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Java集合相关知识
1、Collection的使用
import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; public class Main { public static void main(String[] args) { //创建集合 Collection collection=new ArrayList(); //添加元素 collection.add("苹果"); collection.add("香蕉"); collection.add("西瓜"); //输出元素 System.out.println("元素个数："+collection.size()); System.out.println(collection); //删除元素 //collection.remove("香蕉"); //System.out.println(collection); //清空 //collection.clear(); //遍历集合 //方式1for循环 for(Object object:collection) { System.out.println(object); } //方式2 迭代器 Iterator it=collection.iterator(); while(it.hasNext())//判断是否还有元素 { String s=(String)it.next();//next方法获取元素 System.out.println(s); it.remove();//删除最后一个元素 } System.out.println(collection); //判断 //判断是否包含元素 System.out.println(collection.contains("西瓜")); //判断集合是否为空 System.out.println(collection.isEmpty()); } }- 1
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2、List的使用
List子接口特点：
- 元素有下标；
- 元素可以重复；
- 元素有序；
import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; import java.util.List; import java.util.ListIterator; public class Main { public static void main(String[] args) { List list=new ArrayList(); //添加元素 list.add("小米"); list.add("苹果"); //在指定下标添加 list.add(0,"华为"); System.out.println(list.toString()); //删除元素 //list.remove(1); //list.remove("苹果"); //System.out.println(list.toString()); //遍历 //普通for for(int i=0;i<list.size();i++) { System.out.println(list.get(i)); } //增强for for(Object object:list) { System.out.println(object); } //迭代器 Iterator it=list.iterator(); while(it.hasNext()) { System.out.println(it.next()); } //列表迭代器 ListIterator lit=list.listIterator(); //从前往后遍历，nextIndex返回索引 while(lit.hasNext()) { System.out.println(lit.nextIndex()+":"+lit.next()); } //从后往前遍历 while(lit.hasPrevious()) { System.out.println(lit.previousIndex()+":"+lit.previous()); } //判断是否包含某个元素 System.out.println(list.contains("小米")); //获取元素索引值 System.out.println(list.indexOf("华为")); //subList获取子集合,左闭右开 List list1=list.subList(0,2); System.out.println(list1.toString()); } }- 1
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3、ArrayList
ArrayList特点：
- 是使用数组实现的；
- 元素有下标、可以重复、有序；
- 查询、遍历快；
- 增加、删除慢；
import com.edu.java.Employee; import java.util.ArrayList; import java.util.ListIterator; public class Main { public static void main(String[] args) { ArrayList arrayList=new ArrayList(); Employee e1=new Employee("张三",2000); Employee e2=new Employee("李四",2000); Employee e3=new Employee("王五",2000); //1、添加元素 arrayList.add(e1); arrayList.add(e2); arrayList.add(e3); arrayList.add(e3); //System.out.println(arrayList.toString()); //2、删除元素 //arrayList.remove(e3); //arrayList.remove(new Employee("王五",2000));//这样是删除不了的，相当于创建了一个新对象，除非在Employee中重写了equals方法 // System.out.println(arrayList.toString()); // 3、遍历输出 // 3、1列表迭代器 ListIterator lit= arrayList.listIterator(); //从前往后遍历 while(lit.hasNext()) { System.out.println(lit.nextIndex()+":"+lit.next()); } //从后往前遍历 while(lit.hasPrevious()) { System.out.println(lit.previousIndex()+":"+lit.previous()); } } }- 1
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4、LinkedList双向链表
链表结构实现，增删快，查询慢
import com.edu.java.Employee; import java.util.Iterator; import java.util.LinkedList; import java.util.ListIterator; public class Main { public static void main(String[] args) { LinkedList linkedList=new LinkedList(); //添加元素 linkedList.add(new Employee("小明",2000)); linkedList.add(new Employee("小刚",1000)); linkedList.add(new Employee("小红",3000)); System.out.println(linkedList.toString()); //删除元素 // linkedList.remove(new Employee("小刚",1000)); // System.out.println(linkedList.size()); //遍历 /* for(int i=0;i< linkedList.size();i++) { System.out.println(linkedList.get(i)); } for(Object obj:linkedList) { Employee e=(Employee) obj; System.out.println(e.toString()); } Iterator it=linkedList.iterator(); while(it.hasNext()) { Employee e=(Employee)it.next(); System.out.println(e.toString()); } */ ListIterator listiterator=linkedList.listIterator(); while(listiterator.hasNext()) { Employee e=(Employee) listiterator.next(); System.out.println(e); } //判断 System.out.println(linkedList.contains(new Employee("小明",2000))); System.out.println(linkedList.indexOf(new Employee("小红",3000))); } }- 1
