Java Basics

• as you progress through CS classes, one goal is to deepen your understand computers and computation
  • Python typically implemented in C and/or Java, these languages expose more detail about what the computer is doing
    • 201 → Java, focuses on how data can be efficiently structured at a high level
    • 208 → C, focuses on low-level details of how computer systems function
• learning a second language
• preparing you for future CS coursework
  • forces you to think a lot more about types
• No programming language is perfect, and Java certainly has some weaknesses
  • There are going to be a lot of exceptions and things that don't follow the overall pattern
  • Variety of reasons for this
    • Language has evolved over time since the 1990s
    • Adding some complexity allows programmers to achieve better performance
    • Bad early decision must be kept to maintain compatibility

• among top 3 most-used programming languages (with JavaScript, Python, C)
  • Android
  • finance
  • web infrastructure (e.g., AWS)
  • some scientific software
• as a compiled language, Java offers some advantages
  • compiler detects errors before they happen
  • compiler can optimize our code (compile once, run many times)

How to declare and initialize variables in Java

• Variables must be declared (and given a type) before they are used

• All code in Java exists inside a class

import edu.princeton.cs.algs4.StdIn;  // we must import objects from the algs4 to use them

class Calculator {  // pair of curly braces { } indicate the code inside the class

    public static void main(String[] args) {  // curly braces indicate the code inside the main method
        System.out.print("Input your expression: ");  // prints a message to "standard out" (the terminal) 

        // each call to read___() reads the next portion of input separated by spaces (called a "token")
        // readString() returns it as a string, other versions attempt to convert it to a specific primitive type
        double left = StdIn.readDouble();
        String operator = StdIn.readString();
        double right = StdIn.readDouble();

        // v               v    if statements require the condition be inside parentheses
        if (operator == "+") {  // curly braces also indicate code within an if or an else
            System.out.println(left + right);  // println is like print, but adds a new line at the end
        } else {
            // when a program gets into an invalid state or otherwise encounters an error
            // it should "throw" an appropriate exception with a helpful message
            throw new UnsupportedOperationException(operator + " not an implemented operator");
                                                         // ^^^ can use + to concatenate strings
        }
    }
}

• Mysterious main method incantation
  • The main method is called when we run the Java program, so every program has to start there
  • By the end of Monday's class, we will know what public, static, and void mean
  • For now, we must submit to the Java compiler's inscrutable demands
• This version of our calculator doesn't work! It claims that "+" is not implemented…
  • This is where using == with objects can bite us—it checks whether two references refer to exactly the same address in memory
  • What we want is to compare the content of the two strings
    • Use .equals() for this:

      if (operator.equals("+")) {
      

• Let's upgrade our calculator to let the user do multiple computations and add multiplication to the supported operations

import edu.princeton.cs.algs4.StdIn;

class Calculator {

    public static void main(String[] args) {
        int count = 0;  // keep track of how many expressions we've done, add an indicator to the output

        System.out.print("Input your expression: ");
        // StdIn.isEmpty() returns true if there is no more input
        // so we want to loop while that's not true (hence the use of !)
        // On mac, we can signal "no more input" by typing ctrl-d, and on Windows by typing ctrl-z and hitting enter
        while (!StdIn.isEmpty()) {  // note that while uses parentheses and curly braces like if
            double left = StdIn.readDouble();
            String operator = StdIn.readString();
            double right = StdIn.readