“Education is a Treasure that follows you everywhere.” – Chines Proverb Methods and Functions

“Education is a Treasure that follows you everywhere.” – Chines Proverb If programs are short We can write the code as one contiguous segment The logic is probably simple There are not too many variables Not too likely to make a lot of errors As programs get longer Programming in a single segment gets more and more difficult Logic is more complex Many variables, expressions, control statements

“Education is a Treasure that follows you everywhere.” – Chines Proverb If programs are short Code as one contiguous segment The logic is probably simple There are not too many variables Not too likely to make a lot of errors As programs get longer Programming in a single segment gets difficult Logic is more complex Many variables, expressions, control statements

“Education is a Treasure that follows you everywhere.” – Chines Proverb Difficult to do if ONE BIG PROGRAM: Isolating and fixing bugs need to be modified program Add to program Answer: Break programs into small segments

“Education is a Treasure that follows you everywhere.” – Chines Proverb Method (or function or subprogram) A segment of code that is logically separate from the rest of the program When invoked (i.e. called) control jumps from main to the method and it executes Usually with parameters (arguments) When finished, control reverts to the next statement after the method call

“Education is a Treasure that follows you everywhere.” – Chines Proverb Methods provide us with functional (or procedural) abstraction Do not need to know all of the details of the methods in order to use them. Need to know: 1.What arguments (parameters) we must provide 2.What the effect of the method is (i.e. what does it do?) The actual implementation could be done in several different ways

“Education is a Treasure that follows you everywhere.” – Chines Proverb Ex: Predefined method: sort(Object [] a) There are many ways to sort! EX: word.length(); This allows programmers to easily use methods that they didn’t write.

“Education is a Treasure that follows you everywhere.” – Chines Proverb Java methods have two primary uses: 1. To act as a function, returning a result to the calling code. These methods are declared with return types, and are called within an assignment or expression Ex:X = inScan.nextDouble(); Y = (Math.sqrt(X))/2; 2. To act as a subroutine or procedure, executing code but not explicitly returning a result These methods are declared to be void, and are called as separate stand-alone statements Ex: System.out.println(“Wacky”); Arrays.sort(myData);

“Education is a Treasure that follows you everywhere.” – Chines Proverb There are MANY predefined methods in Java  Look in the online API  These are often called with dot notation: ClassName.methodName(param_list) ClassName is the class in which the method is defined methodName is the name of the method param_list is a list of 0 or more variables or expressions that are passed to the method

“Education is a Treasure that follows you everywhere.” – Chines Proverb Ex: Y = Math.sqrt(X); These are called STATIC methods or CLASS methods They are associated with a class, not with an object.

“Education is a Treasure that follows you everywhere.” – Chines Proverb Some are also called in the following way ClassName.ObjectName.methodName(param_list) ObjectName is the name of a static, predefined object that contains the method Ex: System.out.println(“Hello There”); System is a predefined class out is a predefined PrintStream object within System println is a method within PrintStream These are instance methods – associated with an object – Primarily we will focus on static methods

“Education is a Treasure that follows you everywhere.” – Chines Proverb What if we need to use a method that is not predefined? That’s why we are coders, we are in charge of our universe.

“Education is a Treasure that follows you everywhere.” – Chines Proverb Syntax: public static void methodName(param_list) { // method body } public static retval methodName(param_list) { // method body } Where retval is some Java type When method is not void, there MUST be a return statement

“Education is a Treasure that follows you everywhere.” – Chines Proverb Really simple example: public static void sayWacky() { System.out.println(“Wacky”); } Now in our main program we can have: sayWacky(); for (int i = 0; i < 5; i++) sayWacky(); Note we are not using any parameters in this example

“Education is a Treasure that follows you everywhere.” – Chines Proverb Theparam_list is a way in which we pass values into our methods This enables methods to process different information at different points in the program Makes them more flexible In the method definition (header or signature): List of type identifier pairs, separated by commas Called formal parameters, or parameters In the method call (can be main or another method): List of variables or expressions that match 1-1 with the parameters in the definition Called actual parameters, or arguments

“Education is a Treasure that follows you everywhere.” – Chines Proverb Ex: public static double area(double radius) { double ans = Math.PI * radius * radius; return ans; } In main: double rad = 2.0; double theArea = area(rad); Note: If method is called in same class in which it was defined, we don’t need to use the class name in the call

17 Lecture 7: Parameters  Parameters in Java are passed by value The parameter is a copy of the evaluation of the argument Any changes to the parameter do not affect the argument Main Class 2.0 rad area method radius 2.0 main calls area method value passed from arg. to parameter double theArea = area(rad); theArea double ans = Math.PI * radius * radius; return ans; ans … result returned to main … answer calculated answer returned method completed

“Education is a Treasure that follows you everywhere.” – Chines Proverb Effect of value parameters: Arguments passed into a method cannot be changed within the method, either intentionally or accidentally Good result: Prevents accidental side-effects from methods Bad result: What if we want the arguments to be changed? Ex: swap(A, B) Method swaps the values in A and B - Discuss We can get around this issue when we get into object-oriented programming

“Education is a Treasure that follows you everywhere.” – Chines Proverb Variables declared within a method are local to that method They exist only within the context of the method This includes parameters as well Think of a parameter as a local variable that is initialized in the method call We say the scope of these variables is point in the method that they are declared up to the end of the method

“Education is a Treasure that follows you everywhere.” – Chines Proverb However, Java variables can also be declared within blocks inside of methods In this case the scope is the point of the declaration until the end of that block Be careful that you declare your variables in the correct block

“Education is a Treasure that follows you everywhere.” – Chines Proverb However, Java variables can also be declared within blocks inside of methods  In this case the scope is the point of the declaration until the end of that block  Be careful that you declare your variables in the correct block

22 References and Reference Types Recall primitive types and reference types  Also recall how they are stored With primitive types, data values are stored directly in the memory location associated with a variable With reference types, values are references to objects that are stored elsewhere in memory var1 100 s Hello There

“Education is a Treasure that follows you everywhere.” – Chines Proverb What do we mean by “references”? The data stored in a variable is just the “address” of the location where the object is stored Thus it is separate from the object itself Ex: If I have a Contacts file on my PC, it will have the address of my friend, Joe Schmoe (stored as Schmoe, J.) I can use that address to send something to Joe or to go visit him if I would like However, if I change that address in my Contacts file, it does NOT in any way affect Joe, but now I no longer know where Joe is located However, I can indirectly change the data in the object through the reference. So if reference variables are changed in a method It will also be changed in main.