Collections集合工具类
1. 常用功能
java.utils.Collections是集合工具类，用来对集合进行操作。部分方法如下：
public static <T> boolean addAll(Collection<T> c, T... elements):往集合中添加一些元素。public static void shuffle(List<?> list) 打乱顺序:打乱集合顺序。public static <T> void sort(List<T> list):将集合中元素按照默认规则排序。public static <T> void sort(List<T> list，Comparator<? super T> ):将集合中元素按照指定规则排序。
代码演示：
package com.itheima.demo05.Collections;
import java.util.ArrayList;
import java.util.Collections;
public class Demo01Collections {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
String[] strings = {
"a", "b", "c", "d", "e"};
Collections.addAll(list, strings);
System.out.println(list);
Collections.shuffle(list);
System.out.println(list);
Collections.sort(list);
System.out.println(list);
}
}
2.Comparable接口
/*
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.lang;
import java.util.*;
/**
* This interface imposes a total ordering on the objects of each class that
* implements it. This ordering is referred to as the class's <i>natural
* ordering</i>, and the class's <tt>compareTo</tt> method is referred to as
* its <i>natural comparison method</i>.<p>
*
* Lists (and arrays) of objects that implement this interface can be sorted
* automatically by {@link Collections#sort(List) Collections.sort} (and
* {@link Arrays#sort(Object[]) Arrays.sort}). Objects that implement this
* interface can be used as keys in a {@linkplain SortedMap sorted map} or as
* elements in a {@linkplain SortedSet sorted set}, without the need to
* specify a {@linkplain Comparator comparator}.<p>
*
* The natural ordering for a class <tt>C</tt> is said to be <i>consistent
* with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has
* the same boolean value as <tt>e1.equals(e2)</tt> for every
* <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>. Note that <tt>null</tt>
* is not an instance of any class, and <tt>e.compareTo(null)</tt> should
* throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt>
* returns <tt>false</tt>.<p>
*
* It is strongly recommended (though not required) that natural orderings be
* consistent with equals. This is so because sorted sets (and sorted maps)
* without explicit comparators behave "strangely" when they are used with
* elements (or keys) whose natural ordering is inconsistent with equals. In
* particular, such a sorted set (or sorted map) violates the general contract
* for set (or map), which is defined in terms of the <tt>equals</tt>
* method.<p>
*
* For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
* {@code (!a.equals(b) && a.compareTo(b) == 0)} to a sorted
* set that does not use an explicit comparator, the second <tt>add</tt>
* operation returns false (and the size of the sorted set does not increase)
* because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
* perspective.<p>
*
* Virtually all Java core classes that implement <tt>Comparable</tt> have natural
* orderings that are consistent with equals. One exception is
* <tt>java.math.BigDecimal</tt>, whose natural ordering equates
* <tt>BigDecimal</tt> objects with equal values and different precisions
* (such as 4.0 and 4.00).<p>
*
* For the mathematically inclined, the <i>relation</i> that defines
* the natural ordering on a given class C is:<pre>
* {(x, y) such that x.compareTo(y) <= 0}.
* </pre> The <i>quotient</i> for this total order is: <pre>
* {(x, y) such that x.compareTo(y) == 0}.
* </pre>
*
* It follows immediately from the contract for <tt>compareTo</tt> that the
* quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the
* natural ordering is a <i>total order</i> on <tt>C</tt>. When we say that a
* class's natural ordering is <i>consistent with equals</i>, we mean that the
* quotient for the natural ordering is the equivalence relation defined by
* the class's {@link Object#equals(Object) equals(Object)} method:<pre>
* {(x, y) such that x.equals(y)}. </pre><p>
*
* This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @param <T> the type of objects that this object may be compared to
*
* @author Josh Bloch
* @see java.util.Comparator
* @since 1.2
*/
public interface Comparable<T> {
/**
* Compares this object with the specified object for order. Returns a
* negative integer, zero, or a positive integer as this object is less
* than, equal to, or greater than the specified object.
