06 Java Language and OOP Part VI

Java Language and OOP Part VIBy Hari Christian

Agenda• 01 Threads - Basic• 02 Threads - How to Obtain Threads• 03 Threads - Threads Programming• 04 Threads - Advanced• 05 Threads - Concurrent• 06 Parameters vs Arguments• 07 Generics - Basic• 08 Generics - What Generic Solves• 09 Generics - Generic Code• 10 Generics - Generic Interfaces• 11 Java Code Convention

Concurrency

• Concurrency is the ability to run several programs or several parts of a program in parallel

• If a time consuming task can be performed asynchronously or in parallel, this improve the throughput and the interactivity of the program

Concurrency

• A modern computer has several CPU's or several cores within one CPU

• The ability to leverage these multi-cores can be the key for a successful high-volume application

Process and Threads

• A process runs independently and isolated of other processes

• It cannot directly access shared data in other processes

• The resources of the process, e.g. memory and CPU time, are allocated to it via the operating system.

Process and Threads

• A thread is a so called lightweight process

• It can access shared data of other threads in the same process

• Every thread has its own memory cache. If a thread reads shared data it stores this data in its own memory cache. A thread can re-read the shared data

Process and Threads

• A Java application runs by default in one process

• Within a Java application you work with several threads to achieve parallel processing or asynchronous behavior

Concurrency Issues

• Threads can also access shared data. Therefore you have two basic problems

1. A visibility problem occurs if thread A reads shared data which is later changed by thread B and thread A is unaware of this change

2. An access problem can occur if several thread access and change the same shared data at the same time

Concurrency Issues

• Visibility and access problem can lead to

1. Liveness failure: The program does not react anymore due to problems in the concurrent access of data, e.g. deadlocks

2. Safety failure: The program creates incorrect data

How to Obtain Threads

• There are two ways to obtain a new thread of control in Java

1. Extend the Thread class (only if your class doesn't already extend some other class)

2. Write a class to implement the java.lang.Runnable interface and use it in the Thread constructor


1. Extend class java.lang.Thread and override run():

• Example:class Plum extends Thread {

public void run() { /* more code */ } }

Plum p = new Plum();p.start();


2. Implement the Runnable interface

• Example:class Mango implements Runnable {

public void run() { /* more code */ } } Mango m = new Mango(); Thread t1 = new Thread(m); t1.start();

Threads Programming

public class ThreadsTest1 implements Runnable {public void run() {

for(int i = 0; i < 3; i++) {System.out.println(Thread.currentThread().getName() + " - " + i);

}}

public static void main(String[] args) {ThreadsTest1 r = new ThreadsTest1();Thread a = new Thread(r, "a");Thread b = new Thread(r, "b");Thread c = new Thread(r, "c");

a.start();b.start();c.start();

}}

Threads Programming

public class ThreadsTest2 implements Runnable {public void run() {

for(int i = 1; i <= 100; i++) {System.out.println(i % 10 == 0? i + " = KELIPATAN 10" : i);try {

Thread.sleep(1 * 1000);} catch (InterruptedException e) {}

}}

public static void main(String[] args) {ThreadsTest2 r = new ThreadsTest1();Thread a = new Thread(r, "a");a.start();

}}

Threads Programming

public class Download implements Runnable {public void run() {

System.out.println("START DOWNLOAD");for(int i = 0; i < 10; i++) {

System.out.println("DOWNLOADING " + ((i + 1) * 10) + "%");}System.out.println("FINISH DOWNLOAD");

}

public void start() {System.out.println("START DOWNLOAD");for(int i = 0; i < 10; i++) {

System.out.println("DOWNLOADING " + ((i + 1) * 10) +"%");}System.out.println("FINISH DOWNLOAD");

}}

Threads Programming

public class Upload implements Runnable {public void run() {

System.out.println("START UPLOAD");for(int i = 0; i < 10; i++) {

System.out.println(“UPLOADING " + ((i + 1) * 10) + "%");}System.out.println("FINISH UPLOAD");

