标签：obj rgs 源码 asn java min prime static final
一，list集合排序
对于自定义数据类型，排序可以用Collections集合工具类中的sort()方法，
而要想使用sort()方法则必须实现Comparable接口然后重写里面的compareTo方法或单独一个类实现Comparactor接口重写里面的compare方法
String类型已经实现了Comparable接口
1，实现Comparable接口
public class Teacher implements Comparable<Teacher> { String name; int age; public Teacher(String name, int age) { super(); this.name = name; this.age = age; } @Override public String toString() { return "Teacher [name=" + name + ", age=" + age + "]"; } @Override public int compareTo(Teacher t) { if (age > t.age) { return 1; } if (age < t.age) { return -1; } if (age == t.age) { return name.compareTo(t.name);// 如果age相等则用name排序 } return age; } }
2，实现Comparactor接口
public class Teacher2{ String name; int age; public Teacher2(String name, int age) { super(); this.name = name; this.age = age; } @Override public String toString() { return "Teacher [name=" + name + ", age=" + age + "]"; } }
public class AgeAndNameSort implements Comparator<Teacher2>{ @Override public int compare(Teacher2 t1, Teacher2 t2) { if (t1.age > t2.age) { return 1; } if (t1.age < t2.age) { return -1; } if (t1.age == t2.age) { return t1.name.compareTo(t2.name);// 如果age相等则用name排序 } return t1.age; } }
测试方法
void test6(){ Teacher teacher1 = new Teacher("jack", 23); Teacher teacher2 = new Teacher("mary", 25); Teacher teacher3 = new Teacher("Tom", 25); Teacher teacher4 = new Teacher("Alice", 13); List<Teacher> list = new ArrayList<>(); list.add(teacher1); list.add(teacher2); list.add(teacher3); list.add(teacher4); list.add(teacher2); Collections.sort(list); for (Teacher teacher : list) { System.out.println(teacher); } } void test7(){ Teacher2 teacher1 = new Teacher2("jack", 23); Teacher2 teacher2 = new Teacher2("mary", 25); Teacher2 teacher3 = new Teacher2("Tom", 25); Teacher2 teacher4 = new Teacher2("Alice", 13); AgeAndNameSort ageAndNameSort = new AgeAndNameSort(); List<Teacher2> list = new ArrayList<>(); list.add(teacher1); list.add(teacher2); list.add(teacher3); list.add(teacher4); list.add(teacher2); Collections.sort(list, ageAndNameSort); for (Teacher2 teacher : list) { System.out.println(teacher); } }
测试结果
Teacher [name=Alice, age=13] Teacher [name=jack, age=23] Teacher [name=Tom, age=25] Teacher [name=mary, age=25] Teacher [name=mary, age=25]
3，使用细节
void test3(){ List<Integer> list = new ArrayList<>(); list.add(1); list.add(11); list.add(111); //列表转换为数组 Integer[] ls1 = list.toArray(new Integer[2]); //Integer[] ls2 = (Integer[]) list.toArray();//cannot be cast to [Ljava.lang.Integer; Object[] ls3 = list.toArray(); System.out.println(ls1); //System.out.println(ls4); System.out.println(ls3); } void test4(){ Integer[] in = {33, 44, 55}; //array 转换为list List<Integer> list = Arrays.asList(in);//先转换为list 这个list为只读 没有add方法 List<Integer> arrayList = new ArrayList<>(); arrayList.addAll(list); } void test5(){ //并发修改异常 迭代器相当于集合的副本，当迭代器和集合不一样时（元素个数不同等等）就会报错 //at java.util.ArrayList$Itr.checkForComodification(Unknown Source) List<String> lt = new ArrayList<String>(); lt.add("java"); lt.add("python"); lt.add("php"); // Iterator<String> it = lt.iterator(); // while(it.hasNext()){ // if(it.next().equals("java")){ // lt.add("C++"); // } // System.out.println(it.next());//报错Exception in thread "main" java.util.NoSuchElementException // }
二，set集合
HashSet集合可以去重，但对于自定义数据类型必须重写equals方法和hashCode方法
当你把对象加入到HashSet时，它会使用对象的hashCode值判断对象加入的位置。但是同时也会与其他已经加入的对象的hashCode做对比，如果没有相符的hashCode，HashSet就会假设新对象没有重复。也就是说，如果hashcode时相异的，则HashSet会假设对象不可能是相同的。但有相同hashCode()的对象也不一定相等，所以如果HashCode()找到相同hashcode的两个对象：金加入的对象本来就存在的，它会调用其中一个的equals()来检查hashcode相等的对象是否真的相等。
TreeSet不仅可以去重还可以排序，但排序条件和list集合一样，必须实现Comparable接口或Comparactor接口
1，HashSet重写equals方法和hashCode方法
