Lecture Set 18 Collections New Looping construct 1 1 for each loop Collections 2 1 2 Stack ArrayList CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr Collections in Java Arrays are collections Arrays are objects Arrays are sequences of elements in base type These elements are collected together in one object the array Java includes may other collection mechanisms Arrays good for some applications fixed length sequences not others varying length sequences Other collections tuned for different purposes General observation holds however Collections are objects that contain other objects in a given type We ll study two more in CMSC132 Stack ArrayList CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr 1 1 for each in New construct available in Java 1 5 not available in older versions of Java Use with arrays Use with any iteratable collection Limitations modifications limited access only one can t add items to the list being iterated over can t remove items from the list being iterated over can t replace items in the list being iterated over only a single collection can be traversed at a time can t access the one before or the one after on this iteration limited to forward and one at a time can t traverse the list in the reverse order can t go to every other element or any variation CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr 2 Stacks in Java Recall a stack is a data structure device for holding values FILO First In Last Out Typical operations on a stack push add a new value into the stack pop remove the most recently added value still in stack top return the most recently added value in stack Note Java calls this peek is empty returns true if the stack is currently empty or false otherwise CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr 3 2 Example of stack concept not Java specific Stack s s isempty true s push 3 s isempty false s push 4 s peek 4 s pop s push 5 s peek 5 5 4 3 CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr s 4 Stacks in Java cont Java includes a generic class for stack objects Stack objects contain other objects All objects in stack must have same type Only objects may be stored in stacks no primitive type values Syntax Stack E Stack E is a generic class E is a class variable representing the base type Replace E by a specific type to get a stack of that type of elements Class is in java util package Documentation http java sun com j2se 1 5 0 docs api java util Stack html See example StackExample java Stack String stack new Stack String Creates a stack of strings extend this to be stack of cats extend this to be stack of integer values CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr 5 3 ArrayList Collection Like arrays but support for inserting deleting new elements Sequences of elements All elements must be in same base type Syntax ArrayList E Documentation http java sun com j2se 1 5 0 docs api java util ArrayList html See example ArrayListExample java ArrayList String a new ArrayList String Creates an ArrayList of strings Collections sort may be used on ArrayList String objects Reason String implements Comparable interface ArrayList E implements List E interface CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr 6 Mutable Strings Strings are immutable Sometime mutable strings would be handy Once a String object is created it cannot be altered For String objects reference shallow deep copying why Sometimes a small change needs to be made to a string e g misspelled name Don t want to create a whole new String object in this case StringBuffer Java s class for mutable Strings CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr 7 4 StringBuffer Basics See documentation at http java sun com j2se 1 5 0 docs api java lang StringBuffer html Main methods Note append add characters to end insert add characters in middle delete remove characters append insert return object of type StringBuffer This is alias to object that the methods belong to See StringBufferExample java CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr 8 5
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