}

public void start() {System.out.println("START UPLOAD");for(int i = 0; i < 10; i++) {

System.out.println(“UPLOADING " + ((i + 1) * 10) +"%");}System.out.println("FINISH UPLOAD");

}}

Threads Programming

public class Report implements Runnable {public void run() {

System.out.println("START REPORT");for(int i = 0; i < 10; i++) {

System.out.println(“REPORTING " + ((i + 1) * 10) + "%");}System.out.println("FINISH REPORT");

}

public void start() {System.out.println("START REPORT ");for(int i = 0; i < 10; i++) {

System.out.println(“REPORT ING " + ((i + 1) * 10) +"%");}System.out.println("FINISH REPORT ");

}}

Threads Programming

public class TestThread {public static void main(String[] args) {

TestThread t = new TestThread();t.exampleWithoutThread();

}

private void exampleWithoutThread() {Download d = new Download();Upload u = new Upload();Report r = new Report();

d.start();u.start();r.start();System.out.println("DOING OTHER THINGS");

}}

Threads Programming

public class TestThread {public static void main(String[] args) {

TestThread t = new TestThread();t.exampleWithThread();

}private void exampleWithThread() {

Download d = new Download();Upload u = new Upload();Report r = new Report();

Thread a = new Thread(d, "D");Thread b = new Thread(u, "U");Thread c = new Thread(r, "R");

a.start(); b.start(); c.start();System.out.println("DOING OTHER THINGS");

}}

Threads Programming

public class Account {private int balance = 50;

public void withdraw(int amount) {balance = balance - amount;

}

public int getBalance() {return balance;

}}

Threads Advancedpublic class AccountTest {

private Account account = new Account();

public void run() { for(int i = 0; i < 5; i++) { makeWithdrawal(10); if (account.getBalance() < 0) System.out.println("INSUFFICIENT BALANCE"); }}

private void makeWithdrawal(int amount) { if(account.getBalance() >= amount) { System.out.println(Thread.currentThread().getName() + " is withdrawing"); account.withdraw(amount); System.out.println(Thread.currentThread().getName() + " complete withdrawing"); } else System.out.println(Thread.currentThread().getName() + " not enough: " + account.getBalance());

public static void main(String[] args) { AccountDanger r = new AccountDanger(); Thread a = new Thread(r, "Fred"); Thread b = new Thread(r, "Lucy"); a.start(); b.start();}

}

Threads Advancedpublic class AccountTest {

private Account account = new Account();

public void run() { for(int i = 0; i < 5; i++) { makeWithdrawal(10); if (account.getBalance() < 0) System.out.println("INSUFFICIENT BALANCE"); }}

private synchronized void makeWithdrawal(int amount) { if(account.getBalance() >= amount) { System.out.println(Thread.currentThread().getName() + " is withdrawing"); account.withdraw(amount); System.out.println(Thread.currentThread().getName() + " complete withdrawing"); } else System.out.println(Thread.currentThread().getName() + " not enough: " + account.getBalance());

public static void main(String[] args) { AccountDanger r = new AccountDanger(); Thread a = new Thread(r, "Fred"); Thread b = new Thread(r, "Lucy"); a.start(); b.start();}

}

Threads Advanced

public class Calculation implements Runnable {int total;

public void run() {synchronized (this) {

for(int i = 0; i < 100; i++) {total += 1;

}notifyAll();

}}

}

Threads Advanced

public class TestCalculation {public static void main(String[] args) {

Calculation c = new Calculation();Thread a = new Thread(c);a.start();

synchronized (a) {try {

System.out.println(“Waiting till finish");a.wait();

} catch (Exception e) { }System.out.println("TOTAL = " + t.total);

}}

}

Parameters vs Arguments

• Parameters:public int calculateAge(Date date1, Date date2) {

int result;// more logicreturn result;

}

• Arguments:int age = calculateAge(birthDate, currentDate);

Generics - Basic

• We want to have a parameter that represents a type, not a value

• We'll instantiate objects-of-the-generic-class using different types (Integer, Timestamp, Double, Thread) as the type argument

