Java Fundamentals: Understanding Primitive Types and Reference Types, Assignments of Computer Science

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CS162: Introduction to Computer

Science II

Java Fundamentals

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Primitive Types

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Primitive types

• Primitive types:

bytebyte , short, int, long,short int long

float, double, char, boolean

• Example:

int size = 42;

• size is a primitive variable, i.e., a variable that

t i d t l f th d l d t

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contains a data value of the declared type.

The eight primitive types in Java

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Result of logical operators

AND OR NOT

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DeMorgan’s Laws

!(x && y) == !x || !y

!(x || y)y == !x && !yy

Short-circuit evaluation

int x = 3;

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t 3;

int y = 5;

if ((x == y) && (y > 0)) ...

if ((x < y) || (y < 0)) ...

Short-circuit evaluation

int x = 3;

i tint y = 5 5;

int z = x / y;

if ((z != 0) && ((y / z) < 5))

// OK

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if (((y / z) < 5) && (z != 0))

// crash if DivideByZeroException

// is not handled

Reference types

• All types that are not primitive are reference

or class typesor class types

• Example:

String greeting = "Howdy";

• greeting is a reference variable, i.e., a

variable that contain a reference to (address of)

th l ti f th d t

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the memory location of the data.

Classes

• A class defines the characteristics for all objects

of its typeof its type

• A class gives a general description of

• what an object of the type is (instance data)
• what an object of the type can do (methods)

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Objects / Classes

• An object is an instance of a class.

• A program may have many instancesA program ma ha e man instances

(objects) of one class.

• All instances of the same class have

• The same kinds of data
• The same methods

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The same methods

An Example of a Class

etc. 15

Three instances of the Automobile

class

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Instantiation

bobsCar “Sedan”

A memory cell that contains the memory address of the data for

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bobsCar

Aliases

• It is possible for two variables to reference the same

object. They are aliases

• Example:Example: Automobile myCar = bobsCar;

bobsCar “Sedan”

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myCar 0.

If an object has two (or more) aliases, the object may be referenced and/or modified through any of those aliases.

Invoking methods in a program

• Instead of calling a function to “do something

to” an object (imperative programming), tell

the object to perform one of its actions (object-

based programming).

• Examples:

bobsCar.accelerate(5); int currentSpeed = bobsCar.getSpeed();

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int currentSpeed bobsCar.getSpeed(); if (bobsCar.getFuelLevel() < 0.1) bobsCar.decelerate(10);

Invoking methods in a program

• Valued methods return a single value, and

should be used in an expression.

• Return type of method must be assignment-– Return type of method must be assignment-

compatible in the context of the call

• void methods do not return a value, and

should be used as a single statement.

E l

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• Examples:

bobsCar.accelerate(5); int currentSpeed = bobsCar.getSpeed(); if (bobsCar.getFuelLevel() < 0.1) bobsCar.decelerate(10);

Before calling matchSpeed

bobsCar “Sedan”

2000

suesCar “SUV” 2001

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Call matchSpeed

bobsCar.matchSpeed(suesCar);

bobsCar “Sedan”

2000

suesCar “SUV” 2001

suesCar is associated withother

other is an alias ofsuesCar.

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other 0. 35 9864

After executing

other.accelerate(diff);

bobsCar “Sedan”

2000

suesCar “SUV” 2001

suesCar.speed is changed

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other 0. 55 9864

After matchSpeed terminates

bobsCar “Sedan”

2000

suesCar “SUV” 2001

other no longer exists

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other 0. 55 9864

Before calling matchSpeed *

bobsCar “Sedan”

2000

suesCar “SUV” 2001

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Call matchSpeed *

bobsCar.matchSpeed(suesCar);

suesCar is associated withother

bobsCar “Sedan”

2000

suesCar “SUV” 2001

other is a parameter, an alias ofsuesCar

otherCar is a local variable with the same instance values

asother

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other 0. 35 9864

After executing the statement

other = new Automobile( … );

other refers to a new

suesCar “SUV” 2001

bobsCar “Sedan” (^) otherCar “SUV”

other refers to a new

memory location

other 0. 35 9864

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bobsCar Sedan 2000

55 21405

otherCar SUV 2001

55 9864

After matchSpeed * terminates

suesCar “SUV” 2001

other no longer exists

otherCar and the object it

f d l i t

bobsCar “Sedan” otherCar “SUV”

other 0. 35 9864

referenced no longer exist

suesCar.speed is unchanged

Memory allocated by new is garbage

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otherCar SUV 2001

55 9864

bobsCar Sedan 2000

55 21405

Wrapper Classes

To treat primitive types as objects, you must use

wrapper classes

Primitive Class Wrapper Class byte Byte boolean Boolean char Character double Double

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float Float int Integer long Long short Short

Wrapper Class Example

int size = 42; Integer sizeWrapper = new Integer(size); String sizeString = sizeWrapper.toString();

Integer

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Integer size = 42 value = 42

Wrapper Class Functions

There are lots of other functions you can call on a

wrapper class:pp

• doubleValue() – returns the value as a double
• intValue() – returns the value as an integer

To see the complete list, check out the Java 5.0 API

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Auto-boxing

• In Java 5.0, conversion between primitive

types and the corresponding wrapper classesyp p g pp

is automatic

• This is called auto-boxing

Integer i = 42; // auto-boxing // same as Integer i = new Integer(42)

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int x = i; // auto-unboxing // same as int x = i.intValue();