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
}
}