Programming Model 1 / 22

Transcription

1 Programming Model 1 / 22

2 Outline 1 Basic Structure of a Java Program 2 Primitive Data Types and Expressions 3 Statements 4 Arrays 5 Static Methods 6 APIs 7 Strings 8 Input and Output 2 / 22

3 Basic Structure of a Java Program The Java workflow editor (BlueJ) P.java compiler (javac) P.class JVM (java) output A Java program is either a library of static methods (functions) or a data type definition To create a Java program (class), we use the following programming constructs Primitive data types and expressions Statements Arrays Static methods Strings Input and output (IO) Data abstraction 3 / 22

4 Basic Structure of a Java Program Example (Java program that computes N!) import edu. princeton.cs. algs4. StdOut ; public class Factorial { // Returns the factorial of N. public static int factorial ( int N) { int result = 1; for ( int i = 1; i <= N; i ++) { result *= i; return result ; // Test client. public static void main ( String [] args ) { int N = Integer. parseint ( args [0]); StdOut. println (N + "! = " + Factorial. factorial (N )); $ javac Factorial. java $ java Factorial 5 5! = / 22

5 Primitive Data Types and Expressions A data type (primitive or reference) is a set of values and a set of operations on those values Basic primitive types boolean: true and false values with logical operations (!,, and &&) char: 16-bit characters with arithmetic operations (+, -, *, /, and %) int: 32-bit integers with arithmetic operations double: 64-bit double-precision real numbers with arithmetic operations Other primitive types byte: 8-bit integers with arithmetic operations short: 16-bit integers with arithmetic operations float: 32-bit single-precision real numbers with arithmetic operations long: 64-bit integers with arithmetic operations 5 / 22

6 Primitive Data Types and Expressions An expression is a literal, a variable, or a sequence of allowed operations on literals and/or variables that produces a value Arithmetic operators * and / have higher precedence than + and - Among logical operators,! has the highest precedence, followed by && and then Two operands of the same type can be compared using the relational operators ==,!=, <, <=, >, and >=, producing a boolean result Relational operators have higher precedence than logical operators but lower precedence than arithmetic operators Operators of the same precedence are evaluated left to right Parentheses can be used to override precedence rules Numbers are automatically promoted to a more inclusive type if no information is lost A cast is a type name in parentheses within an expression, and converts the following value into a value of that type 6 / 22

7 Statements A statement defines computation by creating variables, assigning data-type values to them, and controlling the flow of execution A declaration statement associates a variable name with a type <type > <name >; where the initial value for the variable is false for boolean type, 0 for numeric types, and null for reference types An assignment statement associates a data-type value with a variable < name > = < expression >; Declaration and assignment statements can be combined to provide an initial value for a variable < type > < name > = < expression >; 7 / 22

8 Statements A conditional statement is used when different actions are required for different inputs If statement if ( < boolean expression >) { < statements > else if ( < boolean expression >) { < statements >... else { < statements > Conditional operator (really an expression) < boolean expression >? < expression > : < expression > Switch statement switch ( < expression >) { case < value >: < statements > case < value >: < statements >... default : < statements > 8 / 22

9 Statements A loop statement is used for repetitive computations While statement while ( < boolean expression >) { < statements > For statement for ( < initialize >; < boolean expression >; < increment >) { < statements > Do-while statement do { < statements > while ( < boolean expression >); A break statement immediately exits a loop without letting it run to completion; when used as the last statement in a switch case, exits the switch statement preventing fallthrough to the following case A continue statement skips to next iteration of a loop 9 / 22

10 Statements A compound assignment statement is shorthand notation for modifying the value of a variable; for example, i += 5; is equivalent to i = i + 5; The scope of a variable is the statements that follow the declaration in the same block (marked by curly brackets) as the declaration 10 / 22

11 Arrays An array stores a sequence of values that are all of the same type Making an array in Java involves: declaring the array name and type; creating the array; and initializing the array values int [] a; a = new int [10]; for ( int i = 0; i < 10; i ++) { a[i] = i; An array when declared is initialized to null, and once created, each element is initialized to a default value based on the type of the array Initializing declaration int [] a = {1, 1, 2, 3, 5, 8, 13, 21, 34, 55; String [] b = {" Sun ", " Mon ", " Tue ", " Wed ", " Thu ", " Fri ", " Sat "; Example (reversing an array in place) int N = a. length ; for ( int i = 0; i < N / 2; i ++) { double temp = a[i]; a[i] = a[n i]; a[n - i - 1] = temp ; 11 / 22

12 Arrays If we assign one array name to another (aliasing), then both refer to the same array A two-dimensional array in Java is an array of one-dimensional arrays, and may be ragged Initializing declaration for two-dimensional arrays int [][] I = {{1, 0, 0, {0, 1, 0, {0, 0, 1; Example (matrix multiplication C ij = p k=1 A ikb kj, where A is an m p, B is a p n, and C is an m n matrix) int m = A. length ; int p = B. length ; int n = B [0]. length ; double [][] C = new double [m][n]; for ( int i = 0; i < m; i ++) { for ( int j = 0; j < n; j ++) { for ( int k = 0; k < p; k ++) { C[i][j] += A[i][k] * B[k][j]; 12 / 22

