Java Source File Structure — Complete Guide Java Source File Structure A single .java file can hold several classes — but there are strict rules about how many of them can be public, and how that affects the file name. Get this wrong and you'll hit a compile-time error before your code even runs. This post walks through the rules with three quick test cases. The Core Rule A Java program can contain any number of classes, but at most one class can be declared as public. If there is a public class, the name of the program and the name of the public class must match; otherwise we will get a compile time error. Case 1 — No Public Class If there is no public class then we can use any name and there are no restrictions. class A{ } class B{ } class C{ } class D{ } Save the program as any ClassName.java . Case 2 — One Public Class If class B is public, then the name of the program should be B.java ; otherwise we will get a compi...