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Here's the code :

public class EmployeeTest { public static void main(String args[]){ //System.out.println("hello world"); Employee aEmployee = new Employee("David",1000); System.out.println(aEmployee.getName() + aEmployee.getSalary()); } } class Employee // **why can't I put a "public" here** { // Constructor public Employee(String name, double salary) { this.name = name; this.salary = salary; } // Methods public String getName() { return this.name; } public double getSalary() { return this.salary; } // instance field private String name; private double salary; }

My question is : in the second class definition's first line, why can't I put a "public" to define it ? What's the exactly meaning of "public" when using it defines a class ?

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  • You can put a public there if it's in a file called Employee.java Commented Aug 15, 2012 at 9:26

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This is language feature. There must be only one top-level public class per .java file and public class name must match the source java file name.

Basically, non-public types are not accessible outside the package so if you wish to allow type to be used anywhere then make it public.

Never create a type in default package. (Always use package)

Employee.java

package com.abc.model; public class Employee{..}

EmployeeTest.java

package com.abc.test; public class EmployeeTest{ ... }
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Because a Java source file can have at most one top-level public class or interface, and the name of the source file must be the same as the name of that class or interface.

That's a rule that the Java compiler of Oracle's JDK imposes.

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Can you please prove evidence for the "at most one" part? Ive searched the spec over and over and havent found such restriction. Besides that Employee is a top-level class unaffected whether its public or not.
A source file can have zero or one top-level public class or interface. It's not part of the language spec (it's not a language feature as AVD says); it's a restriction in Oracle's Java compiler.
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In Java, there can only be a single public top level class per source file and it needs to be named the same as the file.

This is useful for the compiler when it needs to locate a class definition from outside the package, since it knows the type name, it knows which class file to find the class in. For example. since a jar file is in essence a zip file with class files, this prevents the compiler from having to unzip the entire jar to find a class definition.

The Java language specification §7.6 specifies this as an optional restriction;

If and only if packages are stored in a file system (§7.2), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as .java or .jav) if either of the following is true:

  • The type is referred to by code in other compilation units of the package in which the type is declared.

  • The type is declared public (and therefore is potentially accessible from code in other packages).

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you can define a public class inside a public class which is legal.

public class EmployeeTest { public class Employee { } }
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@joshz, yes you can do like this
Well that is not going to work in his specific case. The inner class must be static.

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