Created: 2023-08-22 21:51
Status: #concept
Subject: Programming
Tags: Programming Language Java Data Type Java Class Java Package Object-Oriented Programming

Java

A strongly-typed Compiled Object-oriented programming language which encodes programs into Java Bytecode which can be Interpreted and ran in any machine with a Java Virtual Machine (JVM).

public class Hello {
  /* this is the "entry point" of the .java program when we run it */
  public static void main(String[] args) {
    System.out.println("You passed in: ", + args[0]);
  }
}

public means that the class Hello's function can be invoked outside the class inside other classes within the same Java Package.
static means that we have to call the function as a Method together with the class name, ie. Hello.main( ... ); without importing the Hello class.
void is the return type of the main Function.
String[] is the type of the Parameter, an array of String objects.
args is the parameter name we can use the access the arguments passed into the function main when we run java Hello.class "I am args" in a CLI.

$ javac Hello.java
$ java Hello someArgument
You passed in: someArgument

Writing A Java Program

Every .java has some constraints before we can compile them into a .class with javac.

We need to declare a Java Package package scope; at the start of the source file which corresponds to the directory/ the code is in, package parent.child corresponds to parent/child/Class.java.
Afterwards, we can write our import statements to import code from other packages.
The filename Name.java must correspond to the main public class Name which contains a public static void main(String[] args) Method as the entrypoint.

package shipping.reports;

import shipping.domain.*; // * denotes the wildcard RegEx character
import java.util.List;
import java.io.*; // imports all files with the 'java.io.' scope prefix

public class VehicleCapacityReport {
    private List vehicles;
    public void generateReport(Writer output) {...}
}

Standard Library Utilities

See java.util and java.lang.

Common Pitfalls

there are no Truthy or Falsy Values when passing other data types like int into a while(bool expr) { ... } loop because they are not compatible types.
- We need to typecast them first via PrimitiveType.valueOf or use Comparison Operators to make a boolean expression.
values are internally UTF-16 Unicode characters.
Java does not have undefined declared variables, it shifts all bits to 0.
ALL non-primitive types (Objects) are Reference Types, so they are passed by Memory Address and are mutable when passed into functions.

Unit Testing

Any class with the suffix Test will be included in the "Test Packages" directory in the Netbeans IDE.

Without it, the tests in the class will not be executed.

import static org.junit.Assert.assertEquals;
import org.junit.Test;

public class CalculatorTest {

    @Test
    public void calculatorInitialValueZero() {
        Calculator calculator = new Calculator();
        assertEquals(0, calculator.getValue());
    }
}

see JUnit Documentation for more info on org.junit.

Java

Writing A Java Program

Standard Library Utilities

Common Pitfalls

Unit Testing

References