比较

Comparable 是排序接口；若一个类实现了 Comparable 接口，就意味着 “该类支持排序”。
而 Comparator 是比较器；我们若需要控制某个类的次序，可以建立一个 “该类的比较器” 来进行排序。
前者应该比较固定，和一个具体类相绑定，而后者比较灵活，它可以被用于各个需要比较功能的类使用。可以说前者属于 “静态绑定”，而后者可以 “动态绑定”。我们不难发现：Comparable 相当于 “内部比较器”，而 Comparator 相当于 “外部比较器”

自定义的类型，谁大谁小默认情况下，系统是无法识别的，引入比较器的目的是自定义对象的比较规则。
类比较:Comparable 接口 （规则一般定义死，修改比较麻烦）
自比较:Comparator 接口 (建议使用，比较灵活)
数值用-：
   s1.getScore()-s2.getScore();
String类型用compare.to():
    s1.getStuName().compareTo(s2.getStuName());
    若是字母比较,如：A a D排序之后为A D a，采用a1.toUpperCase().compare.to(a2.toUpperCase())

实例

1 使用集合工具类对对象进行排序(对象实现Comparable接口）

//1.定义学生类
public class Student implements Comparable<Student>{
    private int id;
    private String stuName;
    private int score;
    public Student() {
        super();
    }
    public Student(int id, String stuName, int score) {
        super();
        this.id = id;
        this.stuName = stuName;
        this.score = score;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getStuName() {
        return stuName;
    }
    public void setStuName(String stuName) {
        this.stuName = stuName;
    }
    public int getScore() {
        return score;
    }
    public void setScore(int score) {
        this.score = score;
    }
    @Override
    public String toString() {
        return "Student [id=" + id + ", stuName=" + stuName + ", score=" + score + "]";
    }
    //定义学生对象的比较规则  arr[j+1]-arr[j]>0
    //1.如果成绩一样，按照编号进行顺序排列 Comparable        
//    @Override
//    public int compareTo(Student stu) {
//        if(stu.getScore()==this.getScore()) {
//            return stu.getId()-this.getId();
//        }
//        return this.getScore()-stu.getScore();
//    }
    //2.按成绩排序，如果成绩一样，按照名字进行顺序排列
    @Override
    public int compareTo(Student stu) {
        if(stu.getScore()==this.getScore()) {
            return this.getStuName().compareTo(stu.getStuName());
        }        
        return this.getScore()-stu.getScore();
    }
}
//2.测试类中添加学生信息，然后进行排序
public static void main(String[] args) {
        ArrayList<Student> list=new ArrayList<>();
        list.add(new Student(3,"jack",89));
        list.add(new Student(1,"rouse",79));
        list.add(new Student(4,"tina",99));
        list.add(new Student(2,"apple",69));
        list.add(new Student(6,"oracle",89));
        list.add(new Student(5,"marry",59));
        System.out.println("排序之前:"+list);
        Collections.sort(list);
        System.out.println("排序之后:"+list);    
        Student stuMax=Collections.max(list);
        System.out.println("最高分:"+stuMax);
        Student stuMin=Collections.min(list);
        System.out.println("最低分:"+stuMin);
    }

运行结果：

排序之前:[Student [id=3, stuName=jack, score=89], Student [id=1, stuName=rouse, score=79], Student [id=4, stuName=tina, score=99], Student [id=2, stuName=apple, score=69], Student [id=6, stuName=oracle, score=89], Student [id=5, stuName=marry, score=59]]
排序之后:[Student [id=5, stuName=marry, score=59], Student [id=2, stuName=apple, score=69], Student [id=1, stuName=rouse, score=79], Student [id=3, stuName=jack, score=89], Student [id=6, stuName=oracle, score=89], Student [id=4, stuName=tina, score=99]]
最高分:Student [id=4, stuName=tina, score=99]
最低分:Student [id=5, stuName=marry, score=59]

2 采用匿名内部类的方式实现Comparator 接口

更加的灵活，一般会采用匿名内部类的方式自定义比较规则(写法更加的简洁)
public static void main(String[] args) {
        ArrayList<Student> list=new ArrayList<>();
        list.add(new Student(3,"jack",89));
        list.add(new Student(1,"rouse",79));
        list.add(new Student(4,"tina",99));
        list.add(new Student(2,"apple",69));
        list.add(new Student(6,"oracle",89));
        list.add(new Student(5,"marry",59));
        //1.顺序排列
        Collections.sort(list,new Comparator<Student>() {
            @Override
            public int compare(Student stu1, Student stu2) {
                return stu1.getScore()-stu2.getScore();
            }    
        });
        System.out.println("按照成绩排序之后:"+list);
        //2.返回最高分
        Student maxStu=Collections.max(list,new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getScore()-o2.getScore();
            }
        });
        System.out.println(maxStu);
    }

运行结果：

按照成绩排序之后:[Student [id=5, stuName=marry, score=59], Student [id=2, stuName=apple, score=69], Student [id=1, stuName=rouse, score=79], Student [id=3, stuName=jack, score=89], Student [id=6, stuName=oracle, score=89], Student [id=4, stuName=tina, score=99]]
Student [id=4, stuName=tina, score=99]

比较器的练习题

创建一个商品类Goods(int id, String goodsName,int num,double price) 99.99 99.89,99.79创建一个测试类定义集合，存入6件商品信息<1>.按照价格进行顺序排列，如果价格一样按照商品的数量排序。<2>.求出最高价格的商品，以及价格最低的商品

public class Goods {
    private int id;
    private String goodsName;
    private int num;
    private double price;
    public Goods() {
    }
    public Goods(int id, String goodsName, int num, double price) {
        this.id = id;
        this.goodsName = goodsName;
        this.num = num;
        this.price = price;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getGoodsName() {
        return goodsName;
    }
    public void setGoodsName(String goodsName) {
        this.goodsName = goodsName;
    }
    public int getNum() {
        return num;
    }
    public void setNum(int num) {
        this.num = num;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    @Override
    public String toString() {
        return "Goods{" +
                "id=" + id +
                ", goodsName='" + goodsName + '\'' +
                ", num=" + num +
                ", price=" + price +
                '}';
    }
}

注意：比较器只能返回int类型，当比较的结果是double类型时，需进行转换判断（>0则互换位置）

Java面试题

comparable与comparator

比较

实例

1 使用集合工具类对对象进行排序(对象实现Comparable接口）

2 采用匿名内部类的方式实现Comparator 接口

比较器的练习题