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part3 jdk17新特性详解

news 来源：原创 2024/5/12 8:12:46

switch case 匹配

多行字符串处理
在这里插入图片描述

public class htmlHandler {public static void main(String[] args) {//jdk17处理多行字符方式String html2= """<html>\<body>\s<h1>helloworld</h1></body></html>""";}
}jdk17提供了两个转义符：\ 置于行尾  转成一行\s 单个空白字符

instance of 增强

jdk17之前，需要强转

在这里插入图片描述

jdk17之后不需要类型强转

public class InstanceOf {public static void main(String[] args) {Object o=2;if(o instanceof  Integer i){System.out.println(i.intValue());}}
}

密封类/限制继承类(sealed class) :

限制子类继承，防止子类变父类；
sealed修饰在父类或者接口上；
子类必须使用final或non-sealed修饰 ;
密封类的子类或父类必须放在同一个包中

父接口：


public abstract   sealed class Animal permits Dog, Cat {
}

子接口：


non-sealed修饰的子类可以被子类继承package com.newfeature;import com.newfeature.Animal;public  non-sealed  class Dog extends Animal {
}package com.newfeature.domain;import com.newfeature.Dog;public class HaShiQi extends Dog {
}final修饰的子类无法被继承package com.newfeature;import com.newfeature.Animal;public final class Cat extends Animal {
}

密封类的优点：1.更加安全，避免不必要的继承； 2.更加可控，可以限制类的包范围

Record类，类似lombok的属性只读对象；提供全参的构造方法，以及只读的get方法，没有set

package com.newfeature;public record RecordUser(Long userId,String userName) {
}package com.newfeature;public class RecordDemo {public static void main(String[] args) {RecordUser recordUser=new RecordUser(1L,"heihe");RecordUser recordUser1=new RecordUser(1L,"heihe");System.out.println("获取属性username:"+recordUser.userName()+","+recordUser.equals(recordUser1));}
}说明：该equal方法是record重新的方法，因此对象比较，内容相同，返回结果为true.

输出：

获取属性username:heihe,true

6.优化空指针异常信息：告诉哪个对象空指针了

创建类Dept,person

package com.newfeature;import com.newfeature.domain.Person;
import lombok.Data;@Data
public class Dept {private Integer DeptNo;private Person person;
}package com.newfeature.domain;import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;@Data
@AllArgsConstructor
@NoArgsConstructor
public class Person {private String name;private Integer age;private Integer height;
}

创建测试类

package com.newfeature;public class NullPointerDemo {public static void main(String[] args) {Dept dept=new Dept();System.out.println(dept.getPerson().getAge());}
}

输出结果：

"C:\Program Files\Java\jdk-17\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2021.3\lib\idea_rt.jar=54479:C:\Program Files\JetBrains\IntelliJ IDEA 2021.3\bin" -Dfile.encoding=UTF-8 -classpath D:\ideaworkplace\ag\target\classes;C:\Users\Lenovo\.m2\repository\org\projectlombok\lombok\1.18.28\lombok-1.18.28.jar com.newfeature.NullPointerDemo
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.newfeature.domain.Person.getAge()" because the return value of "com.newfeature.Dept.getPerson()" is nullat com.newfeature.NullPointerDemo.main(NullPointerDemo.java:8)Process finished with exit code 1