Object-Oriented Programming and Problem Solving Dr. Ramzi Saifan Introduction and basics of Java Slides adapted from Steven Roehrig

Source Files: All java source files must end with the.java extension. A source file must contain at most one top-level public class definition If a public class is present, the class name should match the unextended filename. There are three top-level elements that may appear in a file. None of these elements is required. If they are presented, then they must appear in the following order: Package declaration Import statements Class definitions

No Main Method in the Public Class public class MainClass { } class Demo1 { public static void main (String [] args) { System.out.println("Welcome to Java Programming"); } Compilation command: javac MainClass.java Run-time command: java MainClass Output: java.lang.NoSuchMethodError: main (Exception in thread "main")

No Definition for a Public Class class MainClass { public static void main (String [] args) { System.out.println("Welcome to Java Programming"); } class Demo1 { } Compilation command: javac MainClass.java Run-time command: java MainClass Output: Welcome to Java Programming

Input/Output System.out.print System.out.println JOptionPane.showMessageDialog System.in JOptionPane.showInputDialog java.util.Scanner sc = new Scanner(System.in) int x = Sc.nextInt() String str = Sc.nextLine Output Input

JDK JDK has a huge number of packages. However, each package has a collection of classes and interfaces. Latest Java 2 Sun Release: JDK 1.7 Interactive Java Environments: TextPad Eclipse BlueJ NetBeans

Getting the Java Documentation Point your browser to Bookmark the file so you can always get to it easily Or, just bookmark the index page on Suns Java Web site

Java Operators An operator takes one or more things and produces a resultant thing. Things are usually primitive types, but they are sometimes objects. The things operated upon are called operands. An operator is just a function, but with a different syntax.

A Familiar Example The assignment operator and the addition operator are used (each exactly once!). int i = 3, j = 4, k; k = i + j;

More Operator Facts All operators produce a value. Sometimes they produce side effects, i.e., they change the value of an operand. Evaluation of a statement with several operators follows precedence rules. Use parentheses for readability. (x + y) * z / 3 is different than x + y * z / 3

Java's operator precedence hierarchy: (postfix), -- (postfix),. (dot), [], ( ) (prefix), -- (prefix), + (unary), - (unary), ~ (bitwise inversion operator), ! (boolean complement operator) 3. (type): casting 4. *, /, % 5. + (binary), - (binary) 6. >, >>> (shift operators) 7., = (ordinal operators), instance of 8. = =, != (equality comparison operators) 9. & (bitwise AND operator) 10. ^ (bitwise XOR operator) 11. | (bitwise OR operator) 12. && (AND operator) 13. || (OR operator) 14. ?: (ternary operator) 15. =, +=, -=, *=, /=, %=, >>=, >>=, &=, ^=,!= (assignments operators)

Assignment Is Tricky, Part I public class Number { public int i; } public class Assignment1 { public static void main(String[] args) { Number n1 = new Number(); Number n2 = new Number(); n1.i = 2; n2.i = 5; n1.i = n2.i; n2.i = 10;// what is n1.i? }

Assignment Is Tricky, Part II public class Assignment2 { public static void main(String[] args) { Number n1 = new Number(); Number n2 = new Number(); n1.i = 2; n2.i = 5; n1 = n2; n2.i = 10;// what is n1.i? n1.i = 20;// what is n2.i? }

A Picture Might Help n1n2 reference variablesNumber objects n2.in1.i Before assignment n1 = n2 n1n2 reference variablesNumber objects n2.in1.i After assignment n1 = n

Aliasing In Function Calls public class PassObject { static void f(Number m) { m.i = 15; } public static void main(String[] args) { Number n = new Number(); n.i = 14; f(n);// what is n.i now? }

Math Operators +, -, *, /, % Integer division truncates, i.e., 16/3 = 5 Modulus operator returns remainder on integer division, i.e., 16%3 = 1 Shorthand: x += 4; is the same as x = x + 4; This works for the other arithmetic operators as well.

Auto Increment and Decrement ++ increases by one, and -- decreases by one. Two flavors of each: pre and post: int i = 1, j; j = i++;// j = 1, i = 2 j = ++i;// j = 3, i = 3 j = i--;// j = 3, i = 2 j = --i;// j = 1, i = 1

Booleans and Relational Operators The boolean type has two possible values, true and false. The relational operators >, >=, <, <=, == and != produce a boolean result. >, >=, <, <= are legal for all built-in types except booleans, == and != are legal for all.

Testing for (Non-)Equivalence The == and != operators need to be used with care with objects. public class Equivalence { public static void main(String[] args) { Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1 == n2);// prints false System.put.println(n1 != n2);// prints true }

The equals( ) Operator This exists for all objects (dont need it for built-in types). Integer n1 = new Integer(47); Integer n2 = new Integer(47); System.out.println(n1.equals(n2);// prints true

The equals( ) Operator (cont.) But exists doesnt necessarily mean properly defined! class Number { int i; } : Number n1 = new Number(); Number n2 = new Number(); n1.i = 3; n2.i = 3; System.out.println(n1.equals(n2));// prints false

The equals( ) Operator (cont.) The equals( ) operator is properly defined for most Java library classes. The default behavior is to compare references, so… When you define a class, if youre planning to use equals( ), you need to define it (i.e., override the default behavior).

Logical Operators These are AND (&&), OR (||), and NOT (!). These work on booleans only; if you have old C habits, forget them! Use parentheses freely to group logical expressions. Logical expressions short-circuit; as soon as the result is known, evaluation stops.

Short-Circuiting Example public class ShortCircuit { static boolean test1(int val) {return val < 1;} static boolean test2(int val) {return val < 2;} static boolean test3(int val) {return val < 3;} public static void main(String[] args) { if (test1(0) && test2(2) && test3(2)) System.out.println(Expression is true); else System.out.println(Expression is false); }

Bitwise, Shift, Ternary Operators Bitwise & shift operators manipulate individual bits in integral primitive types. The ternary if-else operator looks like this: boolean-expression ? value0 : value1 The result is either value0 or value1, depending on the truth of the boolean.

The String + Operator The + operator is overloaded for String objects; it means concatenation. It reminds me of good old C++… If an expression begins with a String, then all the following operands of + will be converted into Strings: int x = 0, y = 1, z = 2; String myString = x, y, z ; System.out.println(myString + x + y + z);

A Useful Figure byteshortintlong doublefloat char When multiple types are mixed in an expression, compiler will convert the operands into the higher type: double, float, long, int

Casting A cast produces a temporary new value of a designated type. Implicit and explicit casts: int i = 2; double d= i;// OK, since d can hold all of i float g = F; //! int j = g;// not OK, loses information int k = (int) g;// OK, compiler is reassured