In each practical class you will find three course instructors who have the following duties:

The course is designed to accommodate students with diverse backgrounds in programming ability; consequently Java is taught from first principles in a practical class setting where you can work at your own pace. Students with prior experience at Java should complete this workbook as normal and then seek an extension exercise from the course lecturer.

Each practical class introduces a new programming topic and provides pointers to further background reading. Each exercise is designed to provide experience and insight into the practical effort of designing, building and testing software. The work for each week is cumulative, building on work undertaken for previous weeks, so that by the end of the course you will have written a piece of software capable of playing Conway's Game of Life.^[1] By way of incentive, a screenshot of the graphical user interface of the final piece of software you will write is shown in Figure 1, “Screenshot: Conway's Game of Life”.

Figure 1. Screenshot: Conway's Game of Life

This course uses MCS Linux. If your MCS computer is currently running Windows, please restart the machine and boot into MCS Linux.

Under Unix or Linux, a shell or terminal is often used to control the creation, execution and termination of applications. In this course we will be using a common shell, Bash, to control the Java tools. There are many other Integrated Development Environments (IDEs) available for Java, but we will avoid them in order to provide you with a sound understanding of the fundamentals. We recommend you use the Linux "Text Editor" application to write your Java code—it's simple and has all the features you require—however if you are familiar with Linux or Unix you may use an alternative text editor of your choice.

Bash is a programming language in its own right. Unfortunately we won't have time to explore many of its features in this course, however improving your Bash skills in your own time will pay dividends in later years. If you are interested in finding out more you might like to take a look at the reference manual, but only if you have time.^[2]

crsid@machine:~$

crsid@machine:~$ date
Fri 25 Oct 2013 12:10:12 BST
crsid@machine:~$

crsid@machine:~$ date +"%Y-%m-%d %H:%M:%S"
2013-10-25 12:10:12
crsid@machine:~$

crsid@machine:~$ man date

In this section you will write a simple Java program and use the java tools to compile and execute it.

Java is a compiled language, which means that the Java programs you write must be transformed into another format before execution. To peform this transformation you need a compiler and in this course we will use the java compiler called javac to convert Java source code into Java bytecode. Java bytecode can then be run using the Java runtime program java. Compilers for some languages, such as C, produce machine code as output which can then be run directly on the CPU. The Java designers chose to use the bytecode format because it is portable and can be moved between computers and operating systems without the need for recompilation; the price paid for this portability is the need for a runtime program to assist the computer in the execution of the bytecode. Running the java program launches a Java Virtual Machine which in turn can execute Java bytecode.

Open a copy of Text Editor (or your favourite Linux text editor if you have one). You can do this by selecting the "Dash Home" icon, which is the top icon in the left panel of icons, and typing "Text Editor" into the search box. Click on the Text Editor application icon when it appears to launch the application. The Text Editor application opens with a new blank file. Type the following Java source code into the new file:

class Example {
 public static String message = "Hello, world";
 public static void main(String[] args) {
  System.out.println(message);
 }
}

Save this Java source code to disk using the filename Example.java. (In Java the filename must be the same as the name following the keyword class with the extension '.java' appended to it.)

This example contains a lot of syntax and keywords. Java has quite a lot of essential syntax, even for the most basic program, which you do not need to fully understand this week. You will hear more about it in the Object-Oriented Programming course, and you will gain a thorough understanding of it in future practical classes. Even if it looks complex now, it won't do by the end of the term, and the main message at the moment is don't panic, you're not supposed to understand it!

Having said all this, you might find an overview of the example helpful, so here it is. The phrase class Example { ... } provides a definition of a new data structure, in this case called Example. This data structure, or class, can store values (such as integers, strings, and so on) which in object-oriented programming parlance are called fields; the example class above has a single field called message, which stores an item of type String. A class can also contain program code written as one or more methods; the example above has a single method called main which takes an array of strings (String[] args) and returns nothing (void). This is conceptually similar to an ML function which takes a string list and returns unit. A method which is of the form public static void main(String[] args) is special in Java and indicates the point at which program execution begins.

