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Practice PE1 Question: Expression

Adapted from PE1 of 20/21 Semester 1

Instructions to grab Practice PE Question:

  1. Accept the practice question here
  2. Log into the PE nodes and run ~cs2030s/get-pe1-expression to get the practice question.
  3. There is no submission script

You should see the following in your home directory.

  • The files Test1.java, Test2.java, and CS2030STest.java for testing your solution.
  • The skeleton files for this question: Operand.java InvalidOperandException.java Operation.java

Background

An expression is an entity that can be evaluated into a value.

We consider two types of expression in this question:

  • An operand, which itself is a value.
  • A binary operation, which is a mathematical function that takes in two expressions and produces an output value.

For instance,

  • 3 is an expression that evaluates to 3.
  • 3 + 2 is an expression that evaluates to 5
  • (3 + 2) + 3 is also an expression that evaluates to 8

An operand is not necessarily an integer. It can be of any type. An expression can be evaluated to any type.

Three skeleton files are provided for you: Operand.java, Operation.java, and InvalidOperandException.java. If you need extra classes or interfaces, create the necessary additional Java files yourself.

Operand

Create a class called Operand that encapsulates the operands of an operation. The Operand class can contain references to a value of any reference type.

You may create additional parent classes or interfaces if you think it is appropriate.

The Operand has an eval method that returns its value.

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jshell> new Operand(5).eval()
$.. ==> 5
jshell> new Operand("string").eval()
$.. ==> "string"
jshell> new Operand(true).eval()
$.. ==> true

InvalidOperandException

Create an unchecked exception named InvalidOperandException that behaves as follows:

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jshell> InvalidOperandException e = new InvalidOperandException('!')
jshell> e.getMessage();
$.. ==> "ERROR: Invalid operand for operator !"

The constructor for InvalidOperandException takes in a char which is the corresponding symbol for the operation that is invalid.

Recall that all unchecked exceptions are a subclass of java.lang.RuntimeException. The class RuntimeException has the following constructor:

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RuntimeException(String message)

that constructs a new runtime exception with the specified detail message message. The message can be retrieved by the getMessage() method.

You can test your code by running the Test1.java provided. Make sure your code follows the CS2030S Java style.

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$ javac Test1.java
$ java Test1
$ java -jar ~cs2030s/bin/checkstyle.jar -c ~cs2030s/bin/cs2030_checks.xml *.java

Operation

Create an abstract class called Operation with the following fields and methods:

  • two private fields that correspond to two expressions (an expression is as defined at the beginning of this question).

  • a class factory method of, which returns the appropriate subclass that implements a specific operation. The first parameter of the of methods is a char to indicate the operation to be performed. You need to support three operations:

  • if the first parameter is *, return an operation that performs multiplication on integers

  • if the first parameter is +, return an operation that performs concatenation on strings
  • if the first parameter is ^, return an operation that performs XOR on booleans
  • if the first parameter is none of the above, return null

Note that the operator to perform XOR on two boolean variables is ^.

For instance,

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jshell> Operation o = Operation.of('*', new Operand(2), new Operand(3));
jshell> o.eval()
$.. ==> 6

jshell> Operation o = Operation.of('+', new Operand("hello"), new Operand("world"));
jshell> o.eval()
$.. ==> "helloworld"

jshell> Operation o = Operation.of('^', new Operand(true), new Operand(false));
jshell> o.eval()
$.. ==> true

jshell> Operation.of('!', new Operand(2), new Operand(3));
$.. ==> null

jshell> Operation o1 = Operation.of('*', new Operand(2), new Operand(3));
jshell> Operation o = Operation.of('*', o1, new Operand(4));
jshell> o.eval()
$.. ==> 24

jshell> Operation o2 = Operation.of('*', new Operand(2), new Operand(4));
jshell> Operation o = Operation.of('*', o1, o2);
jshell> o.eval()
$.. ==> 48

If the operands are not of the correct type, eval must throw an unchecked InvalidOperandException exception.

For instance,

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jshell> Operation o = Operation.of('*', new Operand("1"), new Operand(3));
jshell> try {
   ...>   o.eval();
   ...> } catch (InvalidOperandException e) {
   ...>   System.out.println(e.getMessage());
   ...> }
ERROR: Invalid operand for operator *

jshell> Operation o = Operation.of('+', new Operand(1), new Operand(4));
jshell> try {
   ...>   o.eval();
   ...> } catch (InvalidOperandException e) {
   ...>   System.out.println(e.getMessage());
   ...> }
ERROR: Invalid operand for operator +

jshell> Operation o = Operation.of('^', new Operand(false), new Operand(3));
jshell> try {
   ...>   o.eval();
   ...> } catch (InvalidOperandException e) {
   ...>   System.out.println(e.getMessage());
   ...> }
ERROR: Invalid operand for operator ^

jshell> Operation o1 = Operation.of('*', new Operand(1), new Operand(3));
jshell> Operation o2 = Operation.of('^', new Operand(false), new Operand(false));
jshell> Operation o = Operation.of('+', o1, o2);
jshell> try {
   ...>   o.eval();
   ...> } catch (InvalidOperandException e) {
   ...>   System.out.println(e.getMessage());
   ...> }
ERROR: Invalid operand for operator +

jshell> Operation o1 = Operation.of('*', new Operand(1), new Operand("3"));
jshell> Operation o2 = Operation.of('^', new Operand(false), new Operand(false));
jshell> Operation o = Operation.of('+', o1, o2);
jshell> try {
   ...>   o.eval();
   ...> } catch (InvalidOperandException e) {
   ...>   System.out.println(e.getMessage());
   ...> }
ERROR: Invalid operand for operator *

You can test your code by running the Test2.java provided. Make sure your code follows the CS2030S Java style.

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$ javac Test2.java
$ java Test2
$ java -jar ~cs2030s/bin/checkstyle.jar -c ~cs2030s/bin/cs2030_checks.xml *.java