Introduction to Methods

Previously discussed There are similarities in make up of that can help you remember the construct of a class a class in the Java programming language and a book chapter in the English language

Previously discussed (cont.) Here is a graphical depiction of the resemblance:

Previously discussed (cont.) They are similar in the following manner: Class (in Java)Book Chapter (English) A class contains a number of related methods A chapter contains a number of related paragraphs A method contains a number of statements A paragraph contains a number of sentences A statement is the smallest unit of execution in the Java language A sentence is the smallest unit of readable text in the English language

The 2 types (kinds) of methods in Java Older (non-object-oriented) programming languages have only one type (kind) of method Newer programming languages (so called object-oriented languages, explained later) have 2 types (kinds) of methods: 1.Class methods 2.Instance methods

The 2 types (kinds) of methods in Java (cont.) We will now study the class methods We will delay the discussion of the instance methods for later (when we discuss abstract data types).

Why use methods in a computer program Convenience of methods: A statement can only accomplish a very small amount of work A method contains multiple statements. Therefore, a method is a more useful unit of program execution

Steps in using methods in a Java program How to use methods in a Java program: 1.First, you must define the method. How to define a method: Write down the steps (= statements) contained in the method. Attach a name to the steps (= statements)

Steps in using methods in a Java program (cont.) Notes: You only need to define a method once (Remember that in Java, you must define the method inside some class.)

Steps in using methods in a Java program (cont.) 2. After defining the method, you can then call a method using the name of the method This mechanism is called method invocation (an older term is procedure call) When a method is called, the statements inside the corresponding method are executed When all statements in the method has been executed, the execution will resume at the program location of the method call

Steps in using methods in a Java program (cont.) Note: You can invoke a method as many times as you wish

Example on how to use method: find the minimum of 2 floating point numbers We have previously seen an algorithm to find the smaller of 2 values x and y (see: /while2.html) if ( x < y ) { min = x; // x is the smaller value } else { min = y; // y is the smaller value }

Example on how to use method: find the minimum of 2 floating point numbers (cont.) We can create a method to perform the task of "finding the minimum of 2 value as follows: Remember: a method is always contained inside some class in Java Write down the statements in the algorithm Attach a name (e.g.: min) to the statements

Example on how to use method: find the minimum of 2 floating point numbers (cont.) Example: public class ToolBox // Methods must be contained inside a class { /* A method called "min" that contains the statements to find the smaller of 2 values a and b */ public static double min ( double a, double b ) { double min = 0;

Example on how to use method: find the minimum of 2 floating point numbers (cont.) if ( a < b ) { min = a; // a is the smaller value } else { min = b; // b is the smaller value } return(min); // Output of the method }... (You can define more methods inside a class) }

Example on how to use method: find the minimum of 2 floating point numbers (cont.) Explanation: The name of the method is min The method is contained inside the class named ToolBox The complete name of the "min" method is: ToolBox.min

Example on how to use method: find the minimum of 2 floating point numbers (cont.) How to use the method ToolBox.min: public class MyProgram { public static void main(String[] args) { double r; r = ToolBox.min( 1.0, 4.0 ); System.out.println(r); r = ToolBox.min( 3.7, -2.9 ); System.out.println(r); r = ToolBox.min( -9.9, 3.8 ); System.out.println(r); }

Example on how to use method: find the minimum of 2 floating point numbers (cont.) Notice the advantage of using methods: A non-savvy user that wants to use the ToolBox.min method does not need to know the statements contained inside the method ToolBox.min !!! A non-savvy user will only need to know the following in order to use the method: 1.The (complete) name of the method (i.e.: ToolBox.min) 2.What information the method needs to do the task (i.e.: 2 numbers) 3.What information the method returns to the user (i.e.: 1 number)

Example on how to use method: find the minimum of 2 floating point numbers (cont.) Notice the advantage of using methods: In fact, you were a non-savvy user of the methods in Java's Scanner class E.g.: You have used nextDouble() without knowing exactly what statements this method contains !

