Java Collections的sort方法的使用

叁歲伎倆 2024-02-17 21:16 53阅读 0赞

**java.util.Collections.sort(List<T> list)方法的声明**

public static <T extends Comparable<? super T>> void sort(List<T> list) {
            list.sort(null);
        }

java.util.Collections.sort(List<T> list,Comparator<? super T> c **)方法的声明**

public static <T> void sort(List<T> list, Comparator<? super T> c) {
            list.sort(c);
        }

(1)如果是单一的数据存放在集合中,是可以直接排序的

private static void sort1() {
    		List list = new ArrayList();
    		list.add(9);
    		list.add(29);
    		list.add(100);
    		list.add(3);
    		// sort(集合)
    		Collections.sort(list);
    		for (Object object : list) {
    			System.out.println(object);
    		}
    	}

![Center][]

**(2)如果是对象数据存放在集合中,是不可以直接排序的!**

**(a)实体类必须要实现这个接口Comparable**

/**
     * 实体类  必须实现这个接口Comparable
     * 
     * @author Administrator
     *
     */
    public class Student implements Comparable<Student> {
    	// 属性
    	private String name;
    	private int age;
    	private String sex;
    
    	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;
    	}
    
    	public String getSex() {
    		return sex;
    	}
    
    	public void setSex(String sex) {
    		this.sex = sex;
    	}
    
    	/**
    	 * 带参数的构造方法
    	 * 
    	 * @param name
    	 * @param age
    	 * @param sex
    	 */
    	public Student(String name, int age, String sex) {
    		super();
    		this.name = name;
    		this.age = age;
    		this.sex = sex;
    	}
    
    	/**
    	 * 无参的构造方法
    	 */
    	public Student() {
    		super();
    	}
    
    	@Override
    	public String toString() {
    		return "Student [name=" + name + ", age=" + age + ", sex=" + sex + "]";
    	}
    
    	/**
    	 * 重写比较方法
    	 * 按照此比较的东西从小到大排列
    	 */
    	@Override
    	public int compareTo(Student o) {
    		if (this.age < o.age)
    			return -1;
    		if (this.age > o.age) {
    			return 1;
    		}
    		return 0;
    	}
    
    }

测试类:

private static void sort2() {
    		// 1实例化对象
    		Student stu1 = new Student("qq", 22, "F");
    		Student stu2 = new Student("rose", 16, "M");
    		Student stu3 = new Student("boy", 25, "F");
    		Student stu4 = new Student("fei", 18, "M");
    
    		List<Student> list1 = new ArrayList<Student>();
    		list1.add(stu1);
    		list1.add(stu2);
    		list1.add(stu3);
    		list1.add(stu4);
    		// Exception in thread "main" java.lang.ClassCastException:
    		// com.hlx.Student cannot be cast to java.lang.Comparable
    		Collections.sort(list1);
    
    		for (Student student : list1) {
    			System.out.println(student);
    		}
    	}

效果: *从小到大的顺序排序*

**![Center 1][]**

**(b)用Comparator接口**

**注意:实体类不需要实现这个接口**

**测试类:**

private static void mysort1() {
    		// 1实例化对象
    		Student stu1 = new Student("qq", 22, "F");
    		Student stu2 = new Student("rose", 16, "M");
    		Student stu3 = new Student("boy", 25, "F");
    		Student stu4 = new Student("fei", 18, "M");
    
    		List<Student> list1 = new ArrayList<Student>();
    		list1.add(stu1);
    		list1.add(stu2);
    		list1.add(stu3);
    		list1.add(stu4);
    
    		// 这样就可以实现自定义降序或者升序排列list了
    		Collections.sort(list1, new Comparator<Student>() {
    
    			@Override
    			public int compare(Student o1, Student o2) {
    				if (o1.getAge() > o2.getAge()) {
    					return -1;
    				} else if (o1.getAge() < o2.getAge()) {
    					return 1;
    				}
    				return 0;
    			}
    		});
    
    		for (Student student : list1) {
    			System.out.println(student);
    		}
    	}

**效果: *从大到小的顺序排序***

**![Center 2][]**

[Center]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/1cc12d1f040a413c9a6577aee9a3adbb.png
[Center 1]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/c851f80f013843408a4744db0c037376.png
[Center 2]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/e84e67f070594bccbc1c6202c8cf2f37.png