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

Program Execution Methods

• Two main approaches to execute programs:
  1. Compilation: Translates entire program to machine code before execution
  2. Interpretation: Reads and executes program statements one at a time

Java Execution

• Two ways to run Java programs:
  1. Compile to bytecode first, then interpret/compile via JVM
  2. Direct interpretation using Java interpreter

Compilation Process

• Java compiler (javac) compiles source code (.java) to bytecode (.class)
• Basic compilation steps:

  javac Hello.java    # creates Hello.class
  java Hello          # runs the program
  

• Common mistakes to avoid:
  • Confusing javac and java
  • Launch Single-File Source-Code Programs before compiling. E.g. java Hello.java

Java Shell (jshell)

• Interactive tool for Java code evaluation
• Allows direct code testing without compilation
• Useful for learning and experimentation
• Can run in two modes:
  • Interactive mode
  • Script mode with .jsh files

Compiler Functions

• Translates source code to machine code/bytecode
• Checks code for syntax errors
• 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