Assertions in programming

• Validation and Text Manipulation are important aspects of programming.

The assert Statement

• Ensures certain conditions in programs are met while running.
• Programs may behave unexpectedly if incorrect data is passed.
• Errors may make tracing back sources more difficult.
• Code may run with "rotten" data, causing problems.
• Can be accomplished with the assert statement.
• assert statements include a logical expression.
• If the expression is true, the program proceeds normally.
• However, if the expression is false, an Assertion Error occurs, terminating the program.
• Primarily used during development and testing, it can be disabled.
• BlueJ allows assertions by default used as a learning tool.
• This may not always be the case with other IDE's.
• An assertion should never fail in production code.

Assertion Examples

• Student grade should fall between [0, 1].
• Code example double grade = 0.95; assert 0 <= grade && grade <= 1.0; ensures this.
• Can pass a message to be displayed alongside the Assertion Error using: assert condition : message;
• This is useful for debugging unexpected values.
• double grade = -0.95; assert 0 <= grade && grade <= 1.0 : "Grade must be in range [0,1]: " + grade; will display the grade if the condition is not met.

Validating Parameters

• A method expects arguments to be valid.
• Data types of parameters will be enforced automatically.
• Manual checks of parameter values can be done to enforce design rules.
• If an argument with an invalid value is found, an exception of choice can be thrown to terminate the program.
• IllegalArgumentException is a suitable choice.
• This reveals where and why the program failed.
• Example code:

public void setGrade (double grade) {
    boolean valid = (0 <= grade && grade <= 1.0);
    if(!valid){
        throw new IllegalArgumentException("grade must be in range [0,1]: " + grade);
    }
    this.grade = grade;
}

• If grade = -1.0 is inputted an IllegalArgumentException occurs.

Assertion vs Exception

• Assertions and exceptions have similar theory, but different applications.
• Assertions are used to catch coding errors and mistakes.
• Assertions throw Assertion Error exceptions.
• Errors are typically serious, abnormal conditions that are unrecoverable.
• Exceptions are typically used for runtime errors.
• There are many types.
• Exceptions may be reasonable issues that the program may expect and recover from.
• Eg: a failed network connection or incorrect user input.

Text Manipulation toCharArray

• toCharArray() can convert a String to an array of characters.
• Example: "abc".toCharArray(); // generates char[] = {'a', 'b', 'c'};
• Arrays have access to utility functions that Strings may not have.
• Enhanced for loop usage example:

String line = "Hello World";
for(int i=0; i<line.length(); i++){
    if(Character.isDigit(line.charAt(i))){ ... }
    ...
}

char[] lineArray = line.toCharArray();
for(char c: lineArray){
    if(Character.isDigit(c)){ ... }
    ...
}

Text Manipulation - split

• split() method splits a string to an array of strings based on a regular expression (regex).
• Inputting a character to split is the simplest form.
• Example: "John A. Macdonald".split(" "); // yields String[] = {"John", "A.", "Macdonald"}
• Example: "ACS-1904-003".split("-"); // yields String[] = {"ACS", "1904", "003"}

Regular Expression - Regex

• A regular expression is a pattern used for searching purposes.
• [and] specifies any character within the brackets.
• + specifies one or more of a preceding character.
• - specifies a range of characters.

Manipulating Text - Split

• Splitting a string allows iteration over all words in a given test.
• CatenateWords.java and ValidateFormat.java are text examples.

Manipulating Text - StringBuilder

• Strings are immutable, meaning they cannot be changed once initialized.
• However, the value of a String variable can be changed.
• This change allocates new memory area.
• Code example:

String h = "Hello ";
String s = "World";
h += s; // h = "Hello World"

• The value h points to a new location in memory after the change.
• The original value “Hello” may be garbage-collected.
• Excessive text manipulation is inefficient, which is where StringBuilder comes in.
• Methods like append, insert, delete, and reverse are available for StringBuilder and not String.
• Stringbuilder implements replace differently than String.

StringBuilder - Examples

• An instance of StringBuilder is initialized with a memory allotment.

• This allotment grows if needed.

• StringBuilder is stored as an array of characters.

• Extra space allotment allows easy character appending to the end.

• Example declarations:

// By default, 16 bytes are allocated
StringBuilder sb1 = new StringBuilder();

// String and 16 bytes will be allocated
StringBuilder sb2 = new StringBuilder("Hello World");

// Actual number of bytes (8) provided
StringBuilder sb3 = new StringBuilder(8);

• StringBuilder can catenate several strings together using the append() method.
• We can also use StringBuilder to easily check palindrome words.
• A palindrome reads the same forwards and backwards.
• This can be achieved by using the reverse() method.

Explanation of assertions in programming. Assertions validates program data and prevents unexpected behavior. Assertions are used to validate data during the development and testing phases.