Java基础三十二（比较器）_bigdecimal comparator-优快云博客

一、说明：Java中的对象，正常情况下，只能进行比较：== 或！=。不能使用 > 或 <
在开发场景中，需要对多个对象进行排序，言外之意，需要比较对象的大小
如何实现？使用两个接口中的任何一个：Comparable或Comparator

二、Comparable接口与Comparator的使用的对比：
Comparable接口的方式一旦指定，保证Comparable接口实现类的对象都可以比较大小
Comparator接口属于临时性的比较。
Comparable接口的使用举例：自然排序

1.像String、包装类等实现了Comparable接口，重写了compareTo()方法，给出了比较两个对象大小的方式
2.像String、包装类重写compareTo()方法后，进行了从小到大的排序
3.重写compareTo()的规则：
如果当前对象this大于形参队形obj，则返回正整数，如果当前对象this小于形参对象obj，则返回负整数，等于则返回0
4.对于自定义类来说，如果需要排序，可以让自定义类实现Comparable接口，重写compareTo(obj)方法。
在compareTo(obj)方法中指明如何排序

@Test
    public void test1(){
        String[] arr = new String[]{"AA", "CC", "GG", "DD", "JJ"};
        Arrays.sort(arr);
        System.out.println(arr);//[Ljava.lang.String;@6477463f
        System.out.println(Arrays.toString(arr));//[AA, CC, DD, GG, JJ]
    }

    @Test
    public void test2(){
        Goods[] arr = new Goods[6];
        arr[0] = new Goods("lenovoMouse", 34);
        arr[1] = new Goods("xiaomiMouse", 43);
        arr[2] = new Goods("huaweiMouse", 12);
        arr[3] = new Goods("dellMouse", 65);
        arr[4] = new Goods("vivoMouse", 43);
        arr[5] = new Goods("oppoMouse", 65);

        Arrays.sort(arr);
        System.out.println(Arrays.toString(arr));
        //java.lang.ClassCastException: com.java_senior.day02.object_04_compare.Goods cannot be cast to java.lang.Comparable

        System.out.println(Arrays.toString(arr));
        //[Goods{name='huaweiMouse', price=12.0}, Goods{name='lenovoMouse', price=34.0}, Goods{name='xiaomiMouse', price=43.0}, Goods{name='dellMouse', price=65.0}]

    }

public class Goods implements Comparable{

    private String name;
    private double price;

    public Goods() {
    }

    public Goods(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Goods{" +
                "name='" + name + '\'' +
                ", price=" + price +
                '}';
    }

    //指明商品比较大小的方式：价格从低到高排序，再按照产品名称从低到高
    @Override
    public int compareTo(Object o) {
        if (o instanceof Goods){
            Goods goods = (Goods)o;
            //方式一
            if (this.price > goods.price){
                return 1;
            }else if(this.price < goods.price){
                return -1;
            }else{
//                return 0;
                return this.name.compareTo(goods.name);
//                return -this.name.compareTo(goods.name);//从高到低
            }
//            //方式二
//            return Double.compare(this.price, goods.price);
        }
        throw new RuntimeException("传入的数据类型不一致！");
    }
}

Comparator接口的使用：定制排序

1.背景：当元素的类型没有实现java.lang.Comparable接口而又不方便修改代码，
或者实现了接口的排序规则不适合当前的操作，可以考虑使用Comparator的对象来排序。
2.重写compare(object o1, object o2)方法，比较o1和o2的大小：
如果方法返回正整数，则表示o1大于o2；
如果返回0，则相等
返回负整数，表示o1小于o2

public class ComparatorTest {

    @Test
    public void test1(){

        String[] arr = new String[]{"AA", "CC", "GG", "DD", "JJ"};

        Arrays.sort(arr, new Comparator(){

            //按照字符串从大到小排序
            @Override
            public int compare(Object o1, Object o2) {
                if (o1 instanceof String && o2 instanceof String){
                    String s1 = (String)o1;
                    String s2 = (String)o2;
                    return -s1.compareTo(s2);
                }
//                return 0;
                throw new RuntimeException("输入数据类型不一致");
            }
        });
        System.out.println(Arrays.toString(arr));
    }

    @Test
    public void test2(){
        Goods[] arr = new Goods[6];
        arr[0] = new Goods("lenovoMouse", 34);
        arr[1] = new Goods("xiaomiMouse", 43);
        arr[2] = new Goods("huaweiMouse", 12);
        arr[3] = new Goods("dellMouse", 65);
        arr[4] = new Goods("huaweiMouse", 43);
        arr[5] = new Goods("oppoMouse", 65);

        Arrays.sort(arr, new Comparator() {
            //指明商品比较大小的方式：按照产品名称从低到高,价格从高到低排序
            @Override
            public int compare(Object o1, Object o2) {
                if (o1 instanceof Goods && o2 instanceof Goods){
                    Goods g1 = (Goods)o1;
                    Goods g2 = (Goods)o2;
                    if (g1.getName().equals(g2.getName())){
                        return -Double.compare(g1.getPrice(), g2.getPrice());
                    }else {
                        return g1.getName().compareTo(g2.getName());
                    }
                }
//                return 0;
                throw new RuntimeException("输入数据类型不一致");
            }
        });
        System.out.println(Arrays.toString(arr));
        //[Goods{name='dellMouse', price=65.0}, Goods{name='huaweiMouse', price=43.0}, Goods{name='huaweiMouse', price=12.0}, Goods{name='lenovoMouse', price=34.0}, Goods{name='oppoMouse', price=65.0}, Goods{name='xiaomiMouse', price=43.0}]

    }
}

其他常用类的使用

1.System
2.Math
3.BigInteger 和 BigDecimal

public class OtherClassTest {

    @Test
    public void test1() {
        String javaVersion = System.getProperty("java.version");
        System.out.println("java的version:" + javaVersion);//java的version:1.8.0_271

        String javaHome = System.getProperty("java.home");
        System.out.println("java的home:" + javaHome);//java的home:D:\Java\jre

        String osName = System.getProperty("os.name");
        System.out.println("os的name:" + osName);//os的name:Windows 10

        String osVersion = System.getProperty("os.version");
        System.out.println("os的version:" + osVersion);//os的version:10.0

        String userName = System.getProperty("user.name");
        System.out.println("user的name:" + userName);//user的name:chen

        String userHome = System.getProperty("user.home");
        System.out.println("user的home:" + userHome);//user的home:C:\Users\chen

        String userDir = System.getProperty("user.dir");
        System.out.println("user的dir:" + userDir);//user的dir:F:\IDEA\JavaSenior

    }

    @Test
    public void test2(){
        BigInteger bi = new BigInteger("1243324112234324324325235245346567657653");
        BigDecimal bd = new BigDecimal("12435.351");
        BigDecimal bd2 = new BigDecimal("11");
        System.out.println(bi);//1243324112234324324325235245346567657653
        //System.out.println(bd.divide(bd2));
        //java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.
        System.out.println(bd.divide(bd2, BigDecimal.ROUND_HALF_UP));//1130.486
        System.out.println(bd.divide(bd2, 25, BigDecimal.ROUND_HALF_UP));//1130.4864545454545454545454545
    }

}