To compile the source code to bytecode make sure that your current working directory for your Bash shell is the same as the directory containing your source code. For example, if you saved Example.java into the directory called java inside your home directory then you will need to change into that directory before trying to compile the code:

crsid@machine:~$ cd java
crsid@machine:java$

In the rest of this workbook we assume your current working directory is your home directory. To compile your Java source code you should type in the following:

crsid@machine:~$ javac Example.java

The prompt should return without any textual output. If the compiler does print some output it will describe one or more warnings or errors in the program because you have not typed in the source code correctly. In this case take another look at your file and correct any mistakes you find; the output from the compiler may help you to identify where these are.

If your program has compiled correctly, the compiler will have generated a file called Example.class. You can check this exists by using the program ls. The file Example.class is the bytecode version of your Java source code which you can execute with the java program. Run your Java bytecode now as follows:

crsid@machine:~$ java Example
Hello, world
crsid@machine:~$

Notice that we do not include the '.class' file extension when calling the java program. What has happened here is that the program code we wrote describes a class called Example. The compiler has turned this into bytecode and stored it in a file called Example.class. Next we launched java and told it which class to use (not which file). However, in order to use the class the Java Virtual Machine must first load the class' bytecode. One of the means for doing this is to look for a file on disk with the same name and a .class extension and so everything works.

crsid@machine:~$ jar cfe Example.jar Example Example.*

crsid@machine:~$ jar tf Example.jar
META-INF/
META-INF/MANIFEST.MF
Example.class
Example.java
crsid@machine:~$ 

crsid@machine:~$ java -jar Example.jar
Hello, world
crsid@machine:~$ 

In order to manage large projects and prevent different programmers from accidentally giving the same name to two different implementations of a class, Java classes can be placed in a package. By convention, Java package names use an Internet Domain Name Service (DNS) name as a prefix for a package name. Consequently in this course you should package all of your code inside uk.ac.cam.your-crsid where your-crsid should be replaced by your own MCS username. In other words, if you log in to MCS machines with the username arb33 all your java code should have the package prefix uk.ac.cam.arb33.

You must declare which package a class is associated with at the very top of the source file. For example, to place the 'hello world' example above into the package uk.ac.cam.arb33.examples you write the following:

package uk.ac.cam.arb33.examples;

class Example {
 public static String message = "Hello, world";
 public static void main(String[] args) {
  System.out.println(message);
 }
}

The source file should still be saved into a file called Example.java but should be placed inside the directory hierarchy which mirrors the package name. Therefore in the above example, Example.java should be saved inside a directory heirarchy of uk/ac/cam/arb33/examples. Such a directory path can be created with the following Bash commands:

crsid@machine:~$ mkdir uk
crsid@machine:~$ mkdir uk/ac
crsid@machine:~$ mkdir uk/ac/cam
crsid@machine:~$ mkdir uk/ac/cam/arb33/
crsid@machine:~$ mkdir uk/ac/cam/arb33/examples

The electronic manual for mkdir explains how to do this in one step:

crsid@machine:~$ mkdir -p uk/ac/cam/arb33/examples

You should then move Example.java into this location using the mv command:

crsid@machine:~$ mv Example.java uk/ac/cam/arb33/examples

Bash provides command completion to reduce the amount of typing required for commands like these. Simply type the first part of the command or the directory path before pressing the [tab] key on your keyboard. If there is a unique completion then Bash will fill it in for you. If there are multiple completions then simply press tab again to see them or continue typing to disambiguate the options.

To compile a program which is inside a package, the current working directory of the shell must contain the outermost directory of the package (in other words, in the case above, the directory uk should be directly inside the current working directory). The example can then be compiled as follows:

crsid@machine:~$ javac uk/ac/cam/arb33/examples/Example.java

the compiler will put the Example.class file in the same directory as Example.java.

The associated jar file can be built in a similar fashion, except that the name of the class must now include the name of the package it is within:

crsid@machine:~$ jar cfe Example.jar \ 
uk.ac.cam.arb33.examples.Example \  
uk/ac/cam/arb33/examples/Example.*
crsid@machine:~$ 

(Note: the backslash character (\) is used to continue a single command onto an extra line; you do not have to type this character in if you write all the text on a single line.)

