Skip to content

Program and Compiler

Software Programs

  • A program is a collection of data variables and instructions
  • Programs are written in programming languages using keywords, symbols, and names
  • Programming languages provide higher level abstraction for complex instructions
  • Programming languages are used to communicate instructions to a computer.

Compiler and Interpreter Execution Process

  • Java compiler (javac) compiles source code (.java) into bytecode (.class)
  • Compilation: 
    javac Hello.java    # compile Hello.java
  • JVM (java) executes the bytecode by translating it into machine code
  • Execution: 
     java Hello         # runs the program

    Notes: Launch Single-File Source-Code Programs allows running Java files without explicit compilation. E.g. java Hello.java

Java Shell (jshell)

  • jshell is a REPL (Read–Eval–Print Loop) for Java introduced in Java 9.
  • Start it by running: 
    jshell
  • Modes:
    • Interactive mode for experimentation.
    • Script mode using .jsh files.
  • Usage notes:
    • Useful for quick testing, learning, and prototyping without creating full source files.
    • You can declare variables, define methods, and call APIs directly in the REPL.

Compiler Functions

  • Translates source code to machine code/bytecode
  • Checks code for syntax errors
    • Compilation errors are caught during development, allowing programmers to fix them early.
    • Runtime errors are less desirable because they occur while the program is running, potentially affecting users.
    • The goal is to catch as many errors as possible at compile time, rather than during execution.
  • Performs type checking
  • Can be either:
    • Conservative: Reports error if possibility of incorrect statement
    • Permissive: Only reports error if statement is definitely incorrect

Compiler Limitations

  • Cannot detect all runtime errors during compilation
  • Cannot always determine if code will be executed
  • Cannot always determine variable values at compile time

Workflow

  1. Create or edit file containing Java program (Hello.java) with an editor (vim).
  2. Compile source code using a compiler (javac Hello.java) to produce a bytecode file (.class)
  3. Execute the program (java Hello)