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javac and java

Learn to compile and run from the command line.

What is javac?

javac is the Java compiler. It turns your source file (.java) into a .class file containing bytecode that the JVM can read.

javac Main.java

When the command runs, Main.class appears in the same folder. If compilation fails, you get error messages; on success it finishes silently.

javac checks the source code: it catches syntax errors, type mismatches and missing semicolons.

What does the java command do?

The java command runs a compiled .class file on the JVM. You pass the class name without extension:

java Main

This command loads Main.class, finds the main method and runs it. The output appears on the screen.

This two-step flow follows the compile once, run anywhere philosophy. The same .class file runs on Windows, Linux and macOS.

Practice Write a compilable program

Define a Main class with a main method. Print Running with println. This code must compile with javac Main.java and run with java Main.

Sample solution

Main.java
public class Main {
    public static void main(String[] args) {
        System.out.println("Running");
    }
}

Expected output

Running

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What turns a .java file into .class?

  • java
  • javadoc
  • jar
  • javac Correct answer

Explanation: javac is the Java compiler.

2 Complete the compile command

Question 
____ Main.java
Answer 
javac Main.java

Explanation: javac Main.java compiles the file.

3 Match the compile stages

Term Meaning
javac Compiles
.class Bytecode
java Runs
JVM Runtime

All lessons in Beginner

1 Java basics

  1. What is Java?
  2. The main method
  3. Comments
  4. Class skeleton
  5. javac and java
  6. Summary - Java basics

2 Variables

  1. int and double
  2. boolean
  3. String
  4. Naming variables
  5. final variables
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Type casting
  3. String concatenation
  4. Math class
  5. BigInteger and BigDecimal
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Assignment and increment
  6. Summary - Operators

6 String basics

  1. length and charAt
  2. equals and equalsIgnoreCase
  3. indexOf and substring
  4. toUpperCase and trim
  5. String.format
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. else and else if
  3. Nested if
  4. switch
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. do-while
  4. break and continue
  5. for-each
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Filling arrays
  4. Multi-dimensional arrays
  5. Copying arrays
  6. Summary - Arrays

11 Array methods

  1. Arrays.sort
  2. Arrays.toString
  3. Arrays.binarySearch
  4. Arrays.copyOf
  5. Arrays.fill
  6. Summary - Array methods

12 Collections

  1. ArrayList
  2. add and get
  3. HashMap
  4. put and get
  5. HashSet
  6. Summary - Collections

13 Methods

  1. Defining methods
  2. return
  3. void methods
  4. Method overloading
  5. Scope
  6. Summary - Methods

14 Method parameters

  1. Primitive parameters
  2. Object parameters
  3. varargs
  4. Multiple values via array
  5. final parameters
  6. Summary - Method parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String builder

  1. StringBuilder
  2. append
  3. insert and delete
  4. reverse
  5. toString
  6. Summary - String builder

17 Input and output

  1. Scanner
  2. nextInt and nextLine
  3. System.out.printf
  4. BufferedReader
  5. try-with-resources
  6. Summary - Input and output

18 Enum

  1. Declaring enums
  2. values and valueOf
  3. Enum fields
  4. Enum methods
  5. switch with enum
  6. Summary - Enum

19 Exception basics

  1. try/catch
  2. finally
  3. throw
  4. Exception hierarchy
  5. Multiple catch
  6. Summary - Exception basics

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together