The jar file can then be executed using the java program as before:

crsid@machine:~$ java -jar Example.jar
Hello, world
crsid@machine:~$ 

When you become a more proficient Java programmer, you may wish to keep your source files in a separate location from your bytecode files. This is possible, but for now we will forego this complexity and keep all the files in one directory hierarchy.

You have now successfully compiled, packaged and executed a simple Java program. We will now introduce the basic data-types provided in Java and the operators you can use to manipulate them. You will use this knowledge at the end of the practical to complete the assessed exercise.

In Java we use variables to allocate computer memory to store either: (i) a value of a basic, or primitive, type or (ii) a reference to an Object. This workbook focuses on how to create and manipulate variables of a primitive type; you will investigate Objects and references to Objects in future weeks. By the end of this term, you should be able to list and use all the primitive types and explain the behaviour of the operators which act upon them.

Variables in Java are different to values in ML. Values in ML are normally immutable, in other words, once you create them you cannot modify them. In Java it is much more common to program with mutable "values" than immutable ones. A mutable "value" in Java can be modified, possibly many times, after it has been created. Consequently such mutable "values" in Java are called variables in deference to the fact that Java programs commonly modify the contents of a variable during execution.

There are eight primitive storage types in Java which can be used to store values representing truth or falsity, integers of varying size, or floating point numbers. Table 1, “The primitive types” shows the names, sizes and storage capacity of these types. At this stage you should use this table to get an idea of what's there and refer back to it as you need to. By the end of the course we expect you to be able to remember the contents. The meaning of each of the columns in the table is as follows:

We can create a variable of a particular primitive type by writing the name of the type followed by a memorable name. This can be done either within a class definition (in which case it is also called a field) or within a method such as the main method we saw earlier. Below is a simple example which declares two variables, both of type int:

class VariableExample {
 public static int fieldVar;
 public static void main(String[] args) {
  int methodVar;
 }
}

You should choose the names of variables carefully, partly because a good name will improve readability (and therefore is strongly encouraged from a software engineering perspective), but also because some names cannot be used to name variables in Java. The following names are reserved words in Java:

abstract   assert       boolean    break    byte      case      
catch      char         class      const    continue  default    
do         double       else       enum     extends   false     
final      finally      float      for      goto      if           
implements import       instanceof int      interface long
native     new          null       package  private   protected 
public     return       short      static   strictfp  super      
switch     synchronized this       throw    throws    transient  
true       try          void       volatile while

using any of these words as a name for a variable will result in a compile error. If you encounter this problem, the solution is to change the name of the variable. You will learn what many of these reserved words do in this course, but not all. Some of the advanced features of Java will not be taught until the second year.

Integral types

Java has five primitive types which support storage of a range of integral values: byte, char, short, int and long. To explore the primitive integral types you will need to write simple Java programs. To get you started, follow the instructions below, making sure you replace your-crsid with your username. For example, if your username is arb33 you should use arb33 in place of your-crsid.

• Create a new Java source file called PrimitiveInt.java inside the directory uk/ac/cam/your-crsid/examples you created in the last section.

• Enter the following Java program into the file called PrimitiveInt.java:

  package uk.ac.cam.your-crsid.examples;
  
  class PrimitiveInt {
   public static void main(String[] args) {
    int i;                 //create a variable called i
    i = 1;                 //update i with the value 1
    i = i + 1;             //update i with the value of "i+1" 
    System.out.println(i); //print out the current value of i
   }
  }
  

• Compile your program using the javac program.

• Run your program using the java program.

If you have followed the steps correctly, your program should print out the number 2. Make sure you work through the above even if it seems obvious! You will need to modify this program later.

Numeric operators

The first set of operators on primitive types we will consider are the numeric operators. These perform numeric or mathematical operations with values of a primitive type and are written as follows: + (addition), - (subtraction), * (multiplication), / (division), % (modulus or remainder). These are infix binary operators. In this context, the word binary means that the operator takes two operands and infix means that the operator is written between the arguments. For example, a + b uses the infix binary operator + to add the values stored in the variables a and b together; a and b are the left-hand and right-hand operands to the operator +.