13 Static Methods A method encapsulates a computation defined as a sequence of statements A static method is composed of the keywords public static followed by a return type (or void), the signature (method name and a sequence of arguments, each with a declared type), and a body (a sequence of statements enclosed by curly brackets) Example (primality testing) public static boolean isprime ( int N) { if (N < 2) { return false ; for ( int i = 2; i <= N / i; i ++) { if (N % i == 0) { return false ; return true ; A call on a static method is its name followed by expressions that specify argument values in parentheses, separated by commas When a method call is part of an expression, the method computes a value and that value is used in place of the call in the expression A method call followed by a semicolon is a statement that generally causes side effects 13 / 22

14 Static Methods Properties of methods Arguments are passed by value Method names can be overloaded A method has a single return value but may have multiple return statements A method can have side effects A recursive method is one that calls itself, has a base case, addresses subproblems that are smaller in some sense, and does not address subproblems that overlap Example of good recursion (computing N!) public static int factorial ( int N) { return ( N == 0)? 1 : N * factorial ( N - 1); Example of bad recursion (computing N th Fibonacci number) public static int fibonacci ( int N) { return ( N == 0 N == 1)? 1 : fibonacci ( N - 1) + fibonacci ( N - 2); 14 / 22

15 Static Methods A best practice in Java programming is to include a main() method (aka development or test client) in every library of static methods We use static methods from four different kinds of libraries Standard system libraries java.lang.* Imported system libraries such as java.util.arrays Other libraries in the text Standard libraries edu.princeton.cs.algs4.* from the text 15 / 22

16 APIs An application programming interface (API) lists the library name and the return type, signatures, and short descriptions of each of the methods A client is a program that calls a method in another library An implementation is Java code that implements the methods in an API Standard system library (java.lang.math) method description static double abs(double a) a static double max(double a, double b) max(a, b) static double min(double a, double b) min(a, b) static double sin(double theta) sin(θ) static double cos(double theta) cos(θ) static double tan(double theta) tan(θ) static double random() real [0, 1) static double E e static double PI π Imported system library (java.util.arrays) method description put the array in ascending order static void sort(int[] a) 16 / 22

17 APIs Standard libraries from the text edu.princeton.cs.algs4.stdrandom method description static void initialize(long seed) initialize static double random() real [0, 1) static int uniform(int N) integer [0, N) static int uniform(int lo, int hi) integer [lo, hi) real [lo, hi) static boolean bernoulli(double p) true with probability p static double gaussian() real N (0, 1) static double gaussian(double m, double s) real N (m, s) static int discrete(double[] a) i with probability a[i] static void shuffle(double[] a) shuffle the array static double uniform(double lo, double hi) edu.princeton.cs.algs4.stdstats method description largest value smallest value average sample variance sample standard deviation median static double max(double[] a) static double min(double[] a) static double mean(double[] a) static double var(double[] a) static double stddev(double[] a) static double median(double[] a) 17 / 22

18 Strings A string is a sequence of characters (char values) A string literal is a sequence of characters within double quotes, such as "Hello, World" The result of concatenating two strings using the + operator is a single string, the first string followed by the second We use Java library methods such as Integer.parseInt(), Double.parseDouble(), and so on to convert strings to primitives We use the + operator to convert primitives to strings 18 / 22

19 Input and Output Bird s-eye biew of a Java program input P.java output Input types: command-line arguments, standard input, file input Output types: standard output, file output Command-line arguments are listed after the program name as follows $ java P arg_1 arg_2... arg_n and accessed within the main() method of the program via the array args as args[0], args[1],..., args[n - 1] Example import edu. princeton.cs. algs4. StdOut ; public class UseArgument { public static void main ( String [] args ) { StdOut. print ("Hi, "); StdOut. print ( args [0]); StdOut. println (". How are you?"); $ java UseArgument Alice Hi, Alice. How are you? $ java UseArgument Bob Hi, Bob. How are you? 19 / 22

20 Input and Output Standard output (edu.princeton.cs.algs4.stdout) API method description print s, followed by a newline formatted print static void println(string s) static void printf(string f,...) Example import edu. princeton.cs. algs4. StdOut ; import edu. princeton.cs. algs4. StdRandom ; public class RandomSeq { public static void main ( String [] args ) { int N = Integer. parseint ( args [0]); double lo = Double. parsedouble ( args [1]); double hi = Double. parsedouble ( args [2]); for ( int i = 0; i < N; i ++) { double x = StdRandom. uniform ( lo, hi ); StdOut. printf (" %.2 f\n", x); $ java RandomSeq / 22

21 Input and Output Standard input (edu.princeton.cs.algs4.stdin) API Example method description true if no more values, false otherwise read a value of type int read a value of type double read a value of type boolean read a value of type char static boolean isempty() static int readint() static double readdouble() static boolean readboolean() static char readchar() import edu. princeton.cs. algs4. StdIn ; import edu. princeton.cs. algs4. StdOut ; public class Average { public static void main ( String [] args ) { int count = 0; double sum = 0.0; while (! StdIn. isempty ()) { double value = StdIn. readdouble (); sum += value ; count ++; double average = sum / count ; StdOut. println (" Average is " + average ); $ java Average <ctrl -d> Average is / 22

22 Input and Output Output redirection $ java RandomSeq > data. txt $ head -5 data. txt Input redirection $ java Average < data. txt Average is Piping $ java RandomSeq java Average Average is / 22