Serializable Objects

1. Serializable Objects • Serialization is Java’s way of allowing you to write a sequence of bytes representing an object in your program to a file. • Deserialization is the process of reading a file containing serialized objects and ‘reconstituting’ them. See: https://docs.oracle.com/javase/8/docs/api/java/io/Serializable.html • Classes whose objects can be serialized need to: • import java.io.Serializable • implement Serializable • Transientvariables are not written or read • private transient int x; declares an instance variable that will be neither written nor read

2. Serializable Anomalies • When traversing a graph, an object may be encountered that does not support the Serializable interface. In this case a NotSerializableException will be thrown and will identify the class of the non-serializable object. • It is possible for class A to write an object to a file and a different version of class A attempt to read this object back from the file, which causes an InvalidClassException to be thrown on deserialization. • A serializable class can declare its own serialVersionUID explicitly by declaring a field named "serialVersionUID" that must be static, final, and of type long. • ObjectStreamClass.lookup() can be used to retrieve the serialVersionUID for a class. • Java API strongly recommends that all serializable classes explicitly declare serialVersionUID values to prevent version problems that may occur with the system generated serialVersionUID for a class.

3. Reading and Writing Objects • Before writing objects to sequential disk files, instantiate:FileOutputStream fos = newFileOutputStream(“pathname”);BufferedOutputStream bos = new BufferedOutputStream(fos);ObjectOutputStream oos = new ObjectOutputStream(bos); • Before reading objects from sequential disk files, instantiate:FileInputStream fis = newFileInputStream(“pathname”);BufferedInputStream bis = new BufferedInputStream(fis);ObjectInputStream ois = new ObjectInputStream(bis); • To Write the object: oos.writeObject( objectVar ); • To Read the object: objectVar = (objectType)ois.readObject(); • To close the streams: ois.close(); or oos.close(); • Catch exceptions such as IOException, FileNotFoundException

4. Bitwise Operations • Assume: • x is a byte with its eight bits set to 00010101 • y is a byte with its eight bits set to 11110011 • Least significant bit (LSB) is the bit farthest to the right • Then • And: x & y results in 00010001 (a bit is set [has value 1] if both of the bits in the same position are set – LSB of x is 1, LSB of y is 1, so LSB of result is 1) • Or: x | y results in 11110111 (a bit is set if either of the bits in the same position are set) • Xor: x ^ y results in 11100110 (a bit is set if either but not both of the bits in the same position are set) • Not: ~x results in 11101010 (change zeroes to ones and ones to zeroes) • Shl: x <<3 results in 10101000 (bits in x shift left three positions) • Shr: y >>> 3 results in 00011110 (bits in y shift right three positions) • Sar: y >> 3 results in 11111110 (bits in y shift right with sign extension -- the value of the most significant bit (MSB) (1) repeats) • Sar: x >> 3 results in 00000010 (bits in x shift right with sign extension -- the value of the MSB (0) repeats)

5. Why bits? • Embedded controllers and device drivers need to communicate with devices using bitwise operations • Significant storage savings are possible with some applications • A human DNA sequence has billions of letters, where each letter is one of A, C, T, or G. Since there are only 4 values, each letter in the sequence can be stored using only two bits. A byte for each character would require eight times the storage. • Multimedia applications store data in bits. • Sometimes bit operations are faster than using floating point multiplication and division • Databases and operating systems rely heavily on bit manipulation • Encryption algorithms use bit operations Note:Bit operations only on primitive integer values: int, short, character, long

6. Converting binary to decimal • Assume: • x is a byte with its eight bits set to 01010101 • y is a byte with its eight bits set to 00101011 • What displays? • System.out.println(x) displays 85 (26 + 24 + 22 + 20 = 64 + 16 + 4 + 1) • System.out.println(y) displays 43 (25 + 23 + 21 + 20 = 32 + 8 + 2 + 1) • Remember the carry with binary addition • 1 + 1 is 0 with a carry of 1 to the next digit 01010101 + 00000001 = 10000000 -- -128 displays in the demo – see Twos Complement See demos/BitDemo.java To see hex values in the debugger: Window->Preferences: For primitives, browse to Java->Debug->Primitive Display Options

