Java Programming

Last modified Tuesday, 16-Jan-2024 16:50:18 UTC.

Programming Languages
- Machine code, aka Binary
  - What computers understand
  - Written in 1s and 0s, and not easy for humans to understand
  - HelloWorld in machine code
- High-level languages
  - High-level languages provide a very high conceptual model of computing.
  - Humans can understand these, but computers cannot
  - HelloWorld.java
  - HelloWorld.cpp
  - HelloWorld.pl
  - HelloWorld.py
- A compiler translates high-level programs into machine code, assembly code, or byte code. Compilers are provided as machine code programs (magic).
- Interpreted languages
  - Some languages, e.g., byte code and perl, are interpreted to machine code by an interpreter.
  - An interpreter reads a file of instructions, and makes the computer do what the instructions say. For example, a Java byte-code interpreter reads a file of byte-code instructions, and makes the computer do what the byte-code instructions say. Interpreters are provided as machine code programs (magic).
  - HelloWorld.class
  - Java is typically compiled to byte-code and interpreted
- Just In Time (JIT) compilation
  - Bytecode (or even source code) is compiled to machine language on demand, when it's about to be executed.
  - Some delays, but generally better performance overall.
  - See, e.g., Oracle's HotSpot virtual machine.
Concrete Process
Programming in Java - that's what we'll be doing!
- Creating and executing a Java program
  - Find a problem
  - Devise an algorithmic solution (pen, paper, head)
  - Write code, compile, and run in a terminal window
    - Use an editor to type in the Java program, e.g.,
```
prompt> vi HelloWorld.java 
```
    - Use javac to convert the Java to byte code, e.g.,
```
prompt> javac HelloWorld.java 
```
      If there are no mistakes in HelloWorld.java that will create HelloWorld.class, which contains the byte-code.
    - Use java to run the byte code, by interpreting byte code instructions in machine code. Omit the .class (weird, and it shouldn't be, but it is), e.g.,
```
prompt> java HelloWorld 
```
      The output will be on your screen.
  - Compile and run in IntelliJ
    - Use IntelliJ to type in the Java program.
    - The first time, use Run->Run... to compile and run. Subsequently you can use Run->Run (^R) to compile and run.
    - The output will be shown in a subwindow.
Some samples of Java ... a trailer
- Java is a high-level object-oriented language developed by Sun Microsystems (now owned by Oracle).
- HelloWorld.java
- MetricConversion.java
- PizzaCost.java (Papa Johns, Pizza Hut, Dominos)
- CheckbookByDesign.java (eeek)
- Student.java, UndergraduateStudent.java, GraduateStudent.java, UseStudent.java (aaaargh)
Error messages
- If there are errors in a program submitted to javac then error messages will be spat out
- Error messages in IntelliJ
  - Errors are highlighted in the editor window.
  - Hover over each to get information about the error.
- Example: BuggyHelloWorld.java
- Compilation is done in multiple phases, so javac finds all the obvious errors first, and when you've got them all fixed javac will only then look for subtle errors.
- Generally, fixing one error does not mean less errors the next time you compile.

Exercises

What company developed Java?

Exam Style Questions

Explain the differences between high-level, byte-code, and machine-code languages.