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java - 使用 ArrayList 检索属性？

问题描述

*import java.util.*;  
public class UserArrayList {
    public static void main(String[] args) {
        //Creating user defined class objects  
        Student s1=new Student(1,"AAA",13);  
        Student s2=new Student(2,"BBB",14);  
        Student s3=new Student(3,"CCC",15); 
        ArrayList<Student> al=new ArayList<Student>();
        al.add(s1);
        al.add(s2);  
        al.add(s3);  
        Iterator itr=al.iterator();  
        //traverse elements of ArrayList object  
        while(itr.hasNext()){  
            Student st=(Student)itr.next();  
            System.out.println(st.rollno+" "+st.name+" "+st.age);  
        }  
    }
}
class Student{  
    int rollno;  
    String name;  
    int age;  
    Student(int rollno,String name,int age){  
        this.rollno=rollno;  
        this.name=name;  
        this.age=age;  
    }  
}

如何使用 ArrayList 检索 Student 类的单个属性或检查列表中是否存在“AAA”？

标签： javaarraylistcollections

解决方案

刚刚添加了两行，顺便说一句。创建 ArrayList 时，该代码中有错误。你写的“ArayList”是错误的。

方法一。

public static void main(String[] args) {
            //Creating user defined class objects  
            Student s1=new Student(1,"AAA",13);  
            Student s2=new Student(2,"BBB",14);  
            Student s3=new Student(3,"CCC",15); 
            ArrayList<Student> al=new ArrayList<Student>(); // YOU HAVE MISTAKE HERE ArayList<Student>()
            al.add(s1);
            al.add(s2);  
            al.add(s3);  
            Iterator itr=al.iterator();  
            //traverse elements of ArrayList object  
            while(itr.hasNext()){  
                Student st=(Student)itr.next();  
                System.out.println(st.rollno+" "+st.name+" "+st.age);  
                if(st.name.equals("AAA"))  // LINE ADDED
                    System.out.println("present!"); // LINE ADDED

            }  
        }
    }
    class Student{  
        int rollno;  
        String name;  
        int age;  
        Student(int rollno,String name,int age){  
            this.rollno=rollno;  
            this.name=name;  
            this.age=age;  
        }

方法 2 -> Java 8 或更高版本

  Student find = al.stream().filter(s -> s.name.equals("AAA")).findFirst().orElse(null);
        if(find != null)
            System.out.println("AAA is present");

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