The modulus operator (%) might be unfamiliar to you. The operation a % b calculates the remainder after a has been divided by b. This can be restated as follows: a % b finds a number x such that a - x is exactly divisible by b. An alternative way of thinking about the modulus operation uses clock arithmetic: the position of the hour hand twenty-six hours after midnight can be calcuated as 26 % 12, which of course yields the answer two because twenty-six hours after midnight, the hour hand will be be located at position two on the clock face. The answer to 26 % 5 can be found by considering a clock which completes a full revolution every 5 hours (rather than the usual 12) and consequently has only five hours marked on the face. Figure 2, “Clock arithmetic” contains a graphical illustration of 26 % 12 and 26 % 5.

Figure 2. Clock arithmetic

A unary operator is an operator which takes a single argument. The unary operator - (minus) is used to negate a literal or the value stored in a variable. For example, -a changes the sign of the value stored in the variable a.

Every operator in Java produces a result which can then be used as a left-hand or right-hand operand with another operator. The term expression is used to describe a valid construct made up of operators, variables, literal values and methods invocations (which will be explained later in the course); an expression is turned into a statement, or unit of execution, by appending it with a semi-colon (;). The order in which operands are executed in a statement depends on operator precedence, which is reasonably intuitive, but does have the potential to catch you out occasionally. You can override the default operator precendence by using round brackets. For example, the expression 1 + 2 * 3 produces the result 7. If you want the addition to take place before the multiplication you can write (1 + 2) * 3, giving the result 9.

Numeric operators with assignment

In the PrimitiveInt program, the assignment operator (=) is used to assign a new value to the variable i. The assignment operator is another example of a binary infix operator in which the left-hand operand is the name of a variable, and the right-hand operand is a value which the variable is updated to store. As mentioned before, every operator in Java produces a result, and the result of evaluating the assignment operator is the value of the right-hand operand. For example, the expression "i = 1" has the value 1 as its result.

Assignment operator

Write answers to all of the questions found in questions boxes such as this one into a plain text file called answers.txt. You will need to include your answers.txt file in your submission for your first Java Tick. Further information about the submission of the tick can be found at the end of this workbook.

1. Modify PrimitiveInt to demonstrate that the expression a = b returns b as its result. Include your modified or additional line(s) in answers.txt.

The next operators to consider are the unary increment and decrement operators written as ++ and -- respectively. They combine together both an assignment and an addition operation, and the position of the increment and decrement operators with respect to the variable name determines the value returned as a result.

If the increment operator is written before the name of the variable, then it is more accurately called the pre-increment operator. For example, given a variable i, the pre-increment operator is written ++i. The pre-increment operator is equivalent to writing the expression i = i + 1, and therefore ++i increases the value stored in i by one and also returns the increased valued as its result.

In contrast, if the increment operator is written after the name of the variable, then this operator is more accurately called the post-increment operator. For example, for the variable i, the post-increment operator is written i++. The post-increment operator increments the value stored in i by one, but the result returned by the operator is the value stored in i before it was incremented.

The pre-decrement and post-decrement operators behave similarly to the pre- and post-increment operators except that the value stored in the variable is reduced by one rather than increased by one.

Pre- and post-decrement

Add the following code to the main function in your copy of PrimitiveInt

		int j = 10;
		System.out.println(j--);
		System.out.println(j);
	

2. What does the code snippet above print out? Why?

3. If you replace j-- with --j in the above code snippet, what does the program print out?

Java also provides shorthand forms for all numeric operators when combined with assignment. For example, i = i + 10 can be shortened to i += 10 and j = j * k can be shortened to j *= k. A summary of the numeric and assignment operators is shown in Table 2, “Numeric operators and assignment”.

+ - / * %	Binary infix numeric operations
-	Unary negation
++ --	Post-increment, pre-increment, post-decrement, pre-decrement
+= -= *= /= %=	Shorthands for variable updating

Table 2. Numeric operators and assignment

Bitwise operators

In order to understand the next set of operators you need to understand how integer numbers are stored in memory as binary numbers. If you have not seen binary numbers before, this section might take you some time to work through; paying careful attention to binary numbers now will pay dividends later.

