to be compared
* @return a comparator that imposes the reverse of the natural
* ordering on {@code Comparable} objects.
* @see Comparable
* @since 1.8
*/
public static > Comparator reverseOrder() {
return Collections.reverseOrder();
}
/**
* Returns a comparator that compares {@link Comparable} objects in natural
* order.
*
* The returned comparator is serializable and throws {@link
* NullPointerException} when comparing {@code null}.
*
* @param the {@link Comparable} type of element to be compared
* @return a comparator that imposes the natural ordering on {@code
* Comparable} objects.
* @see Comparable
* @since 1.8
*/
@SuppressWarnings("unchecked")
public static > Comparator naturalOrder() {
return (Comparator) Comparators.NaturalOrderComparator.INSTANCE;
}
/**
* Returns a null-friendly comparator that considers {@code null} to be
* less than non-null. When both are {@code null}, they are considered
* equal. If both are non-null, the specified {@code Comparator} is used
* to determine the order. If the specified comparator is {@code null},
* then the returned comparator considers all non-null values to be equal.
*
* The returned comparator is serializable if the specified comparator
* is serializable.
*
* @param the type of the elements to be compared
* @param comparator a {@code Comparator} for comparing non-null values
* @return a comparator that considers {@code null} to be less than
* non-null, and compares non-null objects with the supplied
* {@code Comparator}.
* @since 1.8
*/
public static Comparator nullsFirst(Comparator comparator) {
return new Comparators.NullComparator<>(true, comparator);
}
/**
* Returns a null-friendly comparator that considers {@code null} to be
* greater than non-null. When both are {@code null}, they are considered
* equal. If both are non-null, the specified {@code Comparator} is used
* to determine the order. If the specified comparator is {@code null},
* then the returned comparator considers all non-null values to be equal.
*
* The returned comparator is serializable if the specified comparator
* is serializable.
*
* @param the type of the elements to be compared
* @param comparator a {@code Comparator} for comparing non-null values
* @return a comparator that considers {@code null} to be greater than
* non-null, and compares non-null objects with the supplied
* {@code Comparator}.
* @since 1.8
*/
public static Comparator nullsLast(Comparator comparator) {
return new Comparators.NullComparator<>(false, comparator);
}
/**
* Accepts a function that extracts a sort key from a type {@code T}, and
* returns a {@code Comparator} that compares by that sort key using
* the specified {@link Comparator}.
*
* The returned comparator is serializable if the specified function
* and comparator are both serializable.
*
* @apiNote
* For example, to obtain a {@code Comparator} that compares {@code
* Person} objects by their last name ignoring case differences,
*
*
{@code
* Comparator cmp = Comparator.comparing(
* Person::getLastName,
* String.CASE_INSENSITIVE_ORDER);
* }
*
* @param the type of element to be compared
* @param the type of the sort key
* @param keyExtractor the function used to extract the sort key
* @param keyComparator the {@code Comparator} used to compare the sort key
* @return a comparator that compares by an extracted key using the
* specified {@code Comparator}
* @throws NullPointerException if either argument is null
* @since 1.8
*/
public static Comparator comparing(
Function keyExtractor,
Comparator keyComparator)
{
Objects.requireNonNull(keyExtractor);
Objects.requireNonNull(keyComparator);
return (Comparator & Serializable)
(c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
keyExtractor.apply(c2));
}
/**
* Accepts a function that extracts a {@link java.lang.Comparable
* Comparable} sort key from a type {@code T}, and returns a {@code
* Comparator} that compares by that sort key.
*
* The returned comparator is serializable if the specified function
* is also serializable.
*
* @apiNote
* For example, to obtain a {@code Comparator} that compares {@code
* Person} objects by their last name,
*
*
{@code
* Comparator byLastName = Comparator.comparing(Person::getLastName);
* }
*
* @param the type of element to be compared
* @param the type of the {@code Comparable} sort key
* @param keyExtractor the function used to extract the {@link
* Comparable} sort key
* @return a comparator that compares by an extracted key
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static > Comparator comparing(
Function keyExtractor)
{
Objects.requireNonNull(keyExtractor);
return (Comparator & Serializable)
(c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
}
/**
* Accepts a function that extracts an {@code int} sort key from a type
* {@code T}, and returns a {@code Comparator} that compares by that
* sort key.
*
* The returned comparator is serializable if the specified function
* is also serializable.
*
* @param the type of element to be compared
* @param keyExtractor the function used to extract the integer sort key
* @return a comparator that compares by an extracted key
* @see #comparing(Function)
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static Comparator comparingInt(ToIntFunction keyExtractor) {
Objects.requireNonNull(keyExtractor);
return (Comparator & Serializable)
(c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
}
/**
* Accepts a function that extracts a {@code long} sort key from a type
* {@code T}, and returns a {@code Comparator} that compares by that
* sort key.
*
* The returned comparator is serializable if the specified function is
* also serializable.
*
* @param the type of element to be compared
* @param keyExtractor the function used to extract the long sort key
* @return a comparator that compares by an extracted key
* @see #comparing(Function)
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static Comparator comparingLong(ToLongFunction keyExtractor) {
Objects.requireNonNull(keyExtractor);
return (Comparator & Serializable)
(c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
}
/**
* Accepts a function that extracts a {@code double} sort key from a type
* {@code T}, and returns a {@code Comparator} that compares by that
* sort key.
*
* The returned comparator is serializable if the specified function
* is also serializable.
*
* @param the type of element to be compared
* @param keyExtractor the function used to extract the double sort key
* @return a comparator that compares by an extracted key
* @see #comparing(Function)
* @throws NullPointerException if the argument is null
* @since 1.8
*/
public static Comparator comparingDouble(ToDoubleFunction keyExtractor) {
Objects.requireNonNull(keyExtractor);
return (Comparator & Serializable)
(c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
}
}
利用Comparator接口实现对象数组排序:
class BookComparator implements java.util.Comparator {
@Override
public int compare(Book o1, Book o2) {
if (o1.getPrice() > o2.getPrice()) {
return 1 ;
} else if (o1.getPrice() < o2.getPrice()) {
return -1 ;
} else {
return 0;
}
}
} 修改测试程序:
public class TestDemo {
public static void main(String[] args) throws Exception {
Book books [] = new Book [] {
new Book("经典1",79.8) ,
new Book("经典2",69.8) ,
new Book("经典3",99.8) ,
new Book("经典4",89.8)
} ;
java.util.Arrays.sort(books,(o1,o2)->{
if (o1.getPrice() > o2.getPrice()) {
return 1;
} else if (o1.getPrice() < o2.getPrice()) {
return -1;
} else {
return 0;
}
});
System.out.println(java.util.Arrays.toString(books));
}