learning_java.md

path	title
/learnings/java	Learnings: Java

Table Of Contents

• Dev Environment Tools >
  • Make all the folders in a Java module namespace
• Iteration >
  • Enhanced for loop
• Instance Initializers / Initializer Blocks >
  • Using Initializer Block when instantiating an object
  • Initializer Blocks with Instance Objects
  • Static Initializer Blocks
  • And Anon Inner Classes
• (Faking) Initialization Blocks when creating instances of Object
  • What is going on?
• New in Java 9
  • stream
• Oracle 1Z0-808 Course Prep >
• Casting
  • Static Casting
  • Dynamic Casting
• Development Environment in Docker : >
  • Doing it yourself
  • IntelliJ Instructions on this
• See also:
• Book Recommendations

Dev Environment Tools <<Learning_Dev_Env_Tools>>

Make all the folders in a Java module namespace

function packagePathToFolderPath() {
    mkdir -p $1/main/java
    cd $1/main/java

    mkdir -p $(echo $2 | sed -e 's/\./\//g')
}

$ packagePathToFolderPath $PROJECT_DIR/src com.wilcoxd.myapp.things

Iteration <<Learning_Java_Iteration>>

Enhanced for loop

String[] things = ["One", "Two", "three"]
for ( String currentThing : things) {

}

Works over anything that implements Iterable

Instance Initializers / Initializer Blocks <<Learning_Java_Initialization_Blocks>>

Called at the time the instance's variable initializers are evaluated (after superclass construction, before constructor body).

SO Question/Answer on this

Using Initializer Block when instantiating an object

HashMap<String, String> h = new HashMap<String, String>(){
	h.put("key", "value");
}

Initializer Blocks with Instance Objects

class Thing {
	String myName;
	{
		myName = "Yo";
	}
}

Static Initializer Blocks

class Thing {
	static String myName;

	static {
		myName = "Hello world";
	}
}

And Anon Inner Classes

Have to use this instead of constructors, because don't have constructors (no names!)

(Faking) Initialization Blocks when creating instances of Object

Example:

public void main(String[] args) {
	HashMap<String, String> a = new HashMap<String, String>() {
		{
		put("key", "value");
		}
	};

}

What is going on?

Technically is creating an anon class derived from specified class (outer braces) AND THEN using an initializer block

See also:

• http://wiki.c2.com/?DoubleBraceInitialization
• https://stackoverflow.com/a/31829153/224334 <-- explains what I said above
• https://stackoverflow.com/a/27521360/224334 <-- why this might be a bad idea (class explosion in your classloader, potential memory leaks)

New in Java 9

##@Depreciated <-- has problem: when? Or is there just s better way?

New attributes:

• ForRemoval <---- it' going away
• since <--- when it's considered old

Warnings can be set to warn on since, etc etc

stream

Stream.ofNullable(objOrMaybeNullWhoKnows) <--- makes a stream of the object, or maybe an empty stream because the object might be null

Oracle 1Z0-808 Course Prep <<Learning_Java_Associate_Cert_1Z0-808>>

Casting

Static Casting

Integer myThing = ( Integer )( param );

Dynamic Casting

Integer myThing = Integer::cast( param );

Development Environment in Docker : <<Learning_Java_Docker_Development_Debugging_Environment>>

Doing it yourself

CMD /usr/share/maven/bin/mvnDebug exec:java <-- this exposes a port and you can point a debugger to this image host

IntelliJ Instructions on this

Debugging Java in a Container via IntelliJ

See also:

• Learning_Ops_Java_Docker
• Learning_Ops_SRE_Java

Book Recommendations

• Effective Java
• Learning Java