Java集合框架详解-优快云博客

1 collections工具类

案例1:自然排序

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Test {
	public static void main(String[] args) {
		//所有的集合插入的元素必须是引用类型,Integer类型是int类型的包装类,是int类型的引用类型
		List<Integer> lists = new ArrayList<Integer>();
		//虽然这里插入的是int类型的值,但是底层会在转换成Integer类型后存放到集合中
		lists.add(1);
		lists.add(5);
		lists.add(8);
		lists.add(58);
		lists.add(7);
		lists.add(88);
		lists.add(48);
		
		//排序升序
		Collections.sort(lists);
		System.out.println(lists.toString());
	}
}

2 list与数组之间的互相转换

import java.util.Arrays;
import java.util.List;

public class day12 {
	public static void main(String[] args) {
		String names[] = new String[] {"张三","李四","王五","赵六"};
		//数组转换成list集合
		List<String> list  = Arrays.asList(names);
		//任意引用类型都可以toString()转换成数组
		System.out.println(list.toString());
		
		//list转换成数组
		String newNames[] = (String[])list.toArray();
		for (String name : newNames) {
			System.out.print(" "+name);
		}
	}
}

3 list和map之间的转换

list转换成map

工具类

package day13.util;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import day13.Student;


public class CollectionUtils {

	public static Map<String, Student> getListTranToMap(List<Student> list) {
		Map<String,Student> map = new HashMap<String,Student>();
		for (Student student : list) {
			map.put(student.getName(), student);
		}
		return map;
	}

}

Student类

public class Student implements Comparable<Student>{
	private String name;
	private int age;
	
	public void showStudent(){
		System.out.println("你好我是"+name+",年龄是:"+age);
	}
	
	public Student() {
		super();
	}
	public Student(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}

	@Override
	public int compareTo(Student stu) {
		int num = this.age-stu.age;
		return num;
	}
}

测试

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import day13.util.CollectionUtils;

public class Test {
	public static void main(String[] args) {
		Student stu1 = new Student("马云",45);
		Student stu2 = new Student("马化腾",46);
		Student stu3 = new Student("董明珠",47);
		Student stu4 = new Student("章泽天",28);
		
		List<Student> list = new ArrayList<Student>();
		list.add(stu1);
		list.add(stu2);
		list.add(stu3);
		list.add(stu4);
		//把list转换成map
		Map<String,Student> map =CollectionUtils.getListTranToMap(list);
		
		for (Student student : map.values()) {
			student.showStudent();
		}
	}
}

map转换成list

工具类

public static List<Student> getMapTranToList(Map<String,Student> map) {
		List<Student> list = new ArrayList<Student>();
		for (Student student : map.values()) {
			list.add(student);
		}
		return list;
	}

测试

import java.util.Hashtable;
import java.util.List;
import java.util.Map;

import day13.util.CollectionUtils;

public class Test1 {
	public static void main(String[] args) {
		Student stu1 = new Student("马云",45);
		Student stu2 = new Student("马化腾",46);
		Student stu3 = new Student("董明珠",47);
		Student stu4 = new Student("章泽天",28);
		
		Map<String,Student> map = new Hashtable<String,Student>();
		map.put(stu1.getName(), stu1);
		map.put(stu2.getName(), stu2);
		map.put(stu3.getName(), stu3);
		map.put(stu4.getName(), stu4);
		//map转换list
		List<Student> list = CollectionUtils.getMapTranToList(map);
		
		for (Student student : list) {
			student.showStudent();
		}
	}
}

4 总结

围绕3个集合接口来讲:List、set、map
ArrayList应用场景最广泛,用于大量的遍历有序可重复的数据的场景
ArrayList、linkesList、HashSet、TreeSet、HashMap这几个集合必须学会
hashXXX:都是以哈希算法存储的集合,无序
LinkedXXX:都是双向链表存储的集合,有序,会将值按照插入的顺序存储
TreeXXX:都是以红黑树算法存储的集合,有序,会按照排序方式排序后再存储,排序方式有自然排序和自定义排序规则。

5 扩展

了解一下枚举集合EnumSet、EnumMap

了解一下泛型
pTranToList(Map<String,Student> map) {
List list = new ArrayList();
for (Student student : map.values()) {
list.add(student);
}
return list;
}


* 测试

```java
import java.util.Hashtable;
import java.util.List;
import java.util.Map;

import day13.util.CollectionUtils;

public class Test1 {
	public static void main(String[] args) {
		Student stu1 = new Student("马云",45);
		Student stu2 = new Student("马化腾",46);
		Student stu3 = new Student("董明珠",47);
		Student stu4 = new Student("章泽天",28);
		
		Map<String,Student> map = new Hashtable<String,Student>();
		map.put(stu1.getName(), stu1);
		map.put(stu2.getName(), stu2);
		map.put(stu3.getName(), stu3);
		map.put(stu4.getName(), stu4);
		//map转换list
		List<Student> list = CollectionUtils.getMapTranToList(map);
		
		for (Student student : list) {
			student.showStudent();
		}
	}
}