Important

We have provided some programs to help visualise the conversion between binary and decimal numbers. These can be downloaded from the course webpage.

Counting with decimal, binary and hexadecimal

Binary numbers can be explained by analogy to "normal" numbers. The number system used on a daily basis today is called the decimal number system, sometimes called base 10 arithmetic. The decimal system requires 10 unique digits (0, 1, 2, 3, 4, 5, 6, 7, 8 and 9) to write down a number. For example, the number 6409 has 6 thousands (10³), 4 hundreds (10²), no tens (10¹) and 9 ones (10⁰); hence 6409 can be written as 6 × 10 ³ + 4 × 10² + 0 × 10¹ + 9 × 10⁰.

Binary numbers use base 2 arithmetic and consequently, there are only two digits in binary (0 and 1). Binary arithmetic is often used in computers because it simplifies the hardware design. The binary number 1011 has one eight (2³), no fours (2²), one two (2¹) and one one (2⁰); hence the number 1011 can be written as 1 × 2³ + 0 × 2² + 1×2¹ + 1×2⁰, or 11 in decimal. In the preceding example, the binary number was written so that the leftmost position represented the binary value with the highest power of two and the rightmost position represented the least. This is identical to our "normal" decimal number system. For binary numbers we refer to the leftmost position as the most significant bit and the rightmost position as the least significant bit.

A binary number can be converted into its decimal equivalent by adding up all the powers of 2 where there is a corresponding 1 in the binary representation. In other words, if there is a 1 at position i in the number then you include 2ⁱ in the summation used to calculate the equivalent decimal number. By convention, the least significant bit is found at position 0. Therefore the most significant bit position in an n-bit binary number is n-1.

Hexadecimal notation is used to represent counting in base 16. In this notation we have 16 different digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F). In Java, a literal value is written in hexadecimal by prefixing the number with the characters 0x (a zero and lowercase x) in order to avoid confusion with decimal numbers. For example, the hexadecimal number 0x1FAD has a single four thousand and ninety six (4096 = 16³), fifteen (written as F) two hundred and fifty sixes (256 = 16²), ten (written as A) sixteens (16¹) and thirteen (written as D) ones (16⁰). In other words, 0x1FAD can be written as 1 × 16³ + 15 × 16² + 10 × 16¹ + 13 × 16⁰.

Hexadecimal numbers are popular with programmers because and they are easily converted to binary numbers with simple mental arithmetic and offer a more compact representation than writing a binary number directly. They also allow us to write numbers which spell out (rude) words. Java also supports octal numbers (base 8) and a literal value can be written in octal by prefixing the number with a leading zero. For example, the octal number 077 has the value 63 in decimal.

The bitwise logical operators

Important

We have provided some programs to help visualise the operation of the bitwise operators. These can be downloaded from the course webpage.

The bitwise logical operators perform logical operations on the binary representation of integers. For each of the n bits in the number, the bit at position i in the left-hand operand is combined with the bit in position i in the right-hand operand; these two bits are used to produce the bit at position i in the result. The operators are as follows:

& (bitwise AND)

The output bit is set only if the corresponding bits in the right-hand operand and in the left-hand operand are set;

| (bitwise OR)

The output bit is set if the corresponding bits in left-hand operand or in the right-hand operand are set;

^ (bitwise XOR)

The output bit is set if the corresponding bit in the left-hand operand is different to the bit in right-hand operand.

For example, 23 & 24 gives the result 16, or in binary, 10111 AND 11000 equals 10000.

The shift operators

The shift operators are used to move the bits in a number left (towards the most significant bit) or right (towards the least significant bit). They are written as << and >> for left and right shift respectively. For example, the expression 45>>3 shifts the bits representing the value 45 (written in binary as 101101) right three places, resulting in the value 5 (written in binary as 101).

Right and left shift

Modify your implementation of PrimitiveInt to test your answers to the following questions.

