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Objects ! Java Programming ! ! Example: “Real-world” objects ! ! ! Suppose we want to describe a car in terms of its attributes and functionality Attributes: – tires, steering wheel, doors, engine – model, make, year, color – attributes can have constant variables • all cars have a steering wheel Actions: – brake, stop, drive, reverse, park Java classes ! ! ! ! ! ! Java String class ! ! ! Java is an object-oriented programming language – use objects to define both the data type and the operations that can be applied to the data Objects have attributes and functionality – attributes describe the state of the object – the functionality of an object is the set of actions the object can perform In Java, we define an object’s attributes using variables and its functionality using methods Java objects are created using classes Encapsulation – combining elements to create a new entity A class encapsulates the variables and methods that define an object Instantiation – the act of creating an object – objects are called class instances Java provides many predefined classes You can (and will!) define your own classes Instantiation The String class represents character strings String firstname = “Tammy”; String lastname = “Bailey”; ! ! Creating an object is called instantiation – the new operator is used with class name Example: Create a TextField object Strings are basically an array of characters Strings can be concatenated (added together) using the concatenation operator + String fullname = firstname + “ ” + lastname; ! Can create multiple instances of the same class TextField t = new TextField(); ! TextField t1 = new TextField(); TextField t2 = new TextField(); Exception: The new operator is not required when creating a String 1 Java TextField class ! ! Invoking an object’s methods The TextField class allows the editing and display of a single line of text ! TextField t = new TextField(); ! Methods – setText(String s) • set the text of the field to the string s – String getText() • get the text of the field and assign it to a variable of type string TextField t = new TextField(); t.setText(“Hello”); ! ! ! ! User interaction determines the behavior of the program Program receives user input through mouse and keyboard and performs associated method or action Text fields – edit and display single line of text Buttons – can specify action to occur when button is clicked Action listeners ! ! We would like our applet to do the following: – get text from text field t1 and display it in text field t2 when button b is clicked TextField t1 = new TextField(); TextField t2 = new TextField(); Button b = new Button(“copy text”); b.addActionListener(this); If we want a button to know when it is clicked, we have to enable it to “listen” for user input This is done using the button method addActionListener ! Button b = new Button(“click!”); b.addActionListener(this); ! If we don’t invoke the addActionListener method on a button, nothing will happen when the button is clicked Example ! Syntax – object.verb( data ); – Perform verb on object using data Interactive objects ! Once we create a text field, we can perform actions on it using its methods The variables and methods of an object are accessed using the dot operator Actions ! We specify actions to occur when a button is clicked in the actionPerformed method public void actionPerformed(ActionEvent event) { Object cause = event.getSource(); if(cause == b) { t2.setText(t1.getText()); } } 2 Numeric input ! ! Type wrappers Suppose we want an applet that allows the user to enter two integers and display the minimum – A text field contains a string – If we want to perform numeric operations on the input from a text field, we have to convert the string to a numeric data type – numbers are primitive data types, not objects Can convert using Java type wrappers ! String conversion ! ! ! ! ! ! Convert String str to byte b byte b = Byte.parseByte(str); Convert String str to short s short s = Short.parseShort(str); Convert String str to int i int i = Integer.parseInt(str); Convert String str to long l long l = Long.parseLong(str); Convert String str to float f float f = Float.parseFloat(str); Convert String str to double d double d = Double.parseDouble(str); Convert primitive types into objects Primitive Type byte Wrapper Type Byte short int Short Integer long float double Long Float Double char boolean Character Boolean Action performed for integer input public void actionPerformed(ActionEvent event) { Object cause = event.getSource(); int x = Integer.parseInt(t1.getText()); int y = Integer.parseInt(t2.getText()); if(cause == b) { int min = minimum(x,y); t3.setText(“The minimum is: ” + min); } } 3