TODO

• continue course
• learn about concurrency
• learn about networking
• make a project
• boostrap and thymeleaf

Java-basics-recap

A strong foundation is what makes progress easier and higher goals achievable

Interaces

• An interface is a set of requirements for classes
• Class can choose to conform to one or more interfaces
• Interface is not a class
• Service provider promises caller: "If your class conforms to a particular interface, then i'll perform the service"
• Arrays.sort sorts arrays if element class conforms to the Comparable interface
• Interface definition:

public interface Comparable<T>{
  int compareTo(T other);
}

• Conforming class must provide compareTo method

• If your class does not conform to Comparable interface Arrays.sort will not work

• Since Java 8 you can add static methods in interfaces

• You can put methods in interfaces

• A default method can call an abstract method

• Interfaces clash. If an incerface provides a default method and anohter interface provides the same one (default or not), your must resolve the conflict

• In java 9 you can have private, private static methods to be called by default or static methods

Callbacks

• Callback: Action that should happen at some point in the future

basicstreams package

• BasicStream.java contains an example of a basic stream using lambda to add the values of a list.
• WildCards.java contains an example of an unbound Wildcard. You can see that creating it with any type of object works fine but adding to it is not possible.
• UpperBoundWildcards.java <? extends Number>the idea of using an upper bound on a wild card allows you to tell the compiler what type youre expecting and also allows you to provide elements that are that type of subclasses of that type. Also, you can use all the methods available of that upper bounded class. Example : List<? extends Number> in the method, you can use all methods in Number class also List class
• comparatorexample package <? super Number> For example comparator. You are allowed to compare Numbers, but also superclass of Number like Object

New IO

• Paths class has 2 methods .get(String) where you specify a path
• Files.createFiles (Paths.get("sub1", "sub2","sub3","myfile.txt")) will create myfile.txt at sub1/sub2/sub3
• Files.createDirectories(Paths.get("sub1", "sub2","sub3")); will create sub1/sub2/sub3 folders
• boolean deleted = Files.deleteIfExists(Paths.get("sub1", "sub2","sub3","myfile.txt")); deletes myfile.txt at sub1/sub2/sub3
• Path dataFile = sourceDir.resolve("file.txt"); adds file.txt at the path

Exceptions

• Unchecked exceptions are the ones that are descendants of RuntimeException class.
• Unchecked exception means that it does not require a try catch block
• Checked exceptions require a try/catch block or throws
• Exceptions get printed on standard error while output is on standard out. System.err.println()
• Multiple catch block was added in java 7 catch(ExceptionNoFound1 | ExceptionFoundNot2 e)

Threads

• Executor are the highest level. you Provide threads and it caches starts/stops them

Thread Priorities

• Java assigns to each thread a priority that determines how that thread should be treated with respect to the others. Thread priorities are integers that specify the relative priority of one thread to another. As an absolute value, a priority is meaningless; a higher-priority thread doesn’t run any faster than a lower-priority thread if it is the only thread running. Instead, a thread’s priority is used to decide when to switch from one running thread to the next. This is called a context switch. The rules that determine when a context switch takes place are simple:

• A thread can voluntarily relinquish control. This occurs when explicitly yielding, sleeping, or when blocked. In this scenario, all other threads are examined, and the highest-priority thread that is ready to run is given the CPU.

• A thread can be preempted by a higher-priority thread. In this case, a lower-priority thread that does not yield the processor is simply preempted—no matter what it is doing—by a higher-priority thread. Basically, as soon as a higher-priority thread wants to run, it does. This is called preemptive multitasking.

• For some operating systems, threads of equal priority are time-sliced automatically in round-robin fashion. For other types of operating systems, threads of equal priority must voluntarily yield control to their peers. If they don’t, the other threads will not run.

Syncronization

• you must prevent one thread from writing data while another thread is in the middle of reading it

• The monitor is a control mechanism first defined by C.A.R. Hoare. You can think of a monitor as a very small box that can hold only one thread. Once a thread enters a monitor, all other threads must wait until that thread exits the monitor. In this way, a monitor can be used to protect a shared asset from being manipulated by more than one thread at a time.

• In Java, there is no class “Monitor”; instead, each object has its own implicit monitor that is automatically entered when one of the object’s synchronized methods is called. Once a thread is inside a synchronized method, no other thread can call any other synchronized method on the same object. This enables you to write very clear and concise multithreaded code, because synchronization support is built into the language.

• Java provides a clean, low-cost way for two or more threads to talk to each other, via calls to predefined methods that all objects have. Java’s messaging system allows a thread to enter a synchronized method on an object, and then wait there until some other thread explicitly notifies it to come out.

  • Callable interface in order to set up a series of threads to execute that return something at a later date

  • Default methods allow you to put a method in an existing interface and provide an implementation

  • Tips:

    • When using a new class, check the API documentation to get familiarised with the methods it contains

Private, Default, Protected

• Private anywhere inside the top level curly braces( even in subclasses)
• Protected anywhere in the same package and from any sub-class that's accessing its own type
• public
• default in same package

package a;
class Parent
  protected x; //can be used anywhere in Parent
  
package b;
  class Child extends Parent
  this.x; yes
  
  do stuff(Parent p)
    p.x; doesnt work

When overriding, chaning visibility to something smaller (form public to private) it won't work. Protected is less visible than Default

TOLEARN: Java swing

• interface making in java