*
* <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
* -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
* implies that <tt>x.compareTo(y)</tt> must throw an exception iff
* <tt>y.compareTo(x)</tt> throws an exception.)
*
* <p>The implementor must also ensure that the relation is transitive:
* <tt>(x.compareTo(y)>0 && y.compareTo(z)>0)</tt> implies
* <tt>x.compareTo(z)>0</tt>.
*
* <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
* implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
* all <tt>z</tt>.
*
* <p>It is strongly recommended, but <i>not</i> strictly required that
* <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>. Generally speaking, any
* class that implements the <tt>Comparable</tt> 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."
*
* <p>In the foregoing description, the notation
* <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
* <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
* <tt>0</tt>, or <tt>1</tt> according to whether the value of
* <i>expression</i> is negative, zero or positive.
*
* @param o the object to be compared.
* @return a negative integer, zero, or a positive integer as this object
* is less than, equal to, or greater than the specified object.
*
* @throws NullPointerException if the specified object is null
* @throws ClassCastException if the specified object's type prevents it
* from being compared to this object.
*/
public int compareTo(T o);
}
//String类：继承Comparable接口并重写了comparableTo方法
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
......
/**
* Compares two strings lexicographically.
* The comparison is based on the Unicode value of each character in
* the strings. The character sequence represented by this
* {@code String} object is compared lexicographically to the
* character sequence represented by the argument string. The result is
* a negative integer if this {@code String} object
* lexicographically precedes the argument string. The result is a
* positive integer if this {@code String} object lexicographically
* follows the argument string. The result is zero if the strings
* are equal; {@code compareTo} returns {@code 0} exactly when
* the {@link #equals(Object)} method would return {@code true}.
* <p>
* This is the definition of lexicographic ordering. If two strings are
* different, then either they have different characters at some index
* that is a valid index for both strings, or their lengths are different,
* or both. If they have different characters at one or more index
* positions, let <i>k</i> be the smallest such index; then the string
* whose character at position <i>k</i> has the smaller value, as
* determined by using the < operator, lexicographically precedes the
* other string. In this case, {@code compareTo} returns the
* difference of the two character values at position {@code k} in
* the two string -- that is, the value:
* <blockquote><pre>
* this.charAt(k)-anotherString.charAt(k)
* </pre></blockquote>
* If there is no index position at which they differ, then the shorter
* string lexicographically precedes the longer string. In this case,
* {@code compareTo} returns the difference of the lengths of the
* strings -- that is, the value:
* <blockquote><pre>
* this.length()-anotherString.length()
* </pre></blockquote>
*
* @param anotherString the {@code String} to be compared.
* @return the value {@code 0} if the argument string is equal to
* this string; a value less than {@code 0} if this string
* is lexicographically less than the string argument; and a
* value greater than {@code 0} if this string is
* lexicographically greater than the string argument.
*/
public int compareTo(String anotherString) {
int len1 = value.length;
int len2 = anotherString.value.length;
int lim = Math.min(len1, len2);
char v1[] = value;
char v2[] = anotherString.value;
int k = 0;
while (k < lim) {
char c1 = v1[k];
char c2 = v2[k];
if (c1 != c2) {
return c1 - c2;
}
k++;
}
return len1 - len2;
}
}
//Integer类：继承Comparable接口并重写了comparableTo方法
public final class Integer extends Number implements Comparable<Integer> {
......
/**
* Compares two {@code Integer} objects numerically.
*
* @param anotherInteger the {@code Integer} to be compared.
* @return the value {@code 0} if this {@code Integer} is
* equal to the argument {@code Integer}; a value less than
* {@code 0} if this {@code Integer} is numerically less
* than the argument {@code Integer}; and a value greater
* than {@code 0} if this {@code Integer} is numerically
* greater than the argument {@code Integer} (signed
* comparison).
