Lecture 26 Collections Last time 1 Exceptions Today 1 Collections Stack CMSC 131 Fall 2007 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 Fall 2007 Jan Plane adapted from Bonnie Dorr 1 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 Fall 2007 Jan Plane adapted from Bonnie Dorr 2 1 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 Fall 2007 Jan Plane adapted from Bonnie Dorr s 3 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 Fall 2007 Jan Plane adapted from Bonnie Dorr 4 2
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