CS100Lecture 61 Announcements Homework P1 due on Thursday Homework P2 handed out

CS100Lecture 62 Today’s Topics Review of Friday Review of topics covered so far Scope of variables Introduction to while

CS100Lecture 63 Review of Friday Inheritance: subclass vs. superclass The method toString() Overriding methods super() public vs. private

CS100Lecture 64 Confusion on Strings String s = new String("Millett"); String t = new String("Millett"); if (s == t) System.out.println("s and t are equal!"); String r = "Millett"; String q = "Millett"; if (r == q) System.out.println("r and q are equal!"); if (s == r) System.out.println("s and r are equal!");

CS100Lecture 65 Today: Definitions of basic entities that make up a Java program Syntax of these entities Memorize these Be able to come up with definitions and examples on a test

CS100Lecture 66 Class A “model” or blueprint for the objects (or instances) of the class; a class defines the fields (or variables) and methods of each object of the class. Analogy: a blueprint for a house is a design for a house, many houses (objects) can be built from the same blueprint, but they may differ in color of rooms, wallpaper, etc. Java syntax: public class {declaration of fields and methods } Example: public class C {int x; String s; public C(String sp); {x= 0; s= sp;} public void addToX(int y) {x= x+y;} }

CS100Lecture 67 Variable A named box that can contain the value of some type or class. For a type like int, the value is an integer. For a class, it is the name of (or reference to) an instance of the class Declaration: a definition of the name of the variable and the type or class of value it can containt Syntax: ; Examples: // a variable x that can contain an integer int x; // variable s that can contain the name of an object of class String String s; // a variable that can contain a boolean value (true or false) boolean b;

CS100Lecture 68 Method A parameterized sequence of statements, whose execution performs some task. In Java, there are three kinds of methods: –procedures, –functions, –constructors. The next few slides describe them

CS100Lecture 69 Notes on methods A method should be accompanied by a comment that says what the method does. This is the specification of the method. The comment has to be precise and clear. A potential user of the method should be able to look only at the comment and the list of parameters to know how to use it; they should not have to look at the list of instructions. Example: When you want to bake a cake, you look at the title of a recipe, a short description, and the list of ingredients to determine whether you want to use that recipe --not the list of instructions to bake it.

CS100Lecture 610 Procedure A method that performs some task and doesn’t return a value Java syntax: // comment that explains what the procedure does public void ( ) { sequence of statements to execute } Example: // Raise the salary by n dollars if the salary is <$20000 public void raiseSal(double n); {if (salary < 20000) salary= salary + n; } Example procedure call: raiseSal(20 * y);

CS100Lecture 611 Function A method that performs some task and returns a value. Instead of the keyword void, the type or class of the return value is used. Statement return is used to terminate execution of a function call and return. Syntax: // Comment that explains what the function does. It should include // something like “Yield …” to describe what the function returns. public ( ) {Sequence of statements to execute} Example: // Yield the maximum of x and y public int max (int x, int y); {if (x>= y) return x; return y; } Example function call (within some statement) z = 1 + max(x, y);

CS100Lecture 612 Constructor A method that initializes the fields of an instance of a class when the instance is created. Syntax: // Comment that explains what the constructor does. public ( ) {Sequence of statements to execute} Example (within class Employee): // Constructor: an Employee with name n,salary 0, and year hired d public Employee Employee (String n, int d); {name= n; salary= 0; yearHired= d; } Example use (when creating a new instance): d = new Employee(“Millett”, 1999);

CS100Lecture 613 Execution of Assignment Execution of an assignment statement stores a value in a variable Syntax: = ; Examples: b = 2 + c; s = “Millett” + “ ” + yearHired;

CS100Lecture 614 Block A block is used to unify a sequence of statements into a single statement. Syntax: { sequence of statements } Example: Here is a sequence of statements: a = 10; if (a < c) then a = c; Here is a single statement which is a block: { a = 10; if (a < c) then a = c; }

CS100Lecture 615 Conditional Statement Execution of a conditional statement allows a choice of execution Syntax: if ( ) or if ( ) else

CS100Lecture 616 Subclass A subclass B (say) is a class that extends another class A (say). This means that an instance of B has all the fields and methods that an instance of A has, in addition to the ones declared in B. Syntax: public class extends { declarations of fields and methods } Example: public class VIP extends Employee {private double bonus; // Constructor: an VIP with name n, salary s, bonus b, and year hired d public VIP (String n, double s, double b, int d) {super(n,d); salary= s; bonus= b;} } Note the use of super to call a constructor of the superclass Employee

CS100Lecture 617 Access modifiers Suppose d is an instance of Employee, where class Employee is declared as: public class Employee { int x; … } If the is: –public, then field d.x can be referenced anywhere that d can be referenced. –private, then field d.x can be referenced anywhere within class Employee that d can be referenced –protected, then field d.x can be referenced anywhere within the package in which Employee is declared (packages are discussed later).

CS100Lecture 618 Scope of Variables public class Class1 {public int x; public int y; // Constructor: … public Class1 (int z) {z= z; y= 2*z;} // Set y to the maximum of p and -p public void sety(int p) { int x; x= p; if (p < -p) x= -p; y= x;}} A field of Class1 parameters A local variable of method sety

CS100Lecture 619 More on Scope The scope of a variable is the set of places in which it can be referenced. A variable can be declared only once within a method. We call this a local variable. The scope of a local variable of a method is the sequence of statements following it Example: // Set y to the maximum of p and -p public void sety(int p) {int x; x= p; if (p > -p) x= -p; y= p; } Scope of x

CS100Lecture 620 Another example //... public sety(int p) {if (y= p); {int x; x= p; if (p > -p) x= -p; } y= p; y= -y; } Scope of x x cannot be referenced here

CS100Lecture 621 Scope of parameters The scope of a parameter of a method is the method body. A parameter may not be redeclared within the body. //... public test(int p) {if (y= p); {int x; x= p; if (p > -p) x= -p; } y= p; } Scope of p

CS100Lecture 622 Scope of fields The scope of a field of a class consists of: (1) the bodies of all methods declared in the class and (2) all declarations of fields that follow the declaration of the field. public class Text {int x= 5; int y= x+15; public void test(int p) {if (x= p); {int x= 35; x= p; if (p > -p) x= -p;} x= p; }

CS100Lecture 623 The while loop -- briefly Computer programs are very good at repetition (and don’t get bored) Sometimes it is necessary to repeat a statement or block many times One way to do this is with a while loop A while statement controls how many times another statement/block is executed

CS100Lecture 624 While syntax while ( condition ) while(B) statement;S; Similar to if, but at the end of the statement, the condition is checked again Evaluate B Execute S true false

CS100Lecture 625 Example // use while to print first 10 powers of 2 final static int LIMIT = 10; int count = 1, power_of_two = 1; while (count <= LIMIT) { power_of_two = power_of_two * 2; System.out.println(power_of_two); count = count + 1; }