AP Computer Science A Unit 2 Using Objects

Unit 2: Using Objects Objects: An object is created from a class by calling the class constructor along with the new keyword. The name of a constructor is the same as the name of the class it belongs to. If it is followed by a (possibly empty) list of values then those values are called parameters. The signature of a constructor consists of the name of the constructor along with the list of types of parameters that it expects. When calling the constructor, the parameter list provided must match the signature. Ex: The Rectangle class from the Java standard library contains, among others, the following two constructors: Rectangle(int width, int height) // first constructor and... Rectangle(int x, int y, int width, int height) // second constructor Valid constructor calls: new Rectangle(3, 6) // calls the first constructor new Rectangle(-1, 7, 3, 4) // calls the second constructor Not valid constructor calls: new Rectangle(7.45, 1) // invalid since the first parameter is double new Rectangle(1, 7, 3) // invalid since it has three parameters Creating and Storing Objects (Instantiation): Instantiation for rectangle class: Rectangle myRectangle = new Rectangle(5, 6); myRectangle is a reference variable that refers to an Rectangle object. Calling a Void Method: A method is a storage location for related program statements. When called, a method usually performs a specific task. Ex: Public class Bag { Public void sand() { System.out.println(“Hi”); } } Methods are defined inside of a class and you can define as many methods as needed. Method sand does not return a value! Once you have instantiated an object you can call all the methods contained in the class. Void methods do not return a value. Calling Non-Void Methods: When a method is not void, it has a return type, therefore the method returns a value. Ex: Public class Train { Public int rides ( int ride1, int ride 2) { return ride1 * ride2; } } Return methods are defined inside of a class in the same way you define void methods. Train comp - new Train(); int store = comp.rides(3, 6); String Objects: Concatenation, Literals, and More: A string is a group of characters. Strings are enclosed in “ “. Keep in mind even numerals if enclosed in quotes are considered strings. Strings can be combined or concatenated. The concatenation of strings is shown below: String one= "Hello"; String two= "World"; String concat = one+ " " + two; System.out.print(concat); // Prints Hello World Escape Sequences: Meaning: \” Prints “ \\ Prints \ \n New line String Method: Use: substring(x,y) Returns a section of the string from x to y not including y substring(x) returns a section of the string from x to length-1 charAt(x) Returns the char at spot x length() Returns number of chars indexOf(str) Returns location of String str in the string, searching from spot 0 to spot length-1 indexOf(ch) Returns the location of char ch in the string, searching from spot 0 to spot length-1 lastIndexOf(str) Return the location of String str in the string, searching from spot length-1 to spot 0 lastIndexOf(ch) Returns the location of char ch in the string, searching from spot length-1 to spot 0 equals(s) Checks if this string has same chars as s compareTo(s) Compares this string and s for >,<, and == String Indexes: 0 1 L e 2 m 3 4 o n L is at index or spot 0… etc. Length of this string is 5. Wrapper Classes: Integer and Double: Sometimes it is more convenient to work with objects rather than primitive values because of this, Java provides the wrapper classes Integer and Double. Each of these classes has a constructor that accepts a primitive value of the appropriate type, and a method that returns the primitive value. The Integer class also provides static fields that represent the maximum and minimum values that can be represented by an int. Integer: Double: Integer(int value) Double(double value) Constructs an Integer object with the specified Constructs a Double object with the specified value value int intValue() Returns the stored int value Integer.MAX_VALUE The maximum value that can be represented by an int Integer.MIN_VALUE The minimum value that can be represented by an int. double doubleValue() Returns the stored double value The Math Class: Math class only contains static methods. Method: Description: int abs(int x) Returns the absolute value of x double abs(double x) Returns the absolute value of x double pow(double b, double e) Returns b^e double sqrt(double x) Returns the square root of x double random() Returns a value in the interval [0,1)