Computers, Programs, and the Java Language

Hardware

A computer is organised around four kinds of component.

• CPU (processor). Executes instructions. Its control unit fetches and decodes instructions; its ALU performs arithmetic and logic. Speed is measured in hertz (GHz = billions of cycles per second).
• Main memory (RAM). Holds the program and its data while the program runs. It is volatile: switch the power off and the contents are lost. Access is fast and random (any address costs the same).
• Secondary storage. Hard disk, SSD, USB drive, optical disc. Non-volatile, larger, much slower than RAM. This is where your .java and .class files live.
• Input/output devices. Keyboard, mouse, touchscreen (input); monitor, printer, speakers (output). A touchscreen and a hard disk are both input and output.

Storage units: 1 byte = 8 bits; 1 KB ≈ 10³ bytes; 1 MB ≈ 10⁶; 1 GB ≈ 10⁹; 1 TB ≈ 10¹².

Software

System software manages the machine: the operating system, device drivers, utilities, and the language tools (compiler, JVM). Application software does a job for the user: a browser, a spreadsheet, a game, the program you are about to write.

Languages and translation

• Machine language — binary instructions the CPU executes directly. Different for each processor family.
• Assembly language — mnemonics for machine instructions; needs an assembler.
• High-level language — Java, C, Python: readable, processor-independent, needs a compiler or an interpreter.

A compiler translates the whole program before it runs and reports all syntax errors at once. An interpreter translates and executes one statement at a time.

How Java is built and run

The JVM (Java Virtual Machine) is the program that executes compiled Java code on a specific platform. Because every platform has its own JVM but they all accept the same bytecode, the same .class file runs on Windows, macOS and Linux without recompilation. This is portability, often quoted as write once, run anywhere. Java is therefore both compiled and interpreted: compiled to bytecode, then interpreted (and partly JIT-compiled) by the JVM.

The JDK (Java Development Kit) contains the compiler and development tools; it includes the JRE (Java Runtime Environment), which contains the JVM and the standard class library. You need the JDK to write Java; a user only needs a JRE to run it.

The smallest complete program

public class Hello {                 // class heading; file must be Hello.java
    public static void main(String[] args) {   // entry point of the application
        System.out.println("Hello, world!");   // statement; ends with a semicolon
    }
}

Things to notice, each of which has been an exam question:

• The keyword class is required to declare a class. public is an access modifier, not the declaration keyword.
• A public class must be stored in a file with exactly the same name plus .java.
• The entry point must be public static void main(String[] args). The parameter name is free (args, tracks, moat…), the type is not. String... args (varargs) is also accepted; String args alone is not.
• Java is case sensitive: Main, main and MAIN are three different identifiers.
• Braces { } delimit a block; each statement ends with a semicolon.
• System.out.println(x) prints x and moves to a new line; System.out.print(x) prints x and stays on the same line.
• Comments: // to end of line, /* ... */ over several lines, and /** ... */ for javadoc. Comments do not affect execution — they affect the human who reads your code, including the grader.

Identifiers

An identifier may contain letters, digits, the underscore and the dollar sign; it may not begin with a digit; it may not be a reserved word (class, int, public, new, …); and it may not contain a space or a hyphen. Convention: ClassNamesLikeThis, methodAndVariableNamesLikeThis, CONSTANTS_LIKE_THIS.

Three kinds of error

• Compile-time (syntax) error — the compiler refuses to produce a .class file: missing semicolon, misspelled keyword, undeclared variable, wrong type.
• Run-time error — the program compiles but fails while running: division by zero, using a null reference, indexing past the end of an array.
• Logic error — the program compiles and runs, but produces the wrong result: the wrong formula, < where <= was meant. The compiler cannot help you; only testing and tracing can.