public class Student { String name; int age; public Student(String name, int age) { this.name = name; this.age = age; } public void method1() { } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + age; result = prime * result + ((name == null) ? 0 : name.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Student other = (Student) obj; if (age != other.age) return false; if (name == null) { if (other.name != null) return false; } else if (!name.equals(other.name)) return false; return true; } @Override public String toString() { return "Student [name=" + name + ", age=" + age + "]"; } }
测试类
public class HashSetTest { public static void main(String[] args) { //创建set集合 Set<Student> s1 = new HashSet<>(); //创建学生对象 Student t1 = new Student("xiaoming", 21); Student t2 = new Student("xiaojun", 20); Student t3 = new Student("xiaoming",21); //添加学生对象 s1.add(t1); s1.add(t2); s1.add(t3); //遍历学生对象 for (Student student : s1) { System.out.println(student); } /* * 未实现重写hashCode和equals方法前 * Student [name=xiaoming, age=21] * Student [name=xiaoming, age=21] * Student [name=xiaojun, age=20] * 有重复，默认的hashCode(哈希值返回每个对对象的地址值）比较结果不同， 被认为是不同对象，所以添加成功 * * * 总结： * 对于自定义类型使用HashSet必须正确重写hashCode和equals方法 */ System.out.println("--------使用方法-------"); new HashSetTest().method1(); } public void method1(){ //创建set对象 Set<Integer> s1 = new HashSet<>(); //添加元素 s1.add(11); s1.add(12); s1.add(11); s1.add(14); System.out.println(s1);//[11, 12, 14] 基本引用类型自动去重 } }
测试结果
Student [name=xiaoming, age=21] Student [name=xiaojun, age=20] --------使用方法------- [11, 12, 14]
2，TreeSet集合
public class Teacher{ String name; int age; public Teacher2(String name, int age) { super(); this.name = name; this.age = age; } @Override public String toString() { return "Teacher [name=" + name + ", age=" + age + "]"; } }
/* * 它实现了SortedSet这个接口，所以它有序 * 排序必定要实现comparable接口，目的是保持元素有序 * 可以保持集合有序状态，默认按自然顺序递增排序，也可以按照指定的比较器递增排序 * * 用没有参数的构造函数意味着以对象的comparaTo方法进行排序 * * 不需要覆写hashCode和equals的原因： * TreeSet是红黑树结构，其主要使用的是在插入的时候根据你提供的比较规则插入， 二叉树 */ public class TreeSetTest { public static void main(String[] args) { TreeSet<Teacher> t = new TreeSet<>(); Teacher t1 = new Teacher("Jack", 43); Teacher t2 = new Teacher("Mary", 39); Teacher t3 = new Teacher("Jack", 43); Teacher t4 = new Teacher("John", 23); Teacher t5 = new Teacher("Cal", 23); Teacher t6 = new Teacher("Lory", 99); t.add(t1); t.add(t2); t.add(t3); t.add(t4); t.add(t5); t.add(t6); t.add(t6); /* * 直接进行输出报错： * Teacher cannot be cast to java.lang.Comparable * * 原因： * 用TreeSet存储对象时必须实现Comparable接口，它是一个有序的列表 * * 疑问： * 为何它不用重写hashCode和equals方法？？？？ */ for (Teacher teacher : t) { System.out.println(teacher); } //建立一个自己的nameComparactor class nameCompare implements Comparator<Teacher>{ @Override public int compare(Teacher g0, Teacher g1) { // TODO Auto-generated method stub return g0.name.compareTo(g1.name); } }
三，Map基本使用
//注意事项：使用自定义对象作为key时也要重写hashCode和equals方法 public class MapTest { public static void main(String[] args) { new MapTest().test1(); } void test1(){ //创建map对象 Map<String , String> m1 = new HashMap<>(); //添加元素 映射功能 键如果不存在返回null 如果存在则返回该值进行了修改 m1.put("006", "yuan"); m1.put("002", "dai"); m1.put("005", null); m1.put("0019", "YY"); m1.put("001", "jj"); System.out.println(m1.put("004", "jack")); System.out.println(m1.put("001", "maryan")); // Map.Entry<String, String> ey = (Entry<String, String>) m1.entrySet(); Set<Map.Entry<String, String>> entry1 = m1.entrySet(); //用Map.Entry<>遍历map的键，值 for(Map.Entry<String, String> entry: m1.entrySet()){ String k = entry.getKey(); String v = entry.getValue(); } //获取key生成的集合 Set<String> s1 = m1.keySet(); System.out.println(s1); //获取value生成的集合 Collection<String> c1 = m1.values(); // List<String> l1 = (List<String>) m1.values();//查看源码可知：不能强制转换为list，它不为list 没有索引 System.out.println(c1); // System.out.println(l1); //生成新的集合，TreeMap Map<String, String> m2 = new TreeMap<>(); m2.putAll(m1); System.out.println(m1);// 有序字典 System.out.println(m2);//无序字典 }
标签：obj rgs 源码 asn java min prime static final
原文地址：https://www.cnblogs.com/noperx/p/11372342.html
