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Input

All programs can read input from the keyboard, which programmers call standard input, or "standard in." * In Java, System.in represents standard input.

Use a Scanner to read input. * Store the input value in a variable (or use it immediately). * When we ask the user for input, the program will pause and wait for the user to enter something.

Steps to Reading and Using User Input

  1. Create a Scanner. 

      java.util.Scanner scanner = new java.util.Scanner(System.in);

  2. Prompt the user for input (or they won't know to enter anything). 

      System.out.print("Enter your age: ");

  3. Read the data and store it in a variable of the correct type. 

      int age = scanner.nextInt();  //nextInt() for an int

  4. Use the data. 

      System.out.println("You said your age is " + age);

  5. Close the Scanner. (Once you do this, your program can no longer receive data - so do this as the last thing in your program.) 

      scanner.close();

In Java, you must use different Scanner method calls for reading different data types. 

String str = scanner.next();
int i = scanner.nextInt();
boolean b = scanner.nextBoolean();
double d = scanner.nextDouble();

Drill

screenoutputkeyboardinput/drills/ScannerInput.java * Add a prompt to the code so that the user knows to enter data. * Output the data. * Run the program.

Input Type Errors

If the user does not enter the right type of data, the program could crash with a runtime error, like 

Exception in thread "main" java.util.InputMismatchException
  at java.util.Scanner.throwFor(Scanner.java:864)
  at java.util.Scanner.next(Scanner.java:1485)
  at java.util.Scanner.nextInt(Scanner.java:2117)
  at java.util.Scanner.nextInt(Scanner.java:2076)
  at ScannerInputSolution.main(ScannerInputSolution.java:12)

This is because Java accepts all user input as a String and then tries to convert it to the correct type.

Drill

screenoutputkeyboardinput/drills/ScannerInput.java * Add a statement to output the value read by nextInt(), with some appropriate text. Run the program and input an integer. * Run the program again and try entering a decimal value, or a String (like your first name).

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