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5、Set
特点：无序、无下标，元素不能重复
package com.java.main; import java.util.HashSet; import java.util.Iterator; import java.util.Set; public class TestSet { public static void main(String[] args) { Set<String> set = new HashSet<>(); // 添加元素 set.add("张三"); set.add("李四"); set.add("王五"); //输出所有元素 System.out.println(set.toString()); //删除元素，只能通过值删除 //set.remove("张三"); //System.out.println(set.toString()); // 遍历 // 增强for循环 for(String s : set) { System.out.println(s); } // 迭代器 Iterator<String> iterator = set.iterator(); while(iterator.hasNext()) { System.out.println(iterator.next()); } // 判断是否包含某元素 System.out.println(set.contains("李四")); } }- 1
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6、HashSet
package com.java.main; import java.util.HashSet; import java.util.Iterator; public class TestSet { public static void main(String[] args) { HashSet<Student> hashSet = new HashSet<>(); Student s1 = new Student("张三", 23); Student s2 = new Student("李四", 21); Student s3 = new Student("王五", 24); // 1、添加元素 hashSet.add(s1); hashSet.add(s2); hashSet.add(s3); // 2、遍历输出 Iterator<Student> iter = hashSet.iterator(); while(iter.hasNext()) { System.out.println(iter.next()); } // 尝试添加姓名和年龄相同的对象 // 当直接再添加同一个对象时，HashSet不允许 hashSet.add(s3); System.out.println(hashSet); //当添加另一个对象时，如果没有重写hashCode方法和equals方法，则能够添加相同属性的对象 // 如 hashSet.add(new Student("李四", 21)); // 但是如果重写了hashCode方法和equals方法，则不能再次添加 hashSet.add(new Student("李四", 21)); System.out.println(hashSet); // 此时删除也可以另new一个对象 hashSet.remove(new Student("张三", 23)); System.out.println(hashSet); } } class Student { private String name; private int age; public Student(String aName, int aAge) { name = aName; age = aAge; } public String getName() { return name; } public int getAge() { return age; } // 重写hashCode方法 @Override public int hashCode() { int n1 = this.name.hashCode(); // String中已有hashCode方法 int n2 = this.age; return n1+n2; } // 重写equals方法 @Override public boolean equals(Object obj) { if(this == obj) // 是同一个对象 return true; if(obj == null) return false; if(obj instanceof Student) // 属于同一个类但不是同一个对象，判断二者的属性是否相同 { Student student = (Student)obj; if(this.name.equals(student.getName()) && this.age == student.getAge()) { return true; } } return false; } @Override public String toString() { return "Student [name=" + name + ", age=" + age + "]"; } }- 1
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7、TreeSet
- 基于排列顺序实现元素不重复；
- 实现了SortedSet接口，对集合元素自动排序
- 元素对象的类型必须实现Comparable接口，指定排序规则；
package com.java.main; import java.util.Iterator; import java.util.TreeSet; public class TestTreeSet { public static void main(String args[]) { TreeSet<Studentt> treeSet = new TreeSet<>(); Studentt s1 = new Studentt("张三", 23); Studentt s2 = new Studentt("李四", 21); Studentt s3 = new Studentt("王五", 24); // 添加元素 treeSet.add(s1); treeSet.add(s2); treeSet.add(s3); // 输出 System.out.println(treeSet); // 遍历 Iterator<Studentt> iterator = treeSet.iterator(); while(iterator.hasNext()) { System.out.println(iterator.next()); } // 删除 treeSet.remove(new Studentt("王五", 24)); System.out.println(treeSet); } } class Studentt implements Comparable<Studentt> { private String name; private int age; public Studentt(String aName, int aAge) { name = aName; age = aAge; } public String getName() { return name; } public int getAge() { return age; } // 先按姓名比，再按年龄比较，函数返回0说明两个对象相等 @Override public int compareTo(Studentt o) { int n1 = this.name.compareTo(o.getName()); int n2 = this.age - o.getAge(); return n1 == 0 ? n2 : n1; // n1==0说明姓名相同 } @Override public String toString() { return "Studentt [name=" + name + ", age=" + age + "]"; } }- 1
