Java树映射中的用户定义类对象
我有一个集合(TreeMap),我只想从im传递的对象中打印一个值。这是我正在尝试的代码 Student.javaJava树映射中的用户定义类对象,java,collections,treemap,Java,Collections,Treemap,我有一个集合(TreeMap),我只想从im传递的对象中打印一个值。这是我正在尝试的代码 Student.java class Student { int rollno; String name; int age; Student(int rollno,String name,int age){ this.rollno=rollno; this.name=name;
class Student {
int rollno;
String name;
int age;
Student(int rollno,String name,int age){
this.rollno=rollno;
this.name=name;
this.age=age;
}
public String toString(){//overriding the toString() method
return rollno+" "+name+" "+age;
}
}
StudentTest.java
import java.util.*;
import java.io.*;
class StudentTest{
public static void main(String args[]){
Map<String,Student> al=new TreeMap<String,Student>();
al.put("a1",new Student(101,"Vijay",23));
al.put("a2",new Student(106,"Ajay",27));
al.put("a3",new Student(105,"Jai",21));
Set set = al.entrySet();
// Get an iterator
Iterator i = set.iterator();
// Display elements
while(i.hasNext()) {
Map.Entry me = (Map.Entry)i.next();
System.out.print(me.getKey() + ": ");
System.out.println(me.getValue());
}
System.out.println(al.get("a1"));
System.out.println(al.size());
Set<String> keys =al.keySet();
for(String key : keys)
{
System.out.println(al.get(key));
}
Collection entrySet = al.entrySet();
Iterator<Student> itr = entrySet.iterator();
System.out.println("+++++++"+entrySet);
System.out.println("\n\n\n\n\n");
System.out.println("+++++++"+itr);
System.out.println("\n\n\n\n\n");
while(itr.hasNext())
{
Student st = itr.next();
System.out.println(st.rollno);
}
}
}
import java.util.*;
导入java.io.*;
班级学生测验{
公共静态void main(字符串参数[]){
Map al=新树映射();
al.put(“a1”,新生(101,“Vijay”,23));
al.put(“a2”,新生(106,“Ajay”,27));
al.put(“a3”,新生(105,“Jai”,21));
Set Set=al.entrySet();
//获取迭代器
迭代器i=set.Iterator();
//显示元素
while(i.hasNext()){
Map.Entry me=(Map.Entry)i.next();
System.out.print(me.getKey()+“:”);
System.out.println(me.getValue());
}
System.out.println(al.get(“a1”));
System.out.println(al.size());
Set keys=al.keySet();
用于(字符串键:键)
{
System.out.println(al.get(key));
}
集合entrySet=al.entrySet();
迭代器itr=entrySet.Iterator();
System.out.println(“+entrySet”);
System.out.println(“\n\n\n\n\n”);
System.out.println(“+itr”);
System.out.println(“\n\n\n\n\n”);
while(itr.hasNext())
{
学生st=itr.next();
系统输出打印LN(圣罗尔诺);
}
}
}
请让我知道我尝试的代码错误的解决方案 地图的
entrySet
是一组map.Entry
s
你有这个:
Collection entrySet = al.entrySet();
Iterator<Student> itr = entrySet.iterator();
1:
me.getValue().rollno
immibis me.getValue().rollno将给出错误:----StudentTest.java:22:错误:找不到符号System.out.println(me.getValue().rollno);symbol:variable rollno location:class对象“请让我知道我正在尝试的代码的错误解决方案。”-什么错误?阅读错误信息是开悟的第一步。对不起,伊米比斯没有看它,我教它是粘贴的。错误是:-----线程“main”java.lang.ClassCastException中的异常:java.util.TreeMap$项无法在StudentTest.main(StudentTest.java:42)Student st st=(Student)itr.next()处强制转换为Student;//这是一行感谢immibis分享我正在寻找的正确答案,并纠正我加上一句“永远不要使用原始类型”
Collection<Student> entrySet = al.entrySet(); // ERROR HERE - Set<Map.Entry<String, Student>> cannot be cast to Collection<Student>
Iterator<Student> itr = entrySet.iterator();
Collection<Student> entrySet = al.values();
Iterator<Student> itr = entrySet.iterator();