![Variables in a program
public class Variables {
public static void main(String[] args) {
int num = 2 + 30;
int bottles = 10;
int a = 5;
System.out.println( a );
num = bottles + a;
System.out.println( num );
int variableWithALongName = 789;
int another_long_name = bottles * 2;
System.out.println( " The value of another_long_name is "
+ another_long_name );
}
}
Variables.java](https://image.slidesharecdn.com/pij1-210712003531/85/Pi-j1-2-variable-assignment-8-320.jpg)
![Primitive types
Category Types Size (bits) Precision Example
Integer
byte 8 From +127 to -128 byte b = 65;
char 16 All Unicode characters[1]
char c = 'A’;
char c = 65;
short 16 From +32,767 to -32,768 short s = 65;
int 32 From +2,147,483,647 to -2,147,483,648 int i = 65;
long 64 From +9,223,372,036,854,775,807 to -9,223,372,036,854,775,808 long l = 65L;
Floating-
point
float 32 From 3.402,823,5 E+38 to 1.4 E-45 float f = 65f;
double 64 From 1.797,693,134,862,315,7 E+308 to 4.9 E-324 double d = 65.55;
Other
boolean -- false, true boolean b = true;
void -- -- --](https://image.slidesharecdn.com/pij1-210712003531/85/Pi-j1-2-variable-assignment-11-320.jpg)
Pi j1.2 variable-assignment
- 1. Programming in Java 5-day workshop Variables and assignment Matt Collison JP Morgan Chase 2021 PiJ1.2: Java Basics
- 2. Session overview Primitive types in Java • General purpose • High-level compiled • Strongly typed object-oriented Who uses Java? • popularity TIOBE index • Java history Getting started with Java? • Java download and versions • The javac Java compiler • A first Java program - Hello world
- 3. Lecture overview • Variables • Statements and assignment • Built-in types • Identifiers, namespaces, Naming conventions • What not to do • What to do •Command-line outputs and inputs
- 4. Statements and variables • Each complete instruction is known as a statement • There are multi-line statements and statements over multiple lines later • Statements conclude with a semi-colon int a = 5; System.out.println(a);
- 5. Assignment statements • Each complete instruction is known as a statement • In Java variables are must be created before you assign a value • In an assignment statement the expression on the right hand side of the equals sign is evaluated and assigned to the variable on the left hand side: int a = 5; int newVariable = a – 1 <var> = <expression>
- 6. Assignment, not equality • Note: The equals sign indicates assignment, not equality. Some languages use a different symbol, such as • Or • But even with a plain ‘=‘ there should be no suggestion that the statement can be solved. This rapidly leads to nonsense: • . int bottles <- 10 // Not Java int bottles <- bottles – 1 // Not Java int bottles := 10 // Not Python int bottles := bottles – 1 // Not Python bottles = bottles -1 // Not an equation 0 = -1 // No
- 7. Assignment statements • The right hand side of the assignment statement can be either: • Literal – a quantity that needs no evaluation • Expression – the computed product of an operation int a = 5 int z = 792 String greeting = “Hello” int a = 5 + 6 int z = a * 12 String question = greeting + “Who are you?” Note the speech marks Note the type
- 8. Variables in a program public class Variables { public static void main(String[] args) { int num = 2 + 30; int bottles = 10; int a = 5; System.out.println( a ); num = bottles + a; System.out.println( num ); int variableWithALongName = 789; int another_long_name = bottles * 2; System.out.println( " The value of another_long_name is " + another_long_name ); } } Variables.java
- 9. Variables in a program $ javac Variables.java $java Variables 5 15 The value of another_long_name is 20 • ‘5’ comes from System.out.println(a) • ‘15’ comes from System.out.println(b) • ‘The value of another_long_name is 20’ comes from the final print statement • The only terminal output is from the print call, but all the statements are executed Run the program Variables.java from the command line
- 10. Primitive types
- 11. Primitive types Category Types Size (bits) Precision Example Integer byte 8 From +127 to -128 byte b = 65; char 16 All Unicode characters[1] char c = 'A’; char c = 65; short 16 From +32,767 to -32,768 short s = 65; int 32 From +2,147,483,647 to -2,147,483,648 int i = 65; long 64 From +9,223,372,036,854,775,807 to -9,223,372,036,854,775,808 long l = 65L; Floating- point float 32 From 3.402,823,5 E+38 to 1.4 E-45 float f = 65f; double 64 From 1.797,693,134,862,315,7 E+308 to 4.9 E-324 double d = 65.55; Other boolean -- false, true boolean b = true; void -- -- --
- 12. Integer literals • Integer literals (byte, short, int, long) • Allowed: • decimal: 197 -56 1234_5678 (Java 8 onwards) • octal: 036 • hexadecimal: 0x1a 0X3E1 • binary: 0b11010 (Java 7 onwards) • Not allowed: • 137,879 • Be careful: 0457 (allowed, but its literal decimal value is 111) • The suffix L or l is used to indicate it is of type long (e.g., 234L, OX3E3l). The uppercase L is preferred.
