Welcome to Lesson 18!

Learning Objectives

1. Lesson 17 Practice Exam Questions

Answer the following questions:

• Declare an array called pets, and assign it the values "dog", "cat", "rabbit", "bird" in TWO ways (static and not static)

• Write a for loop to display the pets array to the console

2. Arrays Continued

Arrays and Methods

• When writing a method with an array parameter, we place an empty [] after the datatype or parameter name:

  public static void print(int values[]) { //will work for an array of any size
  

• When we call the method, we do NOT include the []:

  print(data); // name of the array is data
  

• Instead, we pass in the name of the array.

Example program passing an array to a method

    public class ArrayExample {

    public static void print(int values[]){
        for (int i = 0; i < values.length; i++){

            System.out.println(values[i]);

        }

    }

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int[] scores = {90, 89, 76, 55, 91};
        print(scores);
     
   }

}

• Unlike other parameters, you can pass the array into the method and then alter the array inside of the method without needing to return a new array.
  
• For example, what do you think will be the result of running the following program?

import java.util.Scanner;
public class ArrayExample {
   
    public static void assignValues(int values[]){
        for (int i = 0; i < values.length; i++){
            values[i] = i;
        }
    }

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int[]  arr = new int[10];
        assignValues(arr);
     
        for (int i = 0; i < arr.length; i++){
            System.out.println(arr[i]);
        }
    }

}

       

Notice that when you are passing in the array parameter, you are providing the memory address of the array, rather than the values stored there. Therefore, since the method knows where the array is stored in memory, it can make changes to the array directly inside the method. Note that this paradigm is different from the method below, that takes a primitive parameter. The below method assigns the value 4 to the data variable, not the memory address of the variable num. Therefore, any change that is made to data does not affect num.     public static void assignValue(int data){         data = 5; //does not alter num     }     public static void main(String[] args) {         Scanner input = new Scanner(System.in);         int num = 4;         assignValue(num); //does not alter num                   } It is also possible to return an array from a method - though this approach is unnecessary, as shown in the above example of a void method that takes an array parameter. An example of a method returning an array is below: /**  *  * @author Jennifer Parrish  */ import java.util.Scanner; public class ArrayExample {         public static int[] assignValues(int values[]){         for (int i = 0; i < values.length; i++){             values[i] = i;         }         return values;     }     public static void main(String[] args) {         Scanner input = new Scanner(System.in);         int[]  arr = new int[10];         arr = assignValues(arr);         System.out.println("After filling the array:");         for (int i = 0; i < arr.length; i++){             System.out.println(arr[i]);         }             }  }

Activity 18.1: Exploring Arrays (10 pts)

• In this exercise we explore declaring, allocating and assigning values to arrays containing lists of data
• Create a Java project called Arr
  
• Add the following method to the code:

  public static void print(int values[]) {
      for (int i = 0; i < values.length; i++) {
          System.out.print(values[i] + " ");
      }
  }
  

• Compile your code to make sure it has correct syntax.
• Declare and initialize an array for a list of 10 integer scores:

  int scores[] = {90, 91, 92, 93, 94, 95, 96, 97, 98, 99};
  

• After declaring and initializing the array, call the print() method using the code:

  System.out.println("Integer exam scores:");
  print(scores);

• Compile and run the program to make sure you made the changes correctly. When you run the program, the output should look like this:

  Integer exam scores:
  90 91 92 93 94 95 96 97 98 99
  

• Next, write a method, named addExtraCredit. This method uses a for loop to add 5 points to each exam score in the scores array.
• The method should have the following signature:      
  

public static void addExtraCredit(int values[]) {
    //Write the body of the method here
    //Note: Do not print anything in this method
    //No System.out.print statement(s) go here
    
}

• Next, call this method, passing in the scores array.
• Then, call the print method again to display the values, along with the message After adding extra credit:
• Note: Do NOT print inside of the addExtraCredit method. Call the print method to print the array in main. AddExtraCredit should only add extra credit to the scores. It should not print anything inside the body of the method.
• Another way to think of it: This method has one purpose only: to add extra credit
• The print method has one purpose: to print the array
• Structuring the program to have these two separate methods allows for more flexibility. I am free to add extra credit without having to print the array each time I do so. It is my choice of whether to call printArray to print or not after I have added extra credit.
  

Compile and run the program to make sure you made the changes correctly. When you run the program, the new output should look like this:

After adding extra credit:
95 96 97 98 99 100 101 102 103 104

• Declare and initialize an array of double values holding rainfall values (in inches) 23.4, 16.4, 18.9, and 52.7
  
• Write another print() method with one parameter for the array of double values.
  
• After declaring and initializing the array, call the print() method.
• Note that you can re-use the name print for this second method. The compiler will know they are different methods because the parameters are different. This is called method overloading.
  
• Compile and run the program to make sure you made the changes correctly. When you run the program, the output should look like this:

Integer exam scores:
90 91 92 93 94 95 96 97 98 99
After adding extra credit:
95 96 97 98 99 100 101 102 103 104
Double rainfall in inches
23.4 16.4 18.9 52.7

• Declare and allocate an array of char values and assign it the vowels a, e, i, o and u.

