static main in classes Java

Question

I am learning Java and I am currently following a book and this is the code the book provided:

package Person;

public class Person {
    private String lastName;
    private String firstName;
    private int age;

    public Person(String last, String first, int a) {       //constructor
        lastName = last;
        firstName = first;
        age = a;
    }

    public void displayPerson() {
        System.out.print("    Last name: " + lastName);
        System.out.print(", First name: " + firstName);
        System.out.println(", Age: " + age);
    }

    public String getLast(){
        return lastName;
    }
}

class ClassDataArray {
    private Person[] a;
    private int nElems;

    public ClassDataArray(int max){
        a = new Person[max];
        nElems = 0;
    }
    public Person find(String searchName){
        int j;
        for(j = 0; j<nElems; j++){
            if(a[j].getLast().equals(searchName)){
                break;
            }
        }
        if(j == nElems){
            return null;
        }else{
            return a[j];
        }
    }

    public void insert(String last, String first, int age){
        a[nElems] = new Person(last, first, age);
        nElems++;
    }

    public boolean delete(String searchName){
        int j;
        for(j = 0; j<nElems; j++){
            if(a[j].getLast().equals(searchName)){
                break;
            }
        }
        if(j == nElems){
            return false;
        }else{
            for(int k = j; k<nElems; k++){
                a[k] = a[k + 1];
            }
            nElems--;
            return true;
        }
    }

    public void displayA(){
        for(int j = 0; j<nElems; j++){
            a[j].displayPerson();
        }
    }
//////////////////////////////////////////////////////  
    class ClassDataApp{
        public static void main(String[] args){
            int maxSize = 100;
            ClassDataArray arr;
            arr = new ClassDataArray(maxSize);

            arr.insert("Evans","Parry", 24);
            arr.insert("Smith","Lorraine",37);
            arr.insert("Yee","Tom", 43);
            arr.insert("Adams","Henry", 63);
            arr.insert("Hashimoto","Sato",21);
            arr.insert("Stimson","Henry",29);
            arr.insert("Velasquez","Jose",72);
            arr.insert("Lamarque","Henry",54);
            arr.insert("Vang","Minh",22);
            arr.insert("Creswell","Lucinda",18);

            arr.displayA();

            String searchKey = "Stimson";
            Person found;

            found = arr.find(searchKey);
            if(found != null){
                System.out.print("Found ");
                found.displayPerson();
            }else{
                System.out.println("Can't find " + searchKey);
            }
            System.out.println("Deleting Smith, Yee, and Creswell");
            arr.delete("Smith");
            arr.delete("Yee");
            arr.delete("Creswell");

            arr.displayA();
        }
    }   
}

However I am getting the error:

The method main cannot be declared static; static methods can only be declared in a static or top level type

I think creating a separate file would fix this problem but I am following a book so I might be missing a concept? The book doesn't tell me to create another .java file. It is simply 2 classes in the file and one of them contains a static main. Could someone provide some sort of insight please Thank you!

Answer 1

Unless it's a typo, you're missing a closing } before class ClassDataApp{

Answer 2

You should put your classes into different files. You have ClassDataApp nested inside ClassDataArray

Answer 3

Read this : http://docs.oracle.com/javase/tutorial/java/javaOO/nested.html it has a good example. ClassDataApp could be static if it is nested.

Answer 4

public static main(String[] args) will be treated as the entry point into your program only when it is placed in a public class. A .java file can have only one public class.

If you want to keep all of the classes in the same java file, make only ClassDataApp class public.

Otherwise, place them in the same package in different java files and make sure that ClassDataApp is public and the other classes are at least not private.