CS 46A - Lecture 10

Pre-class reading

• Sections 4.4 - 4.6

Calling Static Methods

• A static method does not operate on an object:

  double x = 4;
  double root = x.sqrt(); // Error

• Static methods are declared inside classes
• Call with the name of that class

  double root = Math.sqrt(x); 

• Naming convention: Classes start with an uppercase letter; objects start with a lowercase letter:

  Math
  System.out

Syntax 4.3 Static Method Call

Lecture 10 Clicker Question 1

How do you compute x^y in Java?

1. With the pow instance method that has one explicit parameter: x.pow(y)
2. With the pow instance method that has two explicit parameters: Math.pow(x, y)
3. With the pow static method that has one explicit parameter: x.pow(y)
4. With the pow static method that has two explicit parameters: Math.pow(x, y)

Lecture 10 Clicker Question 2

What best describes the call System.out.println(4)?

1. println is a static method of the class System.out
2. out.println is a static method of the class System
3. println is an instance method with implicit parameter System.out
4. println is an instance method with implicit parameter 4

The String Class

• A string is a sequence of characters
• Strings are objects of the String class
• A string literal is a sequence of characters enclosed in double quotation marks:

  "Hello, World!"

• String length is the number of characters in the String
  • Example: "Harry".length() is 5
• Empty string: ""

Concatenation

• Use the + operator:

  String name = "Dave";
  String message = "Hello, " + name; // message is "Hello, Dave"

• If one of the arguments of the + operator is a string, the other is converted to a string:

  String a = "Agent";
  int n = 7;
  String bond = a + n; // bond is "Agent7"

Concatenation in Print Statements

• Useful to reduce the number of System.out.print instructions

  System.out.print("The total is ");
  System.out.println(total);

  versus

  System.out.println("The total is " + total);

Converting between Strings and Numbers

• Convert to number:

  int n = Integer.parseInt(str);
  double x = Double.parseDouble(x);

• Convert to string:

  String str = "" + n;
  str = Integer.toString(n); 

Substrings

• String greeting = "Hello, World!";
  String sub = greeting.substring(0, 5); // sub is "Hello"

• Supply start and past the end position
• First position is at 0
  
• String sub2 = greeting.substring(7, 12); // sub2 is "World"
• Substring length is past the end - start
  

Lecture 10 Clicker Question 3

What does the following code segment print?

String s = "Agent";
s = s + "00" + s.length();
s = s.substring(0, 5) + s.substring(6, s.length() - 1);
System.out.println(s);

1. Agent007
2. Agent 006
3. Agent06
4. Nothing due to a run-time error

German Keyboard

Thai Alphabet

Chinese Ideographs

Reading Input

• System.in has minimal set of features — it can only read one byte at a time
• In Java 5.0, Scanner class was added to read keyboard input in a convenient manner

• Scanner in = new Scanner(System.in);
  System.out.print("Enter quantity:");
  int quantity = in.nextInt();

• nextDouble reads a double
• nextLine reads a line (until user hits Enter)
• next reads a word (until any white space)

Lecture 10 Clicker Question 4

Suppose in is a Scanner object that reads from System.in, and your program calls

String name = in.next();
int n = in.nextInt();

What is the value of name and n if the user enters Agent 0.07?

1. name is Agent 007, n is 0
2. name is Agent, n is 0
3. name is Agent, n is 0.07
4. Nothing due to a run-time erro

Reading Input From a Dialog Box

Reading Input From a Dialog Box

• String input = JOptionPane.showInputDialog(prompt)
• Convert strings to numbers if necessary:
  int count = Integer.parseInt(input);
• Conversion throws an exception if user doesn't supply a number – see chapter 11