* @since 1.2
*/
public int compareTo(Integer anotherInteger) {
return compare(this.value, anotherInteger.value);
}
/**
* Compares two {@code int} values numerically.
* The value returned is identical to what would be returned by:
* <pre>
* Integer.valueOf(x).compareTo(Integer.valueOf(y))
* </pre>
*
* @param x the first {@code int} to compare
* @param y the second {@code int} to compare
* @return the value {@code 0} if {@code x == y};
* a value less than {@code 0} if {@code x < y}; and
* a value greater than {@code 0} if {@code x > y}
* @since 1.7
*/
public static int compare(int x, int y) {
return (x < y) ? -1 : ((x == y) ? 0 : 1);
}
}
也就是说对于不同的对象（String、Integer、Person）进行排序要有自己规定的排序规则，也就是重写compareTo方法。
package com.itheima.demo05.Collections;
public class Person implements Comparable<Person> {
private String name;
private int age;
private int score;
public Person() {
}
public Person(String name, int age, int score) {
this.name = name;
this.age = age;
this.score = score;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public int getScore() {
return score;
}
public void setScore(int score) {
this.score = score;
}
@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", age=" + age +
", score=" + score +
'}';
}
@Override
public int compareTo(Person o) {
if (this.score != o.score) return o.score - this.score;//不同成绩，按降序排序
else return this.age - o.age;//相同成绩，按年龄降序排序
}
}
package com.itheima.demo05.Collections;
import java.util.ArrayList;
import java.util.Collections;
public class Demo02Sort {
public static void main(String[] args) {
ArrayList<Person> list = new ArrayList<>();
list.add(new Person("张三", 18, 97));//2
list.add(new Person("李四", 18, 95));//4
list.add(new Person("王五", 19, 100));//1
list.add(new Person("赵六麻子", 19, 97));//3
System.out.println(list);
//无法直接调用Collection.sort进行排序
Collections.sort(list, Person::compareTo);
System.out.println(list);
}
}
运行结果
[Person{name=‘张三’, age=18, score=97}, Person{name=‘李四’, age=18, score=95}, Person{name=‘王五’, age=19, score=100}, Person{name=‘赵六麻子’, age=19, score=97}]
[Person{name=‘王五’, age=19, score=100}, Person{name=‘张三’, age=18, score=97}, Person{name=‘赵六麻子’, age=19, score=97}, Person{name=‘李四’, age=18, score=95}]
- 调用对象-被比较对象//降序
- 被比较对象-调用对象//升序
3. Comparator接口
定义一个规则，传入双方对象进行比较。
public interface Comparator<T> {
/**
* 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.<p>
*
* In the foregoing description, the notation
* <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
* <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
* <tt>0</tt>, or <tt>1</tt> according to whether the value of
* <i>expression</i> is negative, zero or positive.<p>
*
* The implementor must ensure that <tt>sgn(compare(x, y)) ==
* -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
* implies that <tt>compare(x, y)</tt> must throw an exception if and only
* if <tt>compare(y, x)</tt> throws an exception.)<p>
*
* The implementor must also ensure that the relation is transitive:
* <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
* <tt>compare(x, z)>0</tt>.<p>
*
* Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
* implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
* <tt>z</tt>.<p>
*
* It is generally the case, but <i>not</i> strictly required that
* <tt>(compare(x, y)==0) == (x.equals(y))</tt>. 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);
package com.itheima.demo05.Collections;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
public class Demo03Sort {
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>();
list.add(2);
list.add(4);
list.add(5);
list.add(1);
System.out.println(list);
Collections.sort(list);
System.out.println("默认排序从小到大：" + list);
Collections.sort(list, new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
return o2 - o1;
}
});
System.out.println("重写Comparator接口：" + list);
}
}
运行结果
[2, 4, 5, 1]
默认排序从小到大：[1, 2, 4, 5]
重写Comparator接口：[5, 4, 2, 1]
- o1-o2,升序
- o2-o1,降序
匿名类实现Comparator接口，能够灵活地对象进行排序。