OCP Chapter 01

• Java Development Kit with compiler javac.
• javac converts .java to .class files.
• Also archiver for .jar files and javadoc

Javac

javac HelloWorld.java

Jar

jar cvf HelloWorld.jar HelloWorld.class

• c: Creates a new archive file.
• v: Generates verbose output to standard output.
• f: Specifies the archive file name.

Javadoc

javadoc -d doc HelloWorld.java

-d: Specifies the destination directory for the generated documentation.

Jdeps

is a command-line tool provided by the Java Development Kit (JDK) to analyze Java class files. ``` jdeps myapp.jar // basic analysis jdeps --verbose myapp.jar // verbose output jdeps --summary myapp.jar // summary jdeps --jdk-internals myapp.jar // detecting internal usage jdeps --generate-module-info . myapp.jar // general module info

- Code Quality: Helps improve code quality by identifying unnecessary dependencies and dependencies on unstable internal APIs.
- Migration to Modules: Facilitates the migration of legacy codebases to the Java module system.
- Maintenance: Assists in maintaining and refactoring large codebases by providing clear insights into dependencies.

## JVM
Java Virtual Machine

## JRE
Java Runtime Enviroment is not anymore in there
jlink

## Java
Assuming you have compiled a Java program named HelloWorld that is in a file named HelloWorld.class, you can run this program using the java command as follows:

java HelloWorld

Assuming your class file is in ./com/example/HelloWorld.class, you would run:

java com.example.HelloWorld

If you're using a JAR file, you can run your application with the -jar option, like so:

java -jar YourApplication.jar

### Common Options
- -cp or -classpath: Specifies a list of directories, JAR archives, and ZIP archives to search for class files.
- -Xmx: Specifies the maximum size of the memory allocation pool (for example, -Xmx512m for 512 megabytes).
- -Xms: Specifies the initial size of the memory allocation pool.
- -version: Displays version information and exits.
- -verbose: Enables verbose output.

## Benefits of Java

- Object Oriented
- Encapsulation
- Platform Independent
- Robust
- Simple
- Secure
- Multithreaded
- Backwards Compatibility

  **OEPRSSMB**

  **SPERMBOS**  


## Class

- Java Class with which you create an Object.  
- An Object is a runtime instance in memory.  

- reference is a variable that points to an object.  

Class members:
-  fields and methods
   - method is an operation that can be called.
   - method signature = methodName + params
   - method declaration has return type
   - return type

public class Demo { }

word with special meaing is a **keyword**
public + class is keyword

A java file can have more classes! But max one needs to be public + same as the file name!

public class Demo {

String name;

public getName() {
    return name;
}

publi void setName(String newName) {
    name = newName;
}

}

### Comments

// until end of line

/* * Multi line comment */

/** * javadoc */

## Main()
public static void main(String[] args)

is startup of a java process managed by the JVM.
JVM allocate memory CPU access file and so on.

toegestane declaratie vd de argumenten:
String[] args  
String args []  
String... args  


## Java Version

javac -version java -version

## Run a Java program


```Java
public class Zoo {

    public static void main(String[] args) {

    }
}

compile and run

javac Zoo.java
java Zoo

vanaf 11 kan ook meteen in 1 file runnen : java Zoo.java maar dan alleen JDK imports

to compile a .java file is needed name of the file must be the same as the containing class results in bytecode in the .class file

public is access modifier static results in Zoo.main()

voor java applicatie static main func is obligotory

String[] args

String args[]

String... args

Als argumenten niet kloppen met run command dan ArrayIndexOutOfBoundException

Random r = new Random();
System.out.println(r.nextInt(12));

4

Imports

Er zijn Java built-in classes

andere classes zijn te vinden in packages die je kan importeren

Conflicts

double same named Class will not compile

Static imports

Redundant imports

Same Name

dan bv 1 importeren en de andere helemaal uitschrijven.

import java.util.Data;

public class Conflicts {
    Date date;
    java.sql.Date sqlDate;

  // .... etc ...
}

Alternate dir for compiling code

javac -d classes packagea/ClassA.java packageb/ClassB.java

zowel bij java als bij javac werken alle drie: - java -cp classes packageb.ClassB - java -classpath classes packageb.ClassB - java --class-path classes packageb.ClassB - -d