Compile time and runtime in Java

Runtime and compile time, these are two programming terms that are more frequently used in java programming language. The programmers specially beginners find it little difficult to understand what exactly they are. So let's understand what these terms means in java with example.

In java running a program happens in two steps, compilation and then execution. The image below shows where does compile time and runtime takes place in execution of a program.

compil time and runtime

What is compile time in Java

After writing the program, programmer needs to compile that program. As soon as the programmer starts compiling the program using javac compiler, the compile time gets started, and it ends when either a .class file is generated after successful compilation or an error is thrown in compilation. In other way we can say, the process of compiling a program is referred as compile time.

For an example if you wrote a program and saved it as, Once you start compiling this program using javac command as javac, the compile time gets started and it ends when a .class file as MyFirstProgram.class is generated or any error is thrown in compilation.

What happens at compile time in java ?

At compile time, the java compiler(javac) takes the source code(.java file) and checks if there is any syntax, type-checking or any semantic errors inside the program. If there is no error, the compiler generates a .class(bytecode) file for that .java file. If there is any compilation error, java compiler displays that error in command window(eg cmd).

What is compile time error in java ?

If a program element(class, method, variable, statements etc) is not written as per it's syntax in java, the compiler throws error for that element while compiling the program. We call these errors as compile time errors as these errors are detected at compile time by the java compiler.

Does java compiler generates .class file even if it throws compilation error ?

No, it will not generate .class file, it will only display the compilation error in console(eg. cmd) window.

Let us see compile time error by an example. The most common mistake that beginners do is, they forget to add semicolon(;) at the end of a statement which results as a compilation error while compiling the program.

 class MyFirstProgram {	
   public static void main(String [] args) {
     System.out.println("first statement") // missing semicolon(;)
     System.out.println("second statement");

Once you compile above program as javac, it will display a compilation error in console window like below :

Compile time Error: error: ';' expected
             System.out.println("first statement") // missing semicolon(;)
 1 error

What is Runtime in Java

As soon as the programmer starts executing the program using java command, runtime gets started and it ends when execution of program ended either successfully or unsuccessfully. In other way the process of running a program is known as runtime.

For an example if you wrote a program and saved it as After compilation when you execute the command java MyFirstProgram for running the program, runtime gets started and it ends when either the output of program is generated or any runtime error is thrown.

What is runtime error in java ?

Errors which comes during the execution(runtime) of a program are known as runtime errors. If a program contains runtime error, it won't run successfully, rather that error will be displayed in command window(eg. cmd) at the time of execution.

Let us suppose if you wrote a program like below, After compilation when you run the below program using java MyFirstProgram command, it will throw a runtime error.

 class MyFirstProgram {	
  public static void main(String [] args) {	
     int num1 = 10;
     int num2 = 0;
     System.out.println(num1/num2); // Runtime error: Divide by zero exception

Runtinme Error:

 Exception in thread "main" java.lang.ArithmeticException: / by zero
         at MyFirstProgram.main(

Difference between runtime and compile time

Compile time is a process in which java compiler compiles the java program and generates a .class file. In other way, in compile time java source code(.java file) is converted in to .class file using java compiler. While in runtime, the java virtual machine loads the .class file in memory and executes that class to generate the output of program.

Difference between compile time error and runtime error

Compile time errors are the error that comes while compiling the program whereas runtime errors are errors that comes at the time of execution(run-time) of the program. An example of compile time error is "not adding a semicolon(;) at the end of a statement" in your java program while "dividing a number by zero" is an example of runtime error.

  • Without compilation you can not run the program, compile first then run.
  • The output of a program is generated in runtime, not in compile time.
  • If you made any changes in program after compilation, you need to compile the program again to see the changes in output.