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import java.io.*;
import java.util.Scanner;

class TextReaderApp {
    public static void main(String[] args)
            throws IOException {
        File data = new File("data.txt");
        Scanner input = new Scanner(data);
        String str = input.nextLine();
        System.out.println(str);

        char ch = input.nextLine().charAt(0);
        System.out.println(ch);

        int x = input.nextInt();
        System.out.println(x);

        double d = input.nextDouble();
        System.out.println(d);

        for (int i = 0; i <= 10; i++) {
            x = input.nextInt();
            System.out.print(x + " ");
        }
        System.out.println();

        input.close();
    }
}

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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");
    }
}

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import java.io.*;
import java.util.Scanner;

public class Files {
    public static void main(String[] args)
            throws IOException {
        File data = new File("sonnet.txt");
        Scanner input = new Scanner(data);

        while (input.hasNextLine()) {
            String line = input.nextLine();
            System.out.println(line);
        }
        input.close();
    }

}

Defining Arrays in Java

• An array is a collection of data items all of the same type
• You declare an array like this:

  dataType[] variableName = new dataType[length];
  

• You can also declare an array like this:

  dataType variableName[] = new dataType[length];
  

• Where:
  • dataType: the data type of all the array items
  • variableName: the name you make up for the array
  • length: the number of data items the array can hold
• For example, the following is the declaration of an array named scores that holds 10 values of type int:

  int[] scores = new int[10];

• When a program executes this statement, it creates 10 contiguous slots in memory like this:

  scores =

• Each of the memory slots can hold one data value
• We can have arrays of any type:

double[] anArrayOfDoubles;
boolean anArrayOfBooleans[];
char[] anArrayOfChars;
String anArrayOfStrings[];

• Once you define the length of an array, it cannot change length later in the program.
  
• When defining an array, you must guess the maximum number of elements you need to store:

  int[] scores = new int[100];
  

• If you need to know the length of an array, you can always use the length property (more on this below):
  

Initializing Array Items

• We specify which slot of an array to access with the [] operator:

  scores[4] = 98;

• The indexes of arrays are numbered starting at 0
  
• We can assign a value to an array element any time after it is declared:

  scores[0] = 90;
  scores[1] = 95;
  scores[2] = 87;
  scores[3] = 89;
  scores[4] = 98;
  

• We can also initialize array elements in the declaration statement:
  • Called static initialization
  • We use a comma-separated list inside curly-braces
• For example:

  int[] scores = { 90, 95, 87, 89, 98 };
  

• This produces the same array as in the previous example
• The compiler computes the size automatically by counting the items in the list

• When an array is declared, Java automatically assigns each element a default value: false for booleans, 0 for numerical types
  

Accessing Array Items

• To access the slots in an array, we must specify which slot to use with the [] operator
• For instance:

  scores[4] = 98;

• The number inside the brackets is called an index or subscript
• In Java, the slots of an array are numbered starting at 0, as shown below:

  scores =

  98

  [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]

• Thus, assignment to the slot with an index of 4 is put into the fifth slot

Using Slots

• We declared our example array with a data type of int:

  int[] scores = new int[10];
  

• Because scores is an array containing int values, we can use a slot, such as scores[4], just like any variable of type int:

  scores[4]++;
  System.out.println(scores[4]);
  

• This includes using a slot as an argument to a method with a parameter of the same type:

  public static void myMethod(int singleScore){//method body}
  ...
  myMethod(scores[4]); 
  

Array Length

• We can always access the length of an array using .length

System.out.println(scores.length);

• Note that the length of the array is always one less than its last index

Image source.

Using Arrays to Collect Data Items

• Note that the index of an array can be any integer value
• Thus, we can use an integer variable for the index
• We can use an integer variable with a loop to read data into the array
  
• Also, we can display the contents of an array using a loop
• The following program shows an example of collecting and displaying data items

/**
 *
 * @author parrishj
 */
import java.util.Scanner;
public class Arrays {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
      
        int[] scores = new int[5];
        System.out.println("Array elements initialized to 0:");
        for (int i =0; i <scores.length; i++){
            System.out.println("Index " + i + ": " + scores[i]);
        }
        System.out.println("\nEnter " + scores.length + " scores:");
        for (int i =0; i <scores.length; i++){
            scores[i] = input.nextInt();
        }
        System.out.println("\nYou entered:");
        for (int i =0; i <scores.length; i++){
            System.out.println("Score " + i + ": " + scores[i]);
        }
    }
    
}

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 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?