Chapter 4 Topics: class declarations method declarations encapsulation instance variables method declarations return types parameters method overloading

The Person class, version 1 public class Person { // state // behavior public void talk() { System.out.println("blah, blah, blah, ...\n"); } public void study (String course) { System.out.println("study, study, study, ..."); System.out.println("I just love studing " + course + "!\n"); public boolean likes (String firstName) { if ( firstName.charAt( firstName.length()-1 ) == 'y' ) return true; else return false;

The Driver class, version 1 public class Driver { public static void main(String [] args) { Person student1 = new Person(); student1.talk(); student1.study("CSCI 201"); if ( student1.likes("Tommy") ) System.out.println("Student #1 likes Tommy"); else System.out.println("Student #1 does not like Tommy"); }

Person class, version 2---adding state public class Person { private int age; private String name; … public String getName() { return name; } public void setName( String newName ) { name = newName; public int getAge() { return age; public void setAge( int newAge ) { age = newAge;

Driver class, version 2 public class Driver { public static void main(String [] args) { Person student1 = new Person(); student1.setName("Sally"); student1.setAge(21); System.out.println("Student #1's name is " + student1.getName() + "\n"); System.out.println("Student #1's age is " + student1.getAge() + "\n"); student1.talk(); student1.study("CSCI 201"); if ( student1.likes("Tommy") ) System.out.println("Student #1 likes Tommy"); else System.out.println("Student #1 does not like Tommy"); }

Person class, version 3 public class Person { private int age; private String name; Person (String startingName, int startingAge) { age = startingAge; name = startingName; } Person () { age = 18; name = "Bob"; … public int getAge() { return age; public void setAge( int newAge ) { age = newAge;

Driver class, version 3 public class Driver { public static void main(String [] args) { Person student1 = new Person(); Person student2 = new Person("Tommy", 23); System.out.println("Student #1's name is " + student1.getName() ); System.out.println("Student #1's age is " + student1.getAge() ); System.out.println("Student #2's name is " + student2.getName() ); System.out.println("Student #2's age is " + student2.getAge() ); if ( student1.likes( student2.getName() ) ) System.out.println("Student #1 likes " + student2.getName() ); else System.out.println("Student #1 does not like " + student2.getName() ); }

Formal Parameters A method has a header and a body. This is the simple version of the setAge method of the Person class: public void setAge(int newAge) { age = newAge; } This method has a parameter called newAge which is an int. When this method is called, it expects to be passed an integer value. newAge is called a formal parameter. It is a placeholder for a value that will be supplied when the method is invoked. } Method header Method body Because the method may be called many times with many different values, we can't know what the value will be. So, we give a name to the value that will come along when the method is called, and write our method code in terms of this name. Switch to BlueJ and run the setAge method of the Person class. Show that BlueJ has to prompt you for the value to be passed to the method, otherwise it couldn't know what the value was supposed to be. Ask the students what will happen if you give it something of the wrong type.

Actual Arguments The method is called by addressing a Person object and asking it to invoke this method. Person friend = new Person(); friend.setAge(20); The integer value 20 is the actual argument that is given to the method when it is invoked. int myAge = 25; friend.setAge(myAge); The setAge method sees only the value 25. It does not see the name of the variable that happens to contain the value of the actual argument. Write on the OHP a few examples of other expressions being passed as actual arguments, e.g. friend.setAge(myAge + 3); friend.setAge(brother.getAge()); friend.setAge(newAge); where newAge happens to be the same name as the name of the formal parameter in the method. Ask students what will happen if you change the name of a formal parameter. Demonstrate. Mention that it's not a good idea to name the parameter the same name as an attribute that you will be referring to in the method. If you do this, you have to go to some trouble to distinguish between them. Don't go into details on this unless somebody asks.

Memory Allocation for Parameters Memory is allocated to the formal parameter at the time the method is called. The memory location is initialised to the actual argument. The memory for the formal parameter is deallocated when the method finishes executing. We call this "going out of scope". Draw a picture on the overhead showing how the memory for the parameter comes into existence at the time the method is called, and disappears again when the method terminates. There will be more on scoping later.

Methods with Multiple Parameters Suppose we had a method that could change all the attributes of a Person object. public void setAllAttributes(String aName, int anAge ) { name = aName; age = anAge; } The call to this method would have to pass two actual arguments, e.g. friend.setAllAttributes("Jack", myAge + 1); A similar method could be used in the assignment to set all the Movie attributes, say. Point out that the values passed to the method can be the results of evaluated expressions. They don't have to be just constants or variables. Give some more examples here.

Matching Formal Parameters with Actual Arguments The actual arguments passed to a method must match the formal parameters in all of the following: number type order The compiler assumes that the first actual argument matches the first formal parameter, the second actual argument matches the second formal parameter, etc. The names of formal parameters do not have to match the names of the actual arguments. The called method does not see the names, it sees only values. Show in BlueJ some examples of calls that won't work with this method, i.e. wrong number of arguments, wrong order, wrong type. Ask students to predict what will happen. Show in BlueJ how the prompts for the setAllAttributes method are in the order written into the code, and that the names and types of the formal parameters are shown also, to make sure you get it right. Show what happens when you leave the quotes off the String for name, and explain why.

Passing Multiple Values of the Same Type public double divideNumbers(int dividend, int divisor) { if (divisor != 0) return (double)dividend / divisor; return 0; } When the method is called, e.g. System.out.println(divideNumbers(12, 3)); the first parameter is taken as the dividend, and the second as the divisor. Show in BlueJ what happens if you put the parameters in the wrong order. This method is in the TestPerson class. Explain without going into detail here that it is necessary to say you want a double answer here, otherwise you get an int, because you are dividing an int by an int. We'll talk more about casting later. FINISH THE FIRST HALF OF THE LECTURE HERE.

Returning Values from Methods A value returned from a method replaces the call to the method. Person aPerson = new Person(); System.out.println(aPerson.getAge()); System.out.println(0); Show on the overhead how this is similar to an arithmetic expression evaluation. The second example is showing how the boolean value returned by the set method should be used to find out whether the change to the attribute really took place, as this may affect what happens next in the program. This was seen in the inputAge method in the week 4 lecture.

Returning Values from Methods When a method returns a value you need to use right away, or “hold on” to it to use it later. Holding on to it: Person aperson = new Person(); String goodName = aperson.getName(); if( aperson.likes(goodName) ) … else Using it right away: if( aperson.likes( aperson.getName() ) )