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也可以用匿名内部类的方式重写compareTo方法
package com.java.main; import java.util.Comparator; import java.util.Iterator; import java.util.TreeSet; public class TestTreeSet { public static void main(String args[]) { TreeSet<Studentt> treeSet = new TreeSet<>(new Comparator<Studentt>() // 匿名内部类的方式重写compareTo方法 { @Override public int compare(Studentt o1, Studentt o2) { int n1 = o1.getName().compareTo(o2.getName()); int n2 = o1.getAge() - o2.getAge(); return n1 == 0 ? n2 : n1; } }); Studentt s1 = new Studentt("张三", 23); Studentt s2 = new Studentt("李四", 21); Studentt s3 = new Studentt("王五", 24); // 添加元素 treeSet.add(s1); treeSet.add(s2); treeSet.add(s3); // 输出 System.out.println(treeSet); // 遍历 Iterator<Studentt> iterator = treeSet.iterator(); while(iterator.hasNext()) { System.out.println(iterator.next()); } // 删除 treeSet.remove(new Studentt("王五", 24)); System.out.println(treeSet); } } class Studentt { private String name; private int age; public Studentt(String aName, int aAge) { name = aName; age = aAge; } public String getName() { return name; } public int getAge() { return age; } @Override public String toString() { return "Studentt [name=" + name + ", age=" + age + "]"; } }- 1
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8、Map
- 用于存储任意键值对；
- 键：无序、无下标、不允许重复；
- 值：无序、无下标、允许重复。
package com.java.main; import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.Set; public class TestMap { public static void main(String[] args) { Map<String, String> map = new HashMap<>(); // 添加元素用put，如果键重复，就会覆盖原来的 map.put("cn", "中国"); map.put("uk", "英国"); map.put("usa", "美国"); // 输出 System.out.println(map.toString()); // 按键删除元素 //map.remove("usa"); //System.out.println(map.toString()); // 遍历 //1、 使用keySet得到所有键的集合，在使用map.get(key)方法得到对应的值 Set<String> keyset = map.keySet(); for(String key : keyset) { System.out.println(key + "---" + map.get(key)); } // 2、使用entrySet得到键、值的映射 Set<Map.Entry<String, String>> entries = map.entrySet(); for(Map.Entry<String, String> entry : entries) { System.out.println(entry.getKey() + "-----" + entry.getValue()); } } }- 1
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9、TreeMap
package com.java.main; import java.util.Comparator; import java.util.Map; import java.util.TreeMap; public class TestTreeMap { public static void main(String[] args) { TreeMap<Teacher, String> treeMap = new TreeMap<>(new Comparator<Teacher>() { public int compare(Teacher o1, Teacher o2) { int n1 = o1.getNo() - o2.getNo(); return n1; } }); Teacher t1 = new Teacher("李四", 123); Teacher t2 = new Teacher("王五", 456); Teacher t3 = new Teacher("赵六", 789); // 添加元素 treeMap.put(t1, "上海"); treeMap.put(t2, "北京"); treeMap.put(t3, "重庆"); // 输出 System.out.println(treeMap.toString()); // 遍历 for(Teacher teacher : treeMap.keySet()) { System.out.println(teacher.toString() + "===" + treeMap.get(teacher)); } // 测试能否再添加编号一样的对象 treeMap.put(new Teacher("李四", 123), "成都"); System.out.println(treeMap.toString()); } } class Teacher { private String name; private int no; public Teacher(String aName, int aNo) { name = aName; no = aNo; } public String getName() { return name; } public int getNo() { return no; } @Override public String toString() { return "Teacher [name=" + name + ", no=" + no + "]"; } }- 1
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- 原文地址：https://blog.csdn.net/qq_57987156/article/details/127167008
