Introduction to Java Programming Language Junji Zhi University of

Introduction to Java Programming Language Junji Zhi University of Toronto 1

Content Java language Syntax “Hello World” program example Compiling, Running and Debugging Java code Inheritance Threading Synchronization 2

Java programming Language Some buzzwords for Java – “Write Once, Run Anywhere” – Simple – Object oriented – Distributed – Multithreaded – Dynamic – Architecture neutral – Portable – High performance – Robust – Secure 3

Example: Hello World Program Everything is in a class One file, one public class In the runnable public class: – public static void main(String [] args) 4

Primitive Data Types Primitive Data Types: byte, short, int, long, float, double, boolean, char Arrays are also a class long [] a new long[5]; – You can get the length by visiting the length field of array object a, like this: a.length String class is very commonly used to represents character strings, for example String s1 “Hello ”, s2 “Wolrd!”; String s3 s1 s2; 5

Operators (same as C/C ) [3] ,-- Auto increment/decrement ,- Unary plus/minus *,/ Multiplication/division % Modulus ,- Addition/subtraction 6

Declaring Variables [3] int n 1; char ch ‘A’; String s “Hello”; Long L new Long(100000); boolean done false; final double pi 3.14159265358979323846; Employee joe new Employee(); char [] a new char[3]; Vector v new Vector(); 7

Compared with C/C [3] Java has no: – pointers – typedef – preprocessor – struct – unions – multiple inheritance – goto – operator overloading – malloc – 8

Declaring a class package Class name Constructor Fields methods 9

Compiling, Running and Debugging Java Programs 10

Java Development Process .java .class JVM execution 11

Installing Java in your machine (1) Downloading Java Development Kit (JDK) from Oracle Java Runtime Environment (JRE) is usually included in the JDK installation file. 12

Installing Java in your machine (2) Setting JAVA HOME (Windows): – E.g., C:\Program Files\Java\jdk1.7.0 45 Setting path and classpath 13

Compile .java File into a .class File (Command Line) 14

Running HelloWorld in Eclipse IDE Eclipse Download from here. 15

Java platform 16

Debugging Java in Eclipse (1) Debugging means “run a program interactively while watching the source code and the variables during the execution.” [5] Set breakpoints to stop the program at the middle of execution Eclipse has a Debug Mode 17

Debugging Java in Eclipse(2) Image courtesy: http://www.vogella.com/tutorials/EclipseDebugging/images/xdebugstart20.gif.pagespeed.ic.SqCELlNeCm.png 18

Debugging Java in Eclipse(3) Table courtesy: http://www.vogella.com/tutorials/EclipseDebugging/article.html 19

Java Inheritance 20

Inheritance in Java Java classes can be derived from other classes, thereby inheriting fields and methods from those classes. 21

Common Root: Object 22

Interface 23

“Multiple Inheritance” 24

A Real World Example: ArrayList http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html 25

Java Threading 26

Java Threading A thread is a thread of execution in a program [6] JVM allows an application to have multiple threads running concurrently. Apache Harmony example: http://harmony.apache.org/subcomponents/drlvm/TM.html 27

Two ways to do threading 1. Extends Thread class 2. Implements Runnable interface http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html 28

Thread lifecycle 29

How to stop a Thread Using Thread.interrupt() method: http://stackoverflow.com/questions/7786305/stopping-a-specific-java-thread 30

Java Synchronization 31

Thread Interference (1) Increment operation is translated to multiple steps by the virtual machine : 1. Retrieve the current value of c. 2. Increment the retrieved value by 1. 3. Store the incremented value back in c. Example from: http://docs.oracle.com/javase/tutorial/essential/concurrency/interfere.html 32

Thread Interference (2) Assume we have 2 threads, A and B. A increments c, and B decrements c. Thread A and B runs together. One possible order of the low-level steps: 1. 2. 3. 4. 5. 6. Thread A: Retrieve c. Thread B: Retrieve c. Thread A: Increment retrieved value; result is 1. Thread B: Decrement retrieved value; result is -1. Thread A: Store result in c; c is now 1. Thread B: Store result in c; c is now -1. Is the result correct? What if the thread A and B are bank transactions? 33

Problem Root Cause Threads are visiting one field (resource) at the same time. Multiple steps of an operation No enforced “happen-before” relationship 34

Solution: synchronized method Example: http://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html 35

synchronized method Enforce the ‘happen-before’ relationship in the method level. Either one of the below instance will happen. But result is always 0, which is correct. 1. 2. 3. 4. 5. 6. Thread A: Retrieve c. Thread A: Increment retrieved value; result is 1. Thread A: Store result in c; c is now 1. Thread B: Retrieve c. Thread B: Decrement retrieved value; result is 0. Thread B: Store result in c; c is now 0. 1. 2. OR 3. 4. 5. 6. Thread B: Retrieve c. Thread B: Decrement retrieved value; result is -1. Thread B: Store result in c; c is now -1. Thread A: Retrieve c. Thread A: Increment retrieved value; result is 0. Thread A: Store result in c; c is now 0. 36

synchronized statements (1) Every object has an intrinsic lock associated with it Primitive types (e.g., int, char) do not have intrinsic locks. We can combine object intrinsic locks and synchronized keyword to create fine-grained synchronization control. 37

synchronized statements (2) http://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html 38

synchronized statement hazards (1) Boolean has only two instances of Boolean If another thread also synchronizes on the same Boolean instance, like this: – private final Boolean someLock Boolean.FALSE; The lock will be reused. The system might be deadlock or unresponsive. It is hard to detect this type of bugs! More examples: https://www.securecoding.cert.org/confluence/display/java/LCK01-J. Do not synchronize on objects that may be reused 39

synchronized statement hazards (2) Another example of the wrong way of using locks: What will happen another thread also synchronizes on an integer instance with the 0 integer value? https://www.securecoding.cert.org/confluence/display/java/LCK01-J. Do not synchronize on objects that may be reused 40

synchronized statement hazards (3) Correct way of using locks: using new to instantiate an object https://www.securecoding.cert.org/confluence/display/java/LCK01-J. Do not synchronize on objects that may be reused 41

References 1. Thinking in Java 4th Ed, Bruce Eckel 2. Oracle Java tutorial (http:// docs.oracle.com/javase/tutorial/index.html) 3. www.cs.drexel.edu/ spiros/teaching/ CS575/slides/java.ppt 4. http:// eclipsetutorial.sourceforge.net/Total Beginn er Companion Document.pdf 5. http://www.vogella.com/tutorials/EclipseDeb ugging/article.html 42