• After declaring and initializing the array, write another print method to display it. Again, this method should be named print(). Then, call this method in main.
  

  Compile and run the program to make sure you made the changes correctly. When you run the program, the output should look like this:

  Integer exam scores:
  90 91 92 93 94 95 96 97 98 99
  After adding extra credit:
  95 96 97 98 99 100 101 102 103 104
  Double rainfall in inches
  23.4 16.4 18.9 52.7
  Char vowels
  a e i o u
  

Writing to a Text File:

• To "write" text to a file, we use a PrintWriter object.
• We can declare the PrintWriter variable in a similar manner to the Scanner variable:
  

  PrintWriter out = new PrintWriter("out.txt"); 
  

• Note that we can either pass in the name of a file directly, as in the example above.

• Or, we can pass in a File object, like with the Scanner:

File outfile = new File("out.txt");
PrintWriter out = new PrintWriter(outfile);

• For consistency with the process for reading from a file using the Scanner, we will practice with the second option above.
• Once we have created a PrintWriter variable, we can call methods to display the data in a file, just like the ones used by System.out
• For exampe:
  

File outfile = new File("out.txt");
PrintWriter out = new PrintWriter(outfile);
out.println("Hello, World!");

out.print("\nGreetings, Universe!");

out.close(); //must close or data will not be written in the file

• Additionally, like with our Scanner objects, we must close the PrintWriter once we are finished with it
• Closing the PrintWriter object is necessary, not just good practice, as the data will not be written in the file until the PrintWriter object is closed.
  

Example program to write out data to a file:

import java.io.*;
public class Files {
    public static void main(String[] args) throws IOException{
    
        File file = new File("scores.txt");
        PrintWriter output = new PrintWriter(file);
       
        output.print("100, 95, 90, 85");
        output.close();  
    }
}

Procedure for Writing a Text File

1. Place import java.io.*; at the top of your program
1. Note that .* is the wildcard. Using a * will import all classes from that package.
2. We need 3 classes here so we will save time by using the * instead of importing all 3 separately (java.io.File, java.io.PrintWriter, and java.io.IOException)
   
2. Add throws IOException as part of the signature for main to avoid compiler errors.
3. Create a File object with a pathname to your file:

   File data = new File("data.txt");
   

4. Connect the file to a file stream:

   PrintWriter out = new PrintWriter(data);
   

   If you want to append data, rather than writing over the contents of the file, then use:

   PrintWriter out = new PrintWriter(data, true);
   

5. Write to the file using the print() and println() methods:

   out.println("This is a string");
   out.println('c');
   out.println(1234);
   out.println(1.234);
   for (int i = 0; i <= 10; i++) {
       out.print(i + ",");
   }
   out.println();
   

6. Close the streams when finished writing:

   out.close();
   

Example Program Writing a Text File

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

import java.io.*; class TextWriterApp { public static void main(String[] args) throws IOException { File data = new File("data.txt"); PrintWriter out = new PrintWriter(data); out.println("This is a string"); out.println('c'); out.println(1234); out.println(1.234); for (int i = 0; i <= 10; i++) { out.print(i + " "); } out.println(); out.close(); System.out.println("Finished writing file"); } }

Activity 18.2: Two Numbers (10 pts)

• In this exercise we write a program that copies two numbers from an input stream to the console
  
• Create a new Java file and name it TwoNumbers.
• Then, let's create a text file to test our program.
  
• In Eclipse, click on the name of the project. Then, go to File->New->File
• Name your text file infile.txt.
• In the file, add a list of numbers each on their own line, like this:

10
20

• We will read from this file in our program.

• Now, we will create a second text file.
• Click on the name of the project again. Then, go to File->New-File
• Name this file outfile.txt
• Now, open TwoNumbers.java.
  
• Add a main method and place the following import statement above main:

  import java.io.*; 
  

• Also, as part of the signature of main(), add throws IOException:

  public static void main(String[] args) throws IOException{
  

• Declare two int variables at the top of main:

int first;

int second;

• Add code to create a new File object:

  File inFile = new File("infile.txt");

• Add statements to read two numbers from the input stream. For example, here is the correct code for reading the first number:

  Scanner input = new Scanner(inFile);
  first = input.nextInt();
  

• Close the stream when finished reading:

  input.close();
  

• Next, we will add some code to write to a file.
• Declare a new File variable for the outfile.txt:

File outFile = new File("outfile.txt");

• Next, declare a PrintWriter variable and connect it to this File:

PrintWriter out = new PrintWriter(outFile);

• Using the PrintWriter, we will write some text into our outfile:

out.println("first=" + first);

//add a line of code to print the second number here

• Lastly, close the PrintWriter:

out.close();

• Compile and run your modified program to make sure you made the changes correctly.
• When you run the code, nothing should appear on the console.
• Instead, open outfile.txt in Eclipse and verify that you have the following contents in your file:

  first=10
  second=20
  

• When you are finshed, submit TwoNumbers.java to Canvas