7. Converting Decimal to Binary • Powers of 2: 26 (64) 25 (32) 24 (16) 23 (8) 22 (4) 21 (2) 20 (1) • In 49: Need a 64? -- no 0, 32? – yes 1, 16 yes 1 (now am upto 48), 8 no 0, 4 no 0, 2 no 0, 1 yes (48+1 == 49) 1 0110001

8. Twos complement representation Non-negative numbers - MSB == 0 Negative numbers – MSB == 1 To compute the twos complement of an integer: Negate (Not) all bits then add one. Find Twos complement of 11110011: • Complementing 11110011 gives 00001100. • Add one to 00001100 gives a twos complement value of 00001101 • In decimal, 00001101 is 23 + 22 + 20 = 8 + 4 + 1 = 13. • Because the number was negative, the original value was -13. Shortcut: flip the digits to the left of the least significant one bit. 0000110 1 (13) – flip bits to the left of least significant 1 bit -> 1111001 1 (-13) 111100 10 (-14) – flip bits to left of least significant 1 bit -> 000011 10 (14)

9. Operations on Bits • We can store YYYYMMDDHHmm in 32 bits (type int) • Fields • mm (6 bits – bits 0 - 5) -- need 60 values • HH (5 bits – bits 6 - 10) – need 24 values • DD (5 bits – bits 11 - 15) – need 31 values • MM (4 bits – bits 16 - 19) -- need 12 values • YYYY (11 bits – bits 20 - 30) – 2047; values • Valid (1 bit – 31) – flag indicating this is a real date -- why? • Bit Mask • an integer with ones in field positions and zeroes elsewhere • int hourM = 31<<6 is the mask for HH • int yyyyM = 4095<<20 is the mask for YYYY • Questions • What are the biggest numbers that can fit in each of the above fields? • In the lab you use longs: • 3L<<35 is the mask for a 2 bit field starting at bit 35 of a long • Why is the 'L' needed in the expression 3L<<35?

10. Bitwise operations using masks Assume that 'int date = 0;' used to create the variable to hold one of these ‘dates’ • Set the year to 2007 • date &= ~(2047<<21); // clear the year field • date |= (2007<<21); // set the new values • Clear the hour field • date &= ~(31<<7); • Toggle a bit (value ^= 1<<5; toggles bit 5) – equivalent to: value = value ^ (1<<5); • Multiply using bits • x << 3 multiplies by 23 = 8. • x + x<<1 + x<<2 multiplies by 1 + 21 + 22 = 7 • Divide using bits • y>>2 divides by 4, handling positive and negative numbers • y>>>2 divides by 4, but only handles positive numbers

11. Quality Assurance • Quality assurance • Design review (walkthroughs), unit tests, integration, test suites, alpha-testing, beta-testing • Unit tests • Black-box testing (tests the interface of this unit by assuming no knowledge of how it is implemented) • White/Glass-box testing (force every path to be taken. Must know how the unit is implemented to test it) • Assertions (statements in the code itself that check data values at the point the statement executes) • Debugging Techniques • Breakpoints, watch variables, single stepping • Tracing (inspecting the call stack)

12. Agile Development Methodology See http://agilemodeling.com/ • Storyboard with users – small amounts of functionality in each release • Write tests (Test driven design) • Develop code • Release code when tests pass • Repeat above steps for next set of functionality. • Continuous integration shops produce daily releases and constantly test with automated test suites.

13. Case Study • Automated testing programs are in high demand. They allow tests to be taken at times convenient to the individual. • Some issues to consider: • Guarantee that the correct person is taking the test. • Dynamic group of questions • Where should the test be taken • Types and numbers of questions • Test bank • Availability of results • How general purpose • Throwing out of bad questions • Artificial intelligence • What steps do we need for a sound design? Lab 3: Create prototype to demonstrate automated true/false tests