Java集合框架Collections原理及用法实例

Java集合框架Collections原理及用法实例

Collections工具类

java里关于聚合的工具类，包含有各种有关集合操作的静态多态方法，不能实例化(把构造函数私有化)

public class Collections {

// Suppresses default constructor, ensuring non-instantiability.

private Collections() {

}

}

和Collection的区别

Collection是接口，提供了对集合对象进行基本操作的通用接口方法，List、Set等多种具体的实现类

Collection是工具类，专门从操作Collection接口实现类里面的元素

常见方法

排序

Sort(List list)

按自然排序的升序排序

public static void main(String[] args) {

List list = new ArrayList<>();

list.add("aaaa");

list.add("zzz");

list.add("gggg");

System.out.println(list);

Collections.sort(list);

System.out.println(list);

}

自定义排序规则

Sort(List list,Comparator c)，由Comparator控制排序逻辑

public static void main(String[] args) {

List list = new ArrayList<>();

list.add("aaaa");

list.add("zzz");

list.add("gggg");

System.out.println(list);

//默认升序

Collections.sort(list, Comparator.naturalOrder());

System.out.println(list);

//降序

Collections.sort(list, Comparator.reverseOrder());

System.out.println(list);

}

随机排序

shuffle(List list)

public static void main(String[] args) {

List list = new ArrayList<>();

list.add("1");

list.add("2");kEaizSA

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);

}

获取最值元素

max(Collection coll)#

默认比较，不适合对象比较

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 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() {

@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() {

@Override

public int compare(Student o1, Student o2) {

return o1.getAge() - o2.getAge();

}

});

System.out.println("mixAgeStudent = "+mixAgeStudent.toString());

}

}

创建不可变集合unmodifiablleXXX()

List list = new ArrayList<>();

list.add("SpringBoot课程");

list.add("架构课程");

list.add("微服务SpringCloud课程"); //设置为只读List集合

list = Collections.unmodifiableList(list);

System.out.println(list);

Set set = new HashSet<>();

set.add("mysql教程");

set.add("linux服务器器教程");

set.add("Git教程");

//设置为只读Set集合

set = Collections.unmodifiableSet(set);

System.out.println(set);

Map map = new HashMap<>();

map.put("key1", "课程1");

map.put("key2", "课程2");

//设置为只读Map集合

map = Collections.unmodifiableMap(map);

System.out.println(map);

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