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)
publicclassCollections{ //Suppressesdefaultconstructor,ensuringnon-instantiability. privateCollections(){ } }
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(Listlist)
按自然排序的升序排序
publicstaticvoidmain(String[]args){ Listlist=newArrayList<>(); 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(Listlist,Comparatorc),由Comparator控制排序逻辑
publicstaticvoidmain(String[]args){ Listlist=newArrayList<>(); 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(Listlist)
publicstaticvoidmain(String[]args){ Listlist=newArrayList<>(); 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); }
获取最值元素
max(Collectioncoll)#
默认比较,不适合对象比较
max(Collectioncoll,Comparatorcomparator)#
Student.java
packagecom.cyb.test; classStudent{ publicStudent(Stringname,intage){ this.name=name; this.age=age; } privateintage; privateStringname; publicvoidsetAge(intage){ this.age=age; } publicintgetAge(){ returnage; } publicvoidsetName(Stringname){ this.name=name; } publicStringgetName(){ returnname; } @Override publicStringtoString(){ return"Student{"+"age="+age+",name='"+name+'\''+'}'; } }
Test03.java
packagecom.cyb.test; importjava.util.ArrayList; importjava.util.Collections; importjava.util.Comparator; importjava.util.List; publicclasstest03{ publicstaticvoidmain(String[]args){ Listlist=newArrayList<>(); list.add(newStudent("jack",26)); list.add(newStudent("tom",29)); list.add(newStudent("mary",32)); list.add(newStudent("tony",19)); list.add(newStudent("smith",41)); System.out.println(list); StudentmaxAgeStudent=Collections.max(list,newComparator (){ @Override publicintcompare(Studento1,Studento2){ returno1.getAge()-o2.getAge(); } }); System.out.println("maxAgeStudent="+maxAgeStudent.toString()); StudentmixAgeStudent=Collections.min(list,newComparator (){ @Override publicintcompare(Studento1,Studento2){ returno1.getAge()-o2.getAge(); } }); System.out.println("mixAgeStudent="+mixAgeStudent.toString()); } }
创建不可变集合unmodifiablleXXX()
Listlist=newArrayList<>(); 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=newHashSet<>(); 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=newHashMap<>(); 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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