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Your First Java Application

Chapter 2. Your First Java Application. Program Concepts. Modern object-oriented programs help us build models to manage the complexity found in a problem domain. The problem domain describes real-world objects and concepts that a computer program is trying to solve. Models.

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Your First Java Application

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  1. Chapter 2 Your First Java Application

  2. Program Concepts • Modern object-oriented programshelp us build models to manage the complexity found in a problem domain. • The problem domain describes real-world objects and concepts that a computer program is trying to solve.

  3. Models • A model is a simplification of a complex system. A good model: • Helps to identify the most important aspects of a problem. • Helps a programmer to focus on the problem he or she is trying to solve instead of on the complexity of the problem’s data.

  4. What is a metaphor? • A metaphor is a word or phrase used in place of another word or phrase to denote a likeness. • Computer programs use “metaphors” to represent real objects.

  5. Objects • Objects represent real-world things: • Ship • Rudder • Wheel • Instruments • Objects have: • properties (characteristics) • methods (behaviors)

  6. Classes and Objects • A class is a definition of a type: • Like a template, a class defines the characteristics and behaviors of the type. • An object is an instance of a class: • Can be instantiated and manipulated • An object’s characteristics are defined by the class that was used to create the object.

  7. Fields • Fields define the properties of a class. • Can be intrinsic types (int, boolean…) • Can be user-defined objects • State is the current value of a field in an object.

  8. Methods • Methods describe the capabilities of the class. • Every method must be called from another method. The only exception is main(), which is called by the OS. • Methods can accept parameters.

  9. Object Relationships • Association • A method of one object calls the method of another object. • Composition • Some objects are composed of other objects.

  10. Specialization • Hierarchies of classes move from a generalized class to a more specific class.

  11. Creating Programs • Syntax and Semantics • A language’s exact keywords, punctuation, and order of terms are called its syntax. • Semantics refers to the meaning of one’s instructions; what the program is trying to accomplish.

  12. Writing a Java Program • Writing a Java program requires five steps: • Analyze • Design • Write • Compile • Test

  13. Compiling the Code • Programs must be compiled in order for the CPU to understand and execute the instructions. • Java byte code is run through the Java Virtual Machine (JVM), which translates the code for the CPU. • .java files are compiled by the javac (java compiler) program into .class files.

  14. Types of Java Programs • Applet • Requires an .HTML file with the <applet> tag to reference the .class file • Can also be run using the appletviewer.exe program file • Application • Run by the java.exe program file • Run without a browser interface, often using the console window

  15. HelloWorld.java Analysis • The keyword import allows existing class libraries to be reused. • The keyword public signifies that a class is visible to other classes. • Braces {} indicate the beginning and end of a block of code. • Braces must always be “balanced” • Align opening and closing braces

  16. More HelloWorld.javaAnalysis • The keyword void signifies that a method does not return a value. • Identifiers are the names given to classes, fields, and methods. • Identifiers are case sensitive. • Identifiers often use camel case notation.

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