Collections工具類

Java里關于聚合的工具類，包含有各種有關集合操作的靜態多態方法，不能實例化(把構造函數私有化)

public class Collections { // Suppresses default constructor, ensuring non-instantiability. private Collections() { }}

和Collection的區別

Collection是接口，提供了對集合對象進行基本操作的通用接口方法，List、Set等多種具體的實現類 Collection是工具類，專門從操作Collection接口實現類里面的元素

常見方法

排序

Sort(List list)

按自然排序的升序排序

public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add('aaaa'); list.add('zzz'); list.add('gggg'); System.out.println(list); Collections.sort(list); System.out.println(list); }

自定義排序規則

Sort(List list,Comparator c)，由Comparator控制排序邏輯

public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add('aaaa'); list.add('zzz'); list.add('gggg'); System.out.println(list); //默認升序 Collections.sort(list, Comparator.naturalOrder()); System.out.println(list); //降序 Collections.sort(list, Comparator.reverseOrder()); System.out.println(list); }

隨機排序

shuffle(List list)

public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add('1'); list.add('2'); list.add('3'); list.add('4'); list.add('5'); list.add('6'); list.add('7'); list.add('8'); list.add('9'); list.add('10'); list.add('J'); list.add('Q'); list.add('K'); System.out.println(list); Collections.shuffle(list); System.out.println(list); }

獲取最值元素

max(Collection coll)#

默認比較，不適合對象比較

max(Collection coll,Comparator comparator)#

Student.java

package com.cyb.test;class Student { public Student(String name, int age) { this.name = name; this.age = age; } private int age; private String name; public void setAge(int age) { this.age = age; } public int getAge() { return age; } public void setName(String name) { this.name = name; } public String getName() { return name; } @Override public String toString() { return 'Student{' + 'age=' + age + ', name=’' + name + ’’’ + ’}’; }}

Test03.java

package com.cyb.test;import java.util.ArrayList;import java.util.Collections;import java.util.Comparator;import java.util.List;public class test03 { public static void main(String[] args) { List<Student> list = new ArrayList<>(); list.add(new Student('jack', 26)); list.add(new Student('tom', 29)); list.add(new Student('mary', 32)); list.add(new Student('tony', 19)); list.add(new Student('smith', 41)); System.out.println(list); Student maxAgeStudent = Collections.max(list, new Comparator<Student>() { @Override public int compare(Student o1, Student o2) {return o1.getAge() - o2.getAge(); } }); System.out.println('maxAgeStudent = '+maxAgeStudent.toString()); Student mixAgeStudent = Collections.min(list, new Comparator<Student>() { @Override public int compare(Student o1, Student o2) {return o1.getAge() - o2.getAge(); } }); System.out.println('mixAgeStudent = '+mixAgeStudent.toString()); }}

創建不可變集合unmodifiablleXXX()

List<String> list = new ArrayList<>(); list.add('SpringBoot課程'); list.add('架構課程'); list.add('微服務SpringCloud課程'); //設置為只讀List集合 list = Collections.unmodifiableList(list); System.out.println(list); Set<String> set = new HashSet<>(); set.add('Mysql教程'); set.add('Linux服務器器教程'); set.add('Git教程'); //設置為只讀Set集合 set = Collections.unmodifiableSet(set); System.out.println(set); Map<String, String> map = new HashMap<>(); map.put('key1', '課程1'); map.put('key2', '課程2'); //設置為只讀Map集合 map = Collections.unmodifiableMap(map); System.out.println(map);

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