Example on how to use method: find the minimum of 2 floating point numbers (cont.) Example Program: (Demo above code) –Prog file: 1/MyProgram.java –File containing the min method: 1/ToolBox.java How to run the program: Right click on both links and save in a scratch directory To compile: javac MyProgram.java To run: java MyProgram

Example on how to use method: find the minimum of 2 floating point numbers (cont.) Output:

Example on how to use method: find the minimum of 2 floating point numbers (cont.) A note on the Java compiler: The Java compiler can detect that the program MyProgram.java uses a method ToolBox.min from the class ToolBox The Java compiler will try locate the file ToolBox.java and compile it automatically So you do not need to compile the Java program ToolBox.java explicitly --- the Java compiler will do that for you.

Method call (invocation) and return $64,000 Question: What happens in a method invocation ???

Method call (invocation) and return (cont.) We must first explain the effect of a return statement: public class ToolBox // Methods must be contained inside a class { /* A method called "min" that contains the statements to find the smaller of 2 values a and b */ public static double min ( double a, double b ) { double min = 0; if ( a < b ) {

Method call (invocation) and return (cont.) min = a; // a is the smaller value } else { min = b; // b is the smaller value } return(min); // ****** A "return" statement ******* }... (You can define more methods inside a class) }

Method call (invocation) and return (cont.) Syntax of a return statement: form 1: return ; form 2: return EXPRESSION ;

Method call (invocation) and return (cont.) Effect of a return statement: The return statement is used to terminate the execution of a method. If a return statement returns an EXPRESSION, then the value (= result) of the EXPRESSION will be used to replace the method call at the point of call. When the program executes a return statement, the method terminates (or exits) The execution will continue at the location where the method was called

Method call (invocation) and return (cont.) What happens in a method invocation: Java program: public class MyProgram { public static void main(String[] args) { double r; r = ToolBox.min( 1.0, 4.0 ); // Invokes the "ToolBox.min" method ! System.out.println(r); r = ToolBox.min( 3.7, -2.9 ); System.out.println(r); r = ToolBox.min( -9.9, 3.8 ); System.out.println(r); }

Method call (invocation) and return (cont.) The ToolBox.min method: public class ToolBox // Methods must be contained inside a class { /* A method called "min" that contains the statements to find the smaller of 2 values a and b */ public static double min ( double a, double b ) { double min = 0; if ( a < b ) { min = a; // a is the smaller value } else { min = b; // b is the smaller value } return(min); // **** Return statement **** }

Method call (invocation) and return (cont.) Execution of the program: Program starts executing in the main method:

Method call (invocation) and return (cont.) The method invocation ToolBox.min(1.0, 4.0) transfers the program execution to the method ToolBox.min

Method call (invocation) and return (cont.) The min method executes and reaches the return statement

Method call (invocation) and return (cont.) Notice that the variable min = 1.0 when the return statement is executed.

Method call (invocation) and return (cont.) The return statement transfers program execution back to the point of invocation

Method call (invocation) and return (cont.) After the method returns, the returned value (1.0) will effectively replace the method invocation In other words, the assignment statement that will be executed is: r = 1.0 ;

Method call (invocation) and return (cont.) Bottom line: The variable r will be assigned with the minimum of the 2 values passed to the ToolBox.min method !!!

Defining a class method Syntax used to define a method:

Defining a class method (cont.) Note: the construct must appear inside some class, so it will look like this:

Defining a class method (cont.) The keyword public tells the Java compiler that we are defining something that had no access restriction The "something" can be one of two things: In other words, you can only define 2 kinds of things inside a class: methods or variables A method, or A variable Explanation:

Defining a class method (cont.) The keyword static tells the Java compiler that we are defining a class typed method (or variable). Note: If you omit the keyword static, you will define an instance method (or variable) which will be discussed later.

Defining a class method (cont.) The RETURN-TYPE is a Java data type (e.g., int, double, etc.) The RETURN-TYPE specifies the type of the data that is returned by the method.

