Constants Variables change, constants don't final = ; final double PI = ; … area = radius * radius * PI; see Liang, p. 32 for full code

Representing numbers Given 3 bits, what numbers can you represent? unsigned: Range 0… (= 0…7) signed: -2 2 … At base, everything in a computer is stored by bits of memory, which can be either on or off (1 or 0). 2 2 * * *1= = * * *0= = * * *1= = 5

Numerical data types: integers Name Range Size int … bits (signed) byte -2 7 … bits (signed) short … bits (signed) We can write big numbers using scientific notation: byte variables can fit numbers from –128 to 127 short variables can fit numbers from –32768 to int variables can fit numbers from – to long … bits (signed) ~ 2.15e+9 ~ 2.15*10 9 long variables can fit ~2.15e+9 times as many numbers as ints

Numerical data types: reals float and double are used to store real numbers: numbers with a (possibly unending!) sequence of decimals; for example, pi: double -1.7e308 … 1.7e (14…15 digits acc.) float -3.4e38 … 3.4e38 32 (6..7 digits accuracy) … float and double can only store a certain number of decimal digits: they can’t store them all! float pi = ; \\ the last 3 is a rounding up double pi = \\the 2 is rounding down

What do these mean? int … bits (signed) short … bits (signed) 2 15 = ( 2x2x2x2x2x2x2x2x2x2x2x2x2x2x2 ) = 32,768 How much bigger is 2 31 than 2 15 ?2 16 times bigger 2 31 = 2,147,483,648 float -3.4e38 … 3.4e38 32 bits (signed) 3.4e38 means 3.4 x (scientific notation) To compare, 2,147,483,648 ~=2.1 x 10 9

Numerical Operators +,-,/,*,% int i1 = ; // i1 becomes 35 double d1 = ; // d1 becomes 33.9 double d2 = 1.0/2.0; // d1 becomes 0.5 int i2 = 1/2; // i1 is 0!!!!! byte i3 = 1/2; // same for all integer types byte i4 = 20%3; // modulus operator. i4 is 2

Literals A basic data type value which appears directly is a literal int i = 34; // 34 is a literal. double d = 5.0; // 5.0 is a literal, /* floating point values are taken to be doubles until proven otherwise*/ float myFloat = 5.0f; // f means make this a float float myDouble = 5.0d; // d means make it a double!!

Shortcut Operators Some operations are so common, we have a shorthand. The following are equivalent: i = i + 1;i += 1; d = d - 1.0;d -= 1.0; f /= 2.0;f = f / 2.0; i++; d--;

A useful string operator When applied to two numbers, the + operator adds those numbers When applied to a string and anything else, the + operator joins the other thing on to the end of that string. String myName = “fintan”; System.out.print(“Welcome ”); System.out.print(myName); System.out.println(“ to Java”); Can be replaced by System.out.println(“Welcome “ + myName + “ to Java”);

Type conversion Number range hierarchy: double > float > int Assigning smaller range variable to larger range variable is ok: int i = 5; float f = i; // ok double d = f; // also ok

Type conversion 2 To assign a larger range value to a smaller variable, use a type cast. This tells the computer to “squeeze” the value into the variable. Caution is always required! You may lose information because you may be putting a big number into a too-small box. float fl = (float)10.1; // double to float. ok. int i = (int)fl; // i now has the value 10! int i2 = 1000; // ok byte b = (byte)i2; // very bad idea!!!!

Casting to integers..... REMEMBER! = loses all information after the decimal point. int i = (int)4.1; // i is now 4 double d = -2.5; i = (int)d; // i is now -2

Character data type type char holds a single character: char c1 = 'A'; char c2 = '7'; char c3 = '\u0041'; // unicode for A Characters map onto numbers: int code = (int)'A'; Write a program to investigate characters and integers....

Special characters char c1 = '\n'; // new line char c2 = '\t'; // tab stop char c3 = '\"'; // double quote More often, we use strings: String s1 = "Hello world"; String s2 = "I said \"Hello world\"\n"; More on Strings later

The boolean data type A variable of type int can have 2 32 different values A variable of type boolean can have 2 different values: boolean b = true; b = false; More on booleans when we consider "if-statements"

Homework Read Liang, p Try exercises 2.2, 2.3, 2.7, 2.9, You will be able to work out the answers for these questions by writing them into Java programs and trying them out.