Java Programming II: Object-Oriented Approach - Stream I/O, Initialization, Variables - Pr, Study notes of Computer Science

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CMSC 132:

Object-Oriented Programming II

Java Constructs Department of Computer ScienceUniversity of Maryland, College Park

Overview

Stream I/O Initialization Blocks Variable Initialization Annotations BitSet class Two-dimensional array

Using Streams

Opening a stream^ Connects program to external data^ Location of stream specified at opening^ Only need to refer to stream Usage 1.^ import java.io.* ; 2.^ Open stream connection 3.^ Use stream

→→→→^ read and / or write Catch exceptions if needed

4.^ Close stream^ Examples^ See fileExamples package

Initialization Block

Definition^ Block of code used to initialize static & instancevariables for class Motivation^ Enable complex initializations for static variables^ Control flow^ Exceptions^ Share code between multiple constructors forsame class

Variable Initialization

Variables may be initialized^ At time of declaration^ In initialization block^ In constructor Order of initialization 1.^ Declaration, initialization block(in the same order as in the class definition) 2.^ Constructor

Variable Initialization – Example class Foo {^ static {^ A = 1;

}^ // static initialization block static int A = 2;

// static variable declaration static {^ A = 3;

}^ // static initialization block {^ B = 4;^ }^

// initialization block private int B = 5;

// instance variable declaration {^ B = 6;^ }^

// initialization block Foo() {^

// constructor A = 7;B = 8;}

// now A = 7, B = 8 }^

// initializations executed in order of number

Reviewing Bit-Operations and

x^11010 y^10110 x and y 10010 or

x^11010 y^10110 x and y 11110 xor

x^11010 y^10110 x and y 01100 Java Bitwise operators^ & and^ |^ or^ ^ exclusive or^ ~ complement

BitSet Class

Implements a vector of bits where the bits of the set are indexed bynonnegative integers Methods^ BitSet()^ – New bit set^ BitSet(int nbits)

• Bit set large enough to represent bits with indices from 0 through nbits – 1 and(BitSet set) - Performs logical

and^ between the current object and the set parameter (current object is updated with the result) or(BitSet set)^ – Performs logical

or^ between the current object and the set parameter (current object is updated with the result) cardinality()^ – Returns number of bits set to 1 flip(int bitIndex)

• Sets the bit at the specified index get(int bitIndex

) – Returns true if the bit at bitIndex is set; false otherwise length()^ – Index of the highest set bit + 1. It returns zero if the BitSet containsno bits set. size()^ – Number of bits space used by the BitSet to represent bit values toString()^ – For every bit set, the decimal representation of that index isincluded in the result. Example (See Computers.java)

Two-Dimensional Arrays of Objects^ Each row in a two-dimensional array is anarray^ The rows can have different lengths^ Defining an array where rows have the samelength

String [ ][ ] data = new String[3][4]; Important: Keep in mind we have created atwo-dimensional array of references to Stringobjects. No String object is present yet. We can also have ragged arrays Example (See Roster.java)