Java Programing

PSC 120Jeff Schank

Let’s Create a Java Program

1. Open Eclipse2. Create a project: File -> New -> Java Project3. Create a package: File -> New -> Package4. Create a Class: File -> New -> Class

How to say “Hello World!”

Let’s Add Some Numbers

Let’s Format the Results

Classes

• Let’s create another class called “Agent”• File -> New -> Class

Data

• Now, let’s add some data—in this case a vocabulary

Methods

• Now, let’s add a method

Let’s Say Something

Let’s Say Something Randomly

Variables and Their Types• As we just saw, we define the objects that will interact in our simulation

by defining classes • Once a class is completely defined, then it can be instantiated many

times– For example, we could define a class called “Person” and then make 1000

persons that interact in our simulation.

• Classes have members that occupy fields in a class• A class can have indefinitely many fields and a field is either occupied by

variables or methods • When defining classes, I prefer to place the variables first and methods

second in a class, but Java does not care how they are ordered • Let’s look at some of the types of variables we can define in a class.

Example MyClass

Access Modifiers

• Variables (and methods) have specifications for how they are accessed

• There are four types of access modifiers: no explicit modifier, public, private, and protected.– public modifier—the field is accessible from all classes.– private modifier—the field is accessible only within its own

class.– protected modifier—the field is accessible within its own

class, package, and subclass.– no explicit modifier—the field is accessible within its own

class and package

Methods• Methods specify how objects do things (how they behave)• Methods also specify how objects interact with other objects• Methods have at least five features:

1. Modifiers—such as public, private, and others listed above.2. The return type—the data type of the value returned by the method,

or void if the method does not return a value.3. The method name—the rules for field names apply to method names

as well, but the convention is a little different.4. The parameter list in parenthesis—a comma-delimited list of input

parameters, preceded by their data types, enclosed by parentheses, (). If there are no parameters, you must use empty parentheses.

5. The method body, enclosed between braces—the method’s code, including the declaration of local variables, goes here.

Example Method

1. Modifier

2. Return Type

3. Method Name

4. Parameter List

5. Body

Example Method

1. Modifier

2. Return Type

3. Method Name

4. Parameter List

5. Body

Example Method

1. Modifier

2. Return Type

3. Method Name

4. Parameter List

5. Body

Logical Operators

1. && means roughly “and”2. || means roughly “or”3. == means roughly “equals”4. ! means roughly “not”5. != means roughly “not equal to”6. > means “greater than”7. >= means “greater than or equal to”8. < means "less than”9. <= means "less than or equal to"

&& and ||

! and !=

Arithmetic Operators

1. + Additive operator but it is also used for String concatenation.

2. – Subtraction operator3. * Multiplication operator4. / Division operator

Examples: +

If-then Statement

Body

Conditions

If-then Example

For Statements

• Probably, the next most commonly used control statement is the for statement.

• For control statements are one of several control statements that allow you to perform a number of operations over and over again for a specified number of steps (the others are while and do-while).

• For statements typically have three statements as arguments and then a body that is repeated (there are variations on this theme).

A common form

Modifier Arguments

Body

Example

Another Example

The maximum value for an integer is 2147483647But, since it does not stop at this value, it would generate an error.

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Scope of a Variable

• The scope of a variable is the region of a program within which, a variable can be referenced.

• In Java, the largest scope a variable can have is at the level of the class.

• So, if variables are declared in a class field, they can be referenced anywhere in the class including inside methods.

Examples

Examples

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