• Each object will be specialized to do work on that specific actual type argument

Generics - What Generic Solves

• Reduce the amount of casting, particularly when using the Java data structure classes known as the collection classes

• Up to JDK 1.4, collections held Objects. Variables of any class type could be put into any collection, because all class types are compatible with the type used in a collection, Object. When you retrieved something from a collection, you had to figure out what type it really was, then cast it back to that.


public static void main(String[] args) {List nonGeneric = new ArrayList();nonGeneric.add(new Student("1", "Hari"));nonGeneric.add(new Student("2", “Solihin"));

for(int i = 0; i < nonGeneric.size(); i++) {Student s = (Student) nonGeneric.get(i);System.out.println(s.getName());

}}


public static void main(String[] args) {List<Student> generic = new ArrayList<Student> ();generic.add(new Student("1", "Hari"));generic.add(new Student("2", “Solihin"));

for(int i = 0; i < generic.size(); i++) {Student s = generic.get(i);System.out.println(s.getName());

}}


• The generic feature in Java lets you tell the compiler about the type that you expect to load into a collection class

• You find out sooner (at compile-time, rather than run-time) about any errors you have made involving the types of objects you put into or take out of collections

• Further, with generics, you catch all the errors. In the bad old days, you only caught the errors you particularly exercised

Generics - What Generic Solvespublic static void main(String[] args) {

List nonGeneric = new ArrayList();nonGeneric.add(new Student("1", "Hari"));nonGeneric.add(new Student("2", “Solihin"));// No compile error, but will error in runtimenonGeneric.add(“Ardi"); for(int i = 0; i < nonGeneric.size(); i++) {

Student s = (Student) nonGeneric.get(i);System.out.println(s.getName());

}}

Generics - What Generic Solvespublic static void main(String[] args) {

List<Student> generic = new ArrayList<Student>();generic.add(new Student("1", "Hari"));generic.add(new Student("2", “Solihin"));// Compile error, so won’t be able to rungeneric.add(“Ardi");for(int i = 0; i < generic.size(); i++) {

Student s = generic.get(i);System.out.println(s.getName());

}}

Generics - Codepublic class Generic<T> {

public void getData(T data) {System.out.println(“Data = " + data.getClass());

}

public static void main(String[] args) {Generic<Integer> t = new Generic<Integer>();t.getData(new Integer(3));

}}



}

public static void main(String[] args) {Generic<String> t = new Generic<String>();t.getData(“”);

}}



}

public static void main(String[] args) {Generic<Student> t = new Generic<Student>();t.getData(“”);

}}

Generics – Class or Interface

• public interface List<E>• boolean add(E o)

• The <E> is a placeholder for the type you pass in

• The E is only a convention. Any valid Java identifier would work

Generics – Class or Interface

• List<Animal> list = new ArrayList<Animal>();• boolean add(Animal a);

• E stands for "Element," and it's used when the template is a collection

• The other main convention is T (stands for "type"), used for, well, things that are NOT collections

Java Code Convention

• Classes and interfaces:– The first letter should be capitalized

– If several words are linked together to form the name, the first letter of the inner words should be uppercase (a format that's sometimes called "camelCase")

– For classes, the names should typically be nouns. For example: Dog, Account, PrintWriter

– For interfaces, the names should typically be adjectives like: Runnable, Serializable


• Methods:– The first letter should be lowercase, and then normal

camelCase rules should be used

– In addition, the names should typically be verb-noun pairs

– For example:• getBalance• doCalculation• setCustomerName


• Variables:– Like methods, the camelCase format should be used,

starting with a lowercase letter

– Sun recommends short, meaningful names, which sounds good to us

– Some examples:• buttonWidth• accountBalance• myString


• Constants: – Java constants are created by marking variables

static and final

– They should be named using uppercase letters with underscore characters as separators

– For example: MIN_HEIGHT, PAGE_SIZE

Thank You

06 Java Language and OOP Part VI

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