Creating, Compiling, and Executing a Java Program

You save a Java program in a .java file and compile it into a .class file. The .class file is executed by the Java Virtual Machine. If your program has compile errors, you have to modify
the program to fix them, and then recompile it. If your program has runtime errors or does not produce the correct result, you have to modify the program, recompile it, and execute it again.
You can use any text editor or IDE to create and edit a Java source-code file. The source file must end with the extension .java and must have the same exact name as the public class name. For example, the file should be named Welcome.java, since the public class name is Welcome.

A Java compiler translates a Java source file into a Java bytecode file. The following command compiles Welcome.java

javac Welcome.java

If there aren’t any syntax errors, the compiler generates a bytecode file with a .class extension. Thus, the preceding command generates a file named Welcome.class.

The Java language is a high-level language, but Java bytecode is a low-level language. The bytecode is similar to machine instructions but is architecture neutral and can run on any platform that has a Java Virtual Machine (JVM). Rather than a physical machine, the virtual machine is a program that interprets Java bytecode. This is one of Java’s primary advantages: Java bytecode can run on a variety of hardware platforms and operating systems. Java source code is compiled into Java bytecode and Java bytecode is interpreted by the JVM. Your Java code may use the code in the Java library. The JVM executes your code along with the code in the library.

To execute a Java program is to run the program’s bytecode. You can execute the bytecode on any platform with a JVM, which is an interpreter. It translates the individual instructions in the bytecode into the target machine language code one at a time rather than the whole program as a single unit. Each step is executed immediately after it is translated. The following command runs the bytecode

java Welcome

Do not use the extension .class in the command line when executing the program. Use java ClassName to run the program. If you use java ClassName.class in the command line, the system will attempt to fetch ClassName.class.class.

If you execute a class file that does not exist, a NoClassDefFoundError will occur. If you execute a class file that does not have a main method or you mistype the main method (e.g., by typing Main instead of main), a NoSuchMethodError will occur.

When executing a Java program, the JVM first loads the bytecode of the class to memory using a program called the class loader. If your program uses other classes, the class loader dynamically loads them just before they are needed. After a class is loaded, the JVM uses a program called the bytecode verifier to check the validity of the bytecode and to ensure that the bytecode does not violate Java’s security restrictions. Java enforces strict security to make sure that Java class files are not tampered with and do not harm your computer.