Java笔记 泛型

泛型：将对象的类型作为参数，指定到其他类或者方法上，从而保证类型转换的安全性和稳定性

 语法结构如下：类或者接口<类型实参> 对象 = new 类<类型实参>( );

泛型集合  

        泛型集合可以约束集合内的元素类型

        典型泛型集合ArrayList<E>、HashMap<K,V>

                <E>、<K,V>表示该泛型集合中的元素类型

                泛型集合中的数据不再转换为Object

public class NewsTitle {
	private int id;
	private String name;
	private String author;
	
	public NewsTitle() {
		super();
	}
 
	public NewsTitle(int id, String name, String author) {
		super();
		this.id = id;
		this.name = name;
		this.author = author;
	}
 
	public int getId() {
		return id;
	}
 
	public void setId(int id) {
		this.id = id;
	}
 
	public String getName() {
		return name;
	}
 
	public void setName(String name) {
		this.name = name;
	}
 
	public String getAuthor() {
		return author;
	}
 
	public void setAuthor(String author) {
		this.author = author;
	}
 
	@Override
	public String toString() {
		return "NewsTitle [id=" + id + ", name=" + name + ", author=" + author+ "]";
	}
}

import java.util.ArrayList;
import java.util.Iterator;
 
public class Test {
 
	public static void main(String[] args) {		
		NewsTitle nt1=new NewsTitle(001,"合肥又发现一名新冠肺炎感染者","合肥日报");
		NewsTitle nt2=new NewsTitle(001,"合肥近日多天有雨","合肥气象局");
		NewsTitle nt3=new NewsTitle(001,"震惊！有一程序员猝死","张三");
		
		//定义一个集合，调用ArrayList类的无参的构造方法,默认构造一个初始容量为 10 的空列表。
		ArrayList<NewsTitle> al=new ArrayList<NewsTitle>();
		
		//将新闻标题的对象存储到al集合中
		al.add(nt1);
		al.add(nt2);
		al.add(nt3);
		
		//获取新闻标题的总数，即获取集合中的元素个数
		int num=al.size();
		System.out.println("新闻的标题总数为"+num);
		
		//void add(int index,Object o):在指定的索引位置添加元素。索引位置必须介于0和列表中元素个数之
		al.add(0, nt3);
		NewsTitle news =al.get(1);
		System.out.println("获取元素："+news);
		
		//boolean contains(Object o):判断列表中是否存在指定元素，如果集合中存在你要找的元素，返回true，否则返回false
		boolean result1=al.contains(nt3);
		System.out.println("集合中你需要找到的元素nt3:"+result1);
		
		//boolean remove(Object o):从列表中删除元素，删除成功返回true，删除失败返回false
		boolean result2=al.remove(nt2);
		System.out.println("删除是否成功："+result2);
		
				
		//toArray():将集合变成数组
		Object[] obj3  =al.toArray();
		for (int i = 0; i < obj3.length; i++) {
			System.out.println("转换成数组："+obj3[i]);
		}
		System.out.println("------------------");
		for(NewsTitle ob:al){
			System.out.println(ob);
		}
		
		//遍历打印出每个新闻的名称
		for(int i=0;i<al.size();i++){
			Object obj=al.get(i);
			NewsTitle newsTitle=(NewsTitle)obj;
			System.out.println(newsTitle.getName());
		}
		
		//清除集合所有的元素
//		al.clear();
		
//		al.clone();
					
		//判断是否为空
		boolean result=al.isEmpty();
		System.out.println(result);
		
		Iterator<NewsTitle> it =al.iterator();
		while(it.hasNext()){
			//取出元素
			NewsTitle nt =it.next();
			System.out.println(nt);
		}		
	}
}

import java.util.HashMap;
import java.util.Map;
import java.util.Set;
 
public class Test {
 
	public static void main(String[] args) {
 
		//创建学生对象
		Student stu1 = new Student("张三", "男");
		Student stu2 = new Student("李四", "男");
		Student stu3 = new Student("如花", "女");
		