- 13. char literals char literals • char type is a numeric type, which means the arithmetic operations are allowed. • Allowed: ’B’, ’.’, ’u5469’, • Special characters: ’n’, ’t’, ’’, ’”, ’”’
- 14. Floating point literals • Floating-point literals (float, double) • Allowed: 6.67 0.1e+3f 7e5 4.5E-19 1E-9f 1e4D • Allowed but not recommended: 3. .76 float height = 0.158; //error: incompatible types float height = 0.158f; float height = 1.58E-1f;
- 15. • int a=012; • int b=23,120; • int c=23 120; • byte d=130; • long e=23120l; • double f=1e-2; • float g = 1.02; • float g = 1.02f; • char h = "B"; • char h = ’B’; • char i = ’u0034’; • boolean j = TRUE; • boolean j = "true"; • boolean j = true; //correct, but ’a’ equals to 10 //error //correct //error: incompatible types //correct, ’L’ is recommended //correct //error: incompatible types //correct //error: incompatible types //correct //correct //error //error //correct
- 16. Constants A constant refers to a data item which cannot be changed. • The keyword final must be used in the declaration of a constant. • E.g., final String MY_MOTTO = "I was struggling to begin, but in the end I overcome and nail it!"; Naming conventions: • Variables: mixed case, lower case first letter • E.g., age, buttonColor, dateDue • Constants: all upper case, words separated by underscore • E.g., PI, MIN VALUE, MAX VALUE
- 17. Challenge • Exercise: Write a program (CircleComputation.java) which prints on the command-line the area and circumference of a circle, given its radius (1.5). Hint: final double PI = 3.14159265;
- 18. Casting primitive types • It is possible to convert one numeral type to another numeral type, refers to casting (or: primitive conversion). • Widening casting (Implicit done) • short a=20; • long b=a; • int c=20; • float d=c; • Narrowing casting (Explicitly done) • long a = 20; • short b = (short) a; • float c = 20; • int d = (int) c; • float hight = (float) 0.158;
- 19. Overflow on casting (be careful) • Overflow: Operations with integer types may produce numbers which are too big to be stored in the primitive types you have allocated. byte d=130; //error: incompatible types byte d= (byte)130; //no error, but d’s value is -126, because of overflow • NOTE: No error message for overflow will be raised, you’ll simply get an unexpected number.
- 20. Learning resources The workshop homepage https://mcollison.github.io/JPMC-java-intro-2021/ The course materials https://mcollison.github.io/java-programming-foundations/ • Session worksheets – updated each week
- 21. Additional resources • Think Java: How to think like a computer scientist • Allen B Downey (O’Reilly Press) • Available under Creative Commons license • https://greenteapress.com/wp/think-java-2e/ • Oracle central Java Documentation – https://docs.oracle.com/javase/8/docs/api/ • Other sources: • W3Schools Java - https://www.w3schools.com/java/ • stack overflow - https://stackoverflow.com/ • Coding bat - https://codingbat.com/java
Editor's Notes
- #7: Computational operation not a formal assertion
- #9: Expressions in a file 5 15 The value of another_long_name is 20
- #11: Primitive types are the building blocks of a language. They are built-in types that enable storage of pure simple values.
- #12: Why does a byte go from -128 to 127? Count from zero.
- #13: 1234_5678 easier to read Octal base 8 numeral system
- #14: Unicode encoding \ escape character
- #21: All resources hang from the ELE pages. How many of you have looked through them?