如何编译具有多个顶级类的单个Java文件

Question

This is my java code:

import java.io.*;
import java.util.*;
public class student
{
    String name;
    int age;
    float cgpa;
}
public class getdata
{
    public static void main(String args[])
    {
        int n;
        Scanner in=new Scanner(System.in);
        n=in.nextInt();
        student[] s=new student[n];
        for(int i=0;i<n;i++)
        {
            s[i]=new student();
            s[i].name=in.nextLine();
            s[i].age=in.nextInt();
            s[i].cgpa=in.nextFloat();
        }
        System.out.println();
        System.out.println("Name\tAge\tCGPA\n");
        for(int i=0;i<n;i++)
        {
            System.out.println(s[i].name+"\t"+s[i].age+"\t"+s[i].cgpa+"\n");
        }
    }
}

When I compile the program, it gives the error that each class should be in a different file. How can i compile this without any error? (I know this is not the best practice, but I need to do this in the same file.)

Answer 1

The general rule is that you can have only one public class in a Java file.

If you want it to be visible from everywhere (this is based on your comment) you can declare the class as public static inside the getdata.

It will be something like this:

public class getdata {
   public static class user {
     //class definition here
  }
}

Now your User class is available for instantiation everywhere.

Answer 2

Make the class Student to an inner class:

import java.io.*;
import java.util.*;
public class getdata
{
    public static void main(String args[])
    {
        int n;
        Scanner in=new Scanner(System.in);
        n=in.nextInt();
        student[] s=new student[n];
        for(int i=0;i<n;i++)
        {
            s[i]=new student();
            s[i].name=in.nextLine();
            s[i].age=in.nextInt();
            s[i].cgpa=in.nextFloat();
        }
        System.out.println();
        System.out.println("Name\tAge\tCGPA\n");
        for(int i=0;i<n;i++)
        {
            System.out.println(s[i].name+"\t"+s[i].age+"\t"+s[i].cgpa+"\n");
        }
    }
    public static class student
    {
        String name;
        int age;
        float cgpa;
    }

}

File name must be getdata.java . And read about java naming convention. Class names should start with uppercase letter.

Answer 3

You can make an inner class as mentioned before, or if you don't need to access this class from outside this file then just remove the public tag. This will allow you to put multiple top level classes in a single file.

Answer 4

You can create a .class file and then simply run class you want to compile

javac FileName.java

java fileName_containsMainMethod

EDIT class cannot be public

eg Consider the following class is saved as name Test.java and contain Two classes under single Name Test.java

class A
{
public static void main(String[] a)
{
System.out.println("Hello from A");
}
}

class B
{
public static void main(String[] a)
{
System.out.println("Hello from B");
}
}

Now to run this you need to do following javac Test.java

To get Output from Class A

java A

To get Output from Class B

java B

NOTE Remove public access modifier from class