		//创建HashMap集合对象
		HashMap<String, Student> hm=new HashMap<String, Student>();
		
		//将学生对象添加到集合
		hm.put("jack", stu1);
		hm.put("tom", stu2);
		hm.put("rose", stu3);
		
		Object obj=hm.get("jack");
		Student stu =(Student)obj;
		System.out.println("jack对应的学员信息为：姓名："+stu.getName()+"，性别："+stu.getGender());
		
		System.out.println("---------------");
		Set<Map.Entry<String, Student>> set=hm.entrySet();
		for (Map.Entry<String, Student> me : set) {
			//获取键值对应的键
			String key=me.getKey();
			//获取键值对应的值
			Student student=me.getValue();
			System.out.println(key+"对应的学员信息为：姓名："+student.getName()+"，性别："+student.getGender());
		}				
	}
}

Collections算法类

        Java集合框架将针对不同数据结构算法的实现都保存在工具类中

        Collections类定义了一系列用于操作集合的静态方法

Collections类常用方法

        Collections和Collection不同，前者是集合的操作类，后者是集合接口

        Collections提供的常用静态方法

                sort():排序

                binarySearch()：查找

                max()\min():查找最大\最小值

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
 
public class Demo01 {
	
	public static void main(String[] args) {
	//创建一个ArrayList集合对象，存储String类型数据
	ArrayList<String> al=new ArrayList<String>();
	
	al.add("fadga");
	al.add("addfag");
	al.add("hmjej");
	al.add("fagevsf");
	al.add("jwryjwyr");
	
	System.out.println("集合排序之前：");
	for (String string : al) {
		System.out.println(string);
	}
	
	//排序
	Collections.sort(al);
	
	System.out.println("集合排序之后：");
	for (String string : al) {
		System.out.println(string);
	}
	
	//查找元素
	int index=Collections.binarySearch(al, "fagevsf");
	System.out.println(index);
	
	//求集合中的最大值最小值
	String max = Collections.max(al);
	System.out.println(max);
	String min = Collections.min(al);
	System.out.println(min);
	
	Collections.reverse(al);
	System.out.println("集合反转之后：");
	Iterator<String> it=al.iterator();
	while(it.hasNext()){
		String str =it.next();
		System.out.println(str);
	}
}
}

Collections排序

        Collections类可以对集合进行排序、查找和替换操作

        实现一个类的对象之间比较大小，该类要实现Comparable接口

                重写compareTo()方法

public class Student implements Comparable<Student> {
 
	private String name;
	private int stuId;
 
	public Student() {
		super();
	}
 
	public Student(String name, int stuId) {
		super();
		this.name = name;
		this.stuId = stuId;
	}
 
	public String getName() {
		return name;
	}
 
	public void setName(String name) {
		this.name = name;
	}
 
	public int getStuId() {
		return stuId;
	}
 
	public void setStuId(int stuId) {
		this.stuId = stuId;
	}
 
	@Override
	public String toString() {
		return "Student [name=" + name + ", stuId=" + stuId + "]";
	}
 
	@Override
	public int compareTo(Student student) {
		//比较此对象与指定对象的顺序。
		return this.stuId-student.stuId;
	}
 
}
 
 
 
import java.util.ArrayList;
import java.util.Collections;
 
public class StudentTest {
 
	public static void main(String[] args) {
		//创建4个Student类对象
		Student stu1 = new Student("张三", 1001);
		Student stu2 = new Student("李四", 1002);
		Student stu3 = new Student("王五", 1003);
		Student stu4 = new Student("赵六", 1004);
		
		//创建ArrayList集合对象
		ArrayList<Student> al = new ArrayList<Student>();
		
		al.add(stu3);
		al.add(stu2);
		al.add(stu4);
		al.add(stu1);
		
		System.out.println("集合排序前：");
		for (Student student : al) {
			System.out.println(student);
		}
		
		Collections.sort(al);		
		System.out.println("集合排序后：");
		for (Student student : al) {
			System.out.println(student);
		}
	}
}