Regular Expressions ("reguläre Ausdrücke", "regexp")

Transcription

1 Regular Expressions ("reguläre Ausdrücke", "regexp") Classes. (dot) Any character \w Any "word character" [A Za z0 9] \W not "word characters" \s white space characters "empty character, CR, LF, TAB \S Non white space characters \d Digits [0 9] \D Non digits [ ] Own class e.g. [A Za z] or [0 9] () Alternatives e.g. (de kr it tk ua) Special characters * Previous pattern repeated 0 to n times + Previous pattern repeated 1 to n times? Previous pattern repeated 0 or 1 times {n, m} Previous pattern repeated between n and m tim {, m} Previous pattern repeated between maximum m {n, } Previous pattern repeated between minimum n t \ Escape Character 1

2 Receipe for JUnit tests New Source folder: Right mouse click on project > new > Java Source Folder Replicate the package structure (same as the src folder) Add JUnit Test case: Right mouse click on new package > new > JUnit Test Case. Pick the class and the method to be tested. Implement the method stubs 2

3 1 2 3 replaceall()! not replace()! $1 and $3 refer to the first and third block identified by the parentheses 3

4 UML to JavaCode => HashSets 1. Using arrays 1. Disadvantage: search gets slower with increasing array size 2. Disadvantage: limit array size 2. Using linear list 1. Advantage: flexible size 2. Disadvantage: search gets slower with increasing length of list 3. Working with HashSets 4

5 Hashsets Insertion Using the hashcode of an object we determine the position of an object in an array. I.e. an object with hashcode = 3 will be place on the third position in the array of the Hashset. If the position is still available we can insert the object right away. However, if the position ("bucket"') is already occupied then there is a so called collission. There are two ways to deal with collision: 1. Insert at the hashcode a linear list and add all objects with same hashcode to this list. 2. Search for next free bucket using a function ("Sondierungsfunktion") Search If an object has to be searched in a hashset we ask the object for the hashcode. If there is no object at the respective position in the hashcode, we can return false (object is not in the array) otherwise we have to either use the "Sondierungsfunktion" or we have to search for the object in the linear list. 5

6 Insert element into a hashset 1. Retrieve the element's hashcode which has to be inserted 2. Evaluate whether the position with the hashcode is empty (check whether it is null) 1. If there is no element at this position then insert the element 2. If there is an element at the respective hashcode check whether this element is the same (equals) 1. If this element is the same, nothing to be done, as the element already is in the array. 2. Otherwise we have collision 1. Option 1: use the exploratory function to search for a free spot e.g. check the next position 2. Option 2: use a linear list that contains all elements with the same hashcode Searching elements in a hashset 1. Retrieve the element's hashcode which has to be inserted 2. Evaluate whether the position with the hashcode is empty (check whether it is null) 1. If there is no element at this position then return false (i.e. element is not in hashet) 2. If there is an element at the respective hashcode check whether this element is the same (equals) 1. If this element is the same, return true 2. If this element is not the same, then use exploratory function 1. If the function finds the element, return true 2. If the function does not find element after one round trip, return false 3. If the function stumbles upon an empty position (null), return false 6

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8 Java Collection Framework Collection No Duplicates No Sequence Set HashSets List LinkedList Duplicates possible Sequence is kept ArrayList Vector (deprecated) SortedSet TreeSets in black: Interfaces in red: Implemention 8

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10 if multiple objects have to be inserted into a sorted set (here: TreeSet) then these objects have to implement the compareto-method. int values can be directly compared, String already have implemented the compareto.-method The compareto-method sorts objects in ascending order, if the return is smaller 0, in descending order otherweise 10