Java常用类库中（ThreadLocal、Comparable比较器、AutoCloseable、Optional空处理）附带相关面试题

package Example1511;
class Message {
    private String inFo;
    public void setInFo(String inFo) {
        this.inFo = inFo;
    }
    public String getInFo() {
        return inFo;
    }
}
class Channel {
    private static final ThreadLocal<Message> ThreadLocal = new ThreadLocal<Message>();
    //    构造方法独立，防止外部访问
    private Channel() {
    }
    //    存放对象到储物盒子ThreadLocal里面
    public static void setMessage(Message msg) {
        ThreadLocal.set(msg);
    }
    //    获取自己盒子里面的对象的信息
    public static void send() {
        System.out.println(Thread.currentThread().getName() + "获取信息:" + ThreadLocal.get().getInFo());
    }
}
public class javaDemo {
    public static void main(String[] args) {
        new Thread(() -> {
            Message msg = new Message();
            msg.setInFo("我是线程A特有的资源哦");
            Channel.setMessage(msg);
            Channel.send();
        }, "线程A").start();
        new Thread(() -> {
            Message msg = new Message();
            msg.setInFo("我是线程B特有的资源哦");
            Channel.setMessage(msg);
            Channel.send();
        }, "线程B").start();
        new Thread(() -> {
            Message msg = new Message();
            msg.setInFo("我是线程C特有的资源哦");
            Channel.setMessage(msg);
            Channel.send();
        }, "线程C").start();
    }
}

package Example1512;
import org.jetbrains.annotations.NotNull;
import java.util.Arrays;
class Student implements Comparable<Student>{
    private int id;
    private String name;
    private int score;
    Student(String name,int id,int score){
        this.name = name;
        this.id = id;
        this.score = score;
    }
//    覆写比较方法
    @Override
    public int compareTo(@NotNull Student o) {
        if (this.score<o.score){
            return 1;
        }
        else if (this.score>o.score){
            return -1;
        }else return 0;
    }
    @Override
    public String toString() {
        return "姓名:"+name + " 学号:" + id +" 总分:"+score;
    }
}
public class javaDemo {
    public static void main(String[] args) {
        Student[] allStudent= new Student[]{
                new Student("张三",210101,560),
                new Student("李四 ",210108,589),
                new Student("王五 ",210116,340),
                new Student("赵六",210153,623)
        };
        Arrays.sort(allStudent);
        System.out.println(Arrays.toString(allStudent));
    }
}

package Example1513;
interface Message extends AutoCloseable{
    public void send();
}
class NetMessage implements Message{
    private String Info;
//    构造函数
    NetMessage(String Info){
        this.Info = Info;
    }
//    判断是否正常建立连接
    public boolean open(){
        System.out.println("建立通信");
        return true;
    }
    @Override
    public void send() {
        if (open()){
                if (Info.contains("马冬梅")){
                    throw new RuntimeException();
                };
            System.out.println("发送消息:"+Info);
        }
    }
    @Override
    public void close() throws Exception {
        System.out.println("自动关闭消息通道");
    }
}
public class javaDemo {
    public static void main(String[] args) {
        try (Message msg = new NetMessage("大爷");) {
            msg.send();
        }catch (Exception e){
            System.out.println("发送失败了捏");
        }
    }
}

public static void main(String[] args) {
    try (Message msg = new NetMessage("大爷")) {
        msg.send();
    } catch (Exception e) {
        System.out.println("发送失败了捏");
    }
}

String str = null;
int length = str.length(); // 触发空指针异常，因为str对象未初始化

package Example1515;
import java.util.Optional;
interface Message{
    public String getContent();
}
class MessageImp implements Message{
    @Override
    public String getContent() {
        return "输出获取到的非空对象信息";
    }
}
class MessageUtil{
//    私立化构造方法防止外部调用
    private MessageUtil(){}
//    获取非空对象
    public static Optional<Message> getMessageImp(){
        return Optional.of(new MessageImp());
    }
//    获取内容
    public static void useMessage(Message msg){
        System.out.println(msg.getContent());
    }
}
public class javaDemo {
    public static void main(String[] args) {
        Optional<Message> test= MessageUtil.getMessageImp();
        if (test.isPresent()){
            Message temp = test.get();
            MessageUtil.useMessage(temp);
        }
    }
}