Defining a class method (cont.) The method-Name is an identifier that is used to identify this method. You use the method-Name to identify the method in the method invocation

Defining a class method (cont.) The pair of brackets (.... ) tells the Java compiler that you want to define a method. You can in fact define 2 different things inside a class: We will discuss variables defined inside a class later in the course. methods variables

Defining a class method (cont.) A definition without the brackets (... ) will define a variable !!! Notice how the Java compiler can tell the difference:

Defining a class method (cont.) The FORMAL-PARAMETER-LIST is a comma-separated list of parameter variables that is passed (= given to) the method as additional information The items in the FORMAL-PARAMETER- LIST has the following form: TYPE variable-name

Defining a class method (cont.) A formal parameter variable is initialized with the value given in the method call It is a way to pass information to the method so it can use the information to perform its designated task

Defining a class method (cont.) The method body completes the method definition The method body is a block (enclosed between {... }) Within the method body, you can write any number of the following things: statements variable definitions --- variables defined inside a method body are called local variables

Defining a class method (cont.) The method header The method header is the first part of the method definition without the method body

Defining a class method (cont.) Example:

Alternative example on defining the min method In the previous example, we put the definition of the min method inside a new class named ToolBox Alternatively, we can define the min method inside the same class as the main method - like this: public class MyProgram { public static double min ( double a, double b ) { double min = 0; if ( a < b ) { min = a; // a is the smaller value

Alternative example on defining the min method (cont.) } else { min = b; // b is the smaller value } return(min); // **** Return statement **** } public static void main(String[] args) { double r; r = MyProgran.min( 1.0, 4.0 ); // The name is now: "MyProgram.min" ! System.out.println(r); r = MyProgram.min( 3.7, -2.9 ); System.out.println(r); r = MyProgram.min( -9.9, 3.8 ); System.out.println(r); }

Alternative example on defining the min method (cont.) Note: Because the definition of min method is contained inside the class MyProgram, the name the method is now: We must use this new name to invoke the min method. MyProgram.min

Alternative example on defining the min method (cont.) Example Program: (Demo above code) –Prog file: 2/MyProgram.java How to run the program: Right click on both links and save in a scratch directory To compile: javac MyProgram.java To run: java MyProgram

A short hand for invoking a method defined inside the same class Notice that in this example: public class MyProgram { public static double min ( double a, double b ) { double m = 0; if ( a < b ) { m = a; // a is the smaller value } else { m = b; // b is the smaller value

A short hand for invoking a method defined inside the same class (cont.) } return(m); // **** Return statement **** } public static void main(String[] args) { double r; r = MyProgram.min( 1.0, 4.0 ); // The name is now: "MyProgram.min" ! System.out.println(r); r = MyProgram.min( 3.7, -2.9 ); System.out.println(r); r = MyProgram.min( -9.9, 3.8 ); System.out.println(r); }

A short hand for invoking a method defined inside the same class (cont.) contains 2 methods The method main invokes (calls) the method min. min main

A short hand for invoking a method defined inside the same class (cont.) Short hand method invocation: We can reference a method defined inside the same class without using the class name

A short hand for invoking a method defined inside the same class (cont.) Example: public class MyProgram { public static double min ( double a, double b ) { double m = 0; if ( a < b ) { m = a; // a is the smaller value } else { m = b; // b is the smaller value }

A short hand for invoking a method defined inside the same class (cont.) return(m); // **** Return statement **** } public static void main(String[] args) { double r; r = min( 1.0, 4.0 ); // ****** Shorthand name "min" used System.out.println(r); r = min( 3.7, -2.9 ); System.out.println(r); r = min( -9.9, 3.8 ); System.out.println(r); }

A short hand for invoking a method defined inside the same class (cont.) Example Program: (Demo above code) –Prog file Scope1.java: 3/MyProgram.java How to run the program: Right click on link and save in a scratch directory To compile: javac MyProgram.java To run: java MyProgram

Organizing methods in classes Fact: But: A method can be placed in any class Imagine placing books on a number of shelves You can place any book on any shelf Then it very difficult to find a desired book !!!

Organizing methods in classes (cont.) Advice: to allow easily location of methods Place related methods inside the same class Give the class a meaningful name Give each method a meaningful name also

Organizing methods in classes (cont.) Example from Java: The sin, cos, tan, log, exp, sqrt, etc methods are all defined inside the same class The name of this class is Math That's why we use: to invoke the sqrt method !!! Math.sqrt(... )

Remaining topics in Methods We still need to discuss some important topics related to methods: 1.The scope and lifetime of local variables 2.How parameters are passed to methods.