4.  Find the value of x and y such that

   i. 0x55>>x equals 5,

   ii. 0x55<<y equals 10880.

5.  Explain why 1<<n gives the value 2ⁿ.

6.  Combine the right shift and bitwise-and operators to give an expression which evaluates to 1 if a variable x has its fifth bit set, and 0 otherwise.

Negative numbers

The binary number system described so far cannot represent negative numbers. Most modern computers use a method called 2's complement which is able to represent both positive and negative numbers. In this scheme the most significant bit in an n-bit binary number represents -2^(n-1) whilst the remaining bits at position i (where 0 ≤ i < n-1) represent the positive values 2ⁱ as in earlier sections. For example, the 8-bit binary number 10001100 is decoded as minus one hundred and twenty eight (-128 = -2⁷) plus eight (8 = 2³), plus four (4 = 2²); in other words -2⁷ + 2³ + 2² = -116. This representation might seem a little odd, but it is appealing from the hardware design point of view (understanding why this is the case is beyond the scope of this course).

The right-shift operator >> performs sign extension, which means that the value of the most siginificant bits added during the right-shift operation are filled with the value of the most significant bit rather then with zeros. The alternative right-shift operator >>> performs a right-shift without sign extension, and therefore the top bits are always filled with zeros.

There is only one unary bitwise operator in Java, tilde (~), which negates every bit found in its argument; in other words it, swaps 1 for 0 and 0 for 1. A table summarising the bitwise operators is shown in Table 3, “Numeric operators and assignment”

& | ^	Bitwise binary infix operators and, or and xor
~	Bitwise unary negation
<< >> >>>	Bitwise left and right shift with and without sign-extension
&= |= ^= <<= >>= >>>=	Shorthand notation for updating the value held in a variable

Table 3. Numeric operators and assignment

Literal values

As you have seen earlier in the workbook, literal values for int can be written in one of three ways:

• in decimal as you might expect (e.g. 52),

• in octal by prefixing the literal with 0 (e.g. 064), or

• in hexadecial by prefixing the literal with 0x (e.g. 0x34).

You can write literal values for long similarly, except that you must postfix the literal with an L (i.e. 52L, 064L and 0x34L). It's not possible to write a literal value of type byte or short. If you want to store a literal into a variable of type short or byte you can do so by assigning an int literal which will be converted to a value of the correct type automatically. For example,

short myShort = 12;

The char type is 16-bits wide and is normally used to store values representing characters, but can also be used to store and retrieve integral values. Java supports unicode,^[4] making programs which support non-latin character sets easy to write. You can embed unicode characters in literal text strings inside Java source code, and even use unicode characters for variable, field, method or class names. You can specify literal values for char variables by:

• placing the character that you want inside single quotes (e.g. 'A'),

• providing a unicode value inside single quotes (e.g. '\u0065'), or

• providing an integer value (e.g. 65).

int a = (1+2)*3;                    //9
int b = 1+2*3;                      //7
boolean t1 = a == b;                //false
boolean t2 = a >= b+2;              //true
boolean t3 = a != b & !(a % 4 > 1); //true
boolean t4 = t1 | t2 & t3;          //true

You will often want to provide comments in your code to document how a particular piece of code works, or provide extra information to other programmers who might read or modify your code. You can write comments in Java in one of two ways:

/* A comment which can span
   multiple lines */

//A comment which contines until the end of the line

A multi-line comment starts with /* and finishes with */; multi-line comments cannot be nested (in other words, a comment cannot contain the characters /* or */ within it. A single line comment starts with // and continues until the end of the line.

If you are writing a large application or library, you will probably want to write more extensive documentation to help other software developers use the library or maintain your software. Java supports javadoc to do this. Javadoc is a command line tool which examines your source code, looking for multi-line comments which start with /** and uses the information held within the comment as well as its location in the source file to generate HTML documentation for the software project. Javadoc is beyond the scope of this course, although the interested reader who wants to know more may like to take a look at Sun's documentation.^[5]

A Java long is an integer with 64 bits. Your task is to write a class called PackedLong which uses the 64 bits inside the java long integer to store 64 boolean values. We will make use of the PackedLong class next week as the basis for our Game of Life implementation. You should create a source file called PackedLong.java inside the directory structure uk/ac/cam/your-crsid/tick1/ where your-crsid is your username. Place the following code inside PackedLong.java:

package uk.ac.cam.your-crsid.tick1; //TODO: replace your-crsid
		
public class PackedLong {
			
 /*
  * Unpack and return the nth bit from the packed number at index position;
  * position counts from zero (representing the least significant bit)
  * up to 63 (representing the most significant bit).
  */
 public static boolean get(long packed, int position) {
  // set "check" to equal 1 if the "position" bit in "packed" is set to 1
  long check = //TODO: complete this statement
  
  return (check == 1);
 }

 /*
  * Set the nth bit in the packed number to the value given
  * and return the new packed number
  */
 public static long set(long packed, int position, boolean value) {
  if (value) {
   // TODO: complete this
   // update the value "packed" with the bit at "position" set to 1
  }
  else {
   // TODO: complete this
   // update the value "packed" with the bit a "position" set to 0 
  }
  return packed;
 }
}

The PackedLong class does not contain a special "main" method. Consequently this means that it cannot be executed directly using the java command line program. Do not add a main method to this class; instead create two extra Java source files inside the same directory structure. Save the following program into uk/ac/cam/your-crsid/tick1/TestBit.java:

package uk.ac.cam.your-crsid.tick1; //TODO: replace your-crsid
		
public class TestBit {
 public static void main(String[] args) throws Exception {
  long currentValue = Long.decode(args[0]);
  int position = Integer.parseInt(args[1]);
  boolean value = PackedLong.get(currentValue, position);
  System.out.println(value);
 }
}

Save the following program into uk/ac/cam/your-crsid/tick1/SetBit.java:

package uk.ac.cam.your-crsid.tick1; //TODO: replace your-crsid

public class SetBit {
 public static void main(String [] args) throws Exception {
  long currentValue = Long.decode(args[0]);
  int position = Integer.parseInt(args[1]);
  boolean value = Boolean.parseBoolean(args[2]);
  currentValue = PackedLong.set(currentValue,position,value);       
  System.out.println(currentValue);
 }
}

Once you have written your PackedLong implementation, you should be able to test your code using the two supplied programs.

The TestBit program accepts two arguments on the command line. The first argument is a long value which represents the 64 boolean bits, and the second argument tells the TestBit program which boolean value to print out. For example, executing

java uk.ac.cam.your-crsid.tick1.TestBit 85 4

should produce the answer "true". If it does not, your implementation of PackedLong is incorrect. The SetBit program accepts three arguments on the command line. The first is the long value which represents the 64 boolean bits, the second argument tells SetBit which bit to set, and the third argument tells SetBit whether the chosen bit in the new value should be set to 1 (to represent true) or 0 (to represent false). For example, executing

java uk.ac.cam.your-crsid.tick1.SetBit 85 2 false

should produce the answer "81". If it does not, then your implementation of PackedLong is incorrect.

Testing your implementation with the two examples above will probably not be sufficient to locate all the errors in your implementation of PackedLong. Therefore you should produce a more complete list of test cases, paying particular attention to sign extension. Once you are happy with your program you should submit a jar file named crsid-tick1.jar to ticks1a-java@cl.cam.ac.uk for testing, remembering to replace crsid with your username. Your jar file should contain the following files:

META-INF/
META-INF/MANIFEST.MF
uk/ac/cam/your-crsid/tick1/SetBit.class
uk/ac/cam/your-crsid/tick1/SetBit.java
uk/ac/cam/your-crsid/tick1/TestBit.class
uk/ac/cam/your-crsid/tick1/TestBit.java
uk/ac/cam/your-crsid/tick1/PackedLong.class
uk/ac/cam/your-crsid/tick1/PackedLong.java
answers.txt

Note: you know how to build a jar file with a specified entry point in order to tell the Java runtime program how to execute the program within the jar file. There are two candidates for the entry point in this tick: TestBit and SetBit. Please make TestBit the entry point for your jar file.