Week 8 -Wednesday
What did we talk about last time? StdDraw practice
We have written lots of code so far It has all been inside of the main() method What about a big program? The main() method is going to get really long and hard to read Sometimes you need to do similar things several times: Read some input and check that it falls within certain values Draw a green hexagon at several different locations Find the square root of several different numbers
Methods allow you to package up some code to run over and over Methods usually take some input (like numbers or Strings ) so that they can be customized Methods often give back an answer (like the square root of a number) Also called functions in some other languages
More modular programming Break a program into separate tasks Each task could be assigned to a different programmer Code reusability Use code over and over Even from other programs (like Math.sqrt() ) Less code duplication Improved readability Each method can do a few, clear tasks
Required syntax Type of Type of Type of 1 st argument last argument answer public static type name(type arg1,…,type argn) { statement1; statement2; Name of Name of … 1 st argument last argument statementm; } Name of method Code done by method
Given two integers, find the smaller: public static int min(int a, int b) { if( a < b ) { return a; } else { return b; } }
static methods are methods that are not connected to a particular object They are just supposed to perform some simple task We are going to focus on static methods until next week For now, just taken it as a given that the definition of every method will include the static keyword
It is possible to divide methods into two types: Value returning methods Void methods Value returning methods give an answer: int small = min(x, y);
Void methods are declared with void as their return type public static void help(int times) { for(int i = 0; i < times; ++i){ System.out.println("Help!"); } } Void methods just do something If you try to save the value they give back, there will be a compiler error
Like most code in Java, the code inside of a method executes line by line Of course, you are allowed to put if statements and loops inside methods You can also put in return statements A method will stop executing and jump back to wherever it was called from when it hits a return The return statement is where you put the value that will be given back to the caller
Defining a method is only half the game You have to call methods to use them Calling a method means giving a method the parameters (or arguments ) it needs and then waiting for its answer By now, you have done many method calls System.out.println() Math.sqrt() You can call your own methods the same way
Proper syntax for calling a static method gives first the name of the class that the method is in, a dot, the name of the method, then the arguments Class.name(arg1, arg2, arg3); If the method is in the same class as the code calling it, you can leave off the Class. part If it is a value returning method, you can store that value into a variable of the right type
If a static method is in another class, you have to put the name of the class first, followed by a dot We've seen a few of these already: double length = Math.sin(angle); double value = Math.random(); double[] samples = StdAudio.read(file); By convention, class names in Java always start with a capital letter
Variables from outside of the method don't exist unless they've been passed in as parameters public static int add(int x, int y){ int z = x + y; return z; } No matter how complex a program is, inside this method, only x , y , and z variables exist
A magical process called binding happens which copies the values from the calling code into the parameters of the method The calling code can use variables with the same names, with different names, or even just literals The method does not change the values of the variables in the original code Remember, it only has copies of the values
int a = 10; int x = 3; int y = add( 5, a ); //y contains 15 now public static int add(int x, int y){ int z = x + y; //5 + 10 return z; } No connection between the two different x 's and y 's
1. Start a method with the keywords public static 2. Next comes the return type 3. Then the name of the method 4. Then, in parentheses, the arguments you want to give to the method 5. Inside braces, put the body of the method 6. Include a return statement that gives back a value if needed
A palindrome is a word or phrase spelled the same forwards and backwards: "bob" "radar" "satanoscillatemymetallicsonatas" Write a method with the following header that returns true if a String is a palindrome and false otherwise public static boolean isPalindrome(String phrase) {
Keep reading Chapter 8 More method practice
Keep reading Chapter 8 of the textbook Keep working on Project 3 Due next Friday
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