SE-1010 Dr. Mark L. Hornick 1 Defining Your Own Classes Part 3

SE-1010 Dr. Mark L. Hornick 2 A local variable is a variable that is declared within a method declaration Local variables are accessible only from the method in which they are declared. In this example, output and message are local variables accessible only from within printSomeMessage public void printSomeMessage( String message ) { // textMessage is a local String variable String output = “The message is “ + message ; System.out.println(output ); }

SE-1010 Dr. Mark L. Hornick 3 In this example, output is a local variable public class Student { private String firstName; … public String toString(){ String output; output = “My name is ” + firstName; return output; } } Local variables are only visible to the method in which they are defined.

SE-1010 Dr. Mark L. Hornick 4 In this example, x, y, and z are all local variables public int add( int x, int y ){ int z = x + y; return z; } Local variables are only visible to the method in which they are defined.

SE-1010 Dr. Mark L. Hornick 5 Rules for Local Variables A local variable is a variable that is declared within a method declaration. The parameters of a method are also considered local variables to the method. Local variables are accessible only from the method in which they are declared. Memory space for local variables is allocated only during the execution of the method. When the method execution completes, memory space will be cleared.

SE-1010 Dr. Mark L. Hornick 6 A local variable with the same name as a class attribute will hide the attribute within the scope of the local variable public class Student { private String someString; // attribute … public void method1(){ String someString; // local var hides attribute someString = “hello”; // uses local variable } public void method2(){ someString = “se1010”; // uses attribute } Local variables are only visible to the method in which they are defined.

SE-1010 Dr. Mark L. Hornick 7 The this keyword is used to distinguish an object’s attribute value from a local variable in the case of ambiguity public class Student { private String someString; // attribute … public void method1(){ String someString; // local var hides attribute someString = “hello”; // uses local variable this.someString = “there” // uses attribute for “this” object } public void method2(){ someString = “se1010”; // uses attribute } The best approach, however, is to avoid local variables that have the same name as attributes.

SE-1010 Dr. Mark L. Hornick 8 A local variable is a variable that is declared within a method declaration Local variables are accessible only from the method in which they are declared. In this example, output and message are local variables accessible only from within printSomeMessage public void printSomeMessage( String message ) { // textMessage is a local String variable String output = “The message is “ + message ; System.out.println(output ); }

SE-1010 Dr. Mark L. Hornick 9 Passing primitive arguments to methods When a method containing primitive arguments is called, the value of the actual argument is passed to the matching formal argument, and separate memory space (the Stack) is allocated to store this value. This way of passing the value of arguments is called a pass-by-value, or call-by-value, scheme.

SE-1010 Dr. Mark L. Hornick 10 Example of pass-by-value/call-by-value argument passing from caller to called method // this method is part of the main class public static void main(String args[]) { Calculator calc1 = new Calculator(); int sum = calc1.add( 3, 4); }.... // This method is part of the Calculator class public int add( int x, int y) { return x+y; // return the sum } The values 3 and 4 are passed as the actual arguments The formal arguments x and y receive copies of the values provided by the caller The value resulting from the addition of x+y is passed back to the caller, where it is copied into the variable sum

SE-1010 Dr. Mark L. Hornick 11 The datatype of an argument must be assignment-compatible with the data type of the matching formal parameter When it can, the compiler will provide automatic upcasts of the argument to the datatype of the matching parameter For example, if you supply an int argument where a long parameter is expected, the compiler will upcast the int to a long But the compiler will complain if you try to pass a long argument into an int parameter  This is called a downcast, and will not be supplied by the compiler

SE-1010 Dr. Mark L. Hornick 12 Passing and Returning Objects Passing and returning objects follows the same process as passing and returning primitive data types. The only difference is that with objects, the value being passed is the reference (or address) to an object. When a variable is an object name, the reference of the variable is the address in memory where the object is stored.

SE-1010 Dr. Mark L. Hornick 13 Review: Primitive variables vs. Object variables The only difference between a variable for a primitive and a variable for objects is the contents in the memory locations. For primitives, a variable contains the numerical value itself. For objects, a variable contains an address where the object is stored. 10 name x String object

SE-1010 Dr. Mark L. Hornick 14 Example of pass-by-reference, or call- by-reference // this method is part of the main class public static void main(String args[]) { String first = “Joe”; student.setFirstName( first ); }... // This method is part of the Student class public void setFirstName( String name) { firstname = name; } The identifier first references a String object and is passed as an actual argument The formal argument name receives the reference to the String object

SE-1010 Dr. Mark L. Hornick 15 Another example of pass-by- reference, or call-by-reference // this method is part of the main class public static void main(String args[]) { String s = student.getFirstName( ); }... // This method is part of the Student class public String getFirstName( ) { return firstName; } The identifier s receives a reference to a String object The method returns a copy of the reference to (the address of) the String object containing the name

SE-1010 Dr. Mark L. Hornick 16 Dot notation When you call a public method of an object, you must use dot notation to specify both the method and the object you are calling it on Dot notation is optional when you call a method from another method if the two methods belong to the same object. If dot notation is used, use the reserved word this to refer to the same object.

SE-1010 Dr. Mark L. Hornick 17 Calling a method belonging to the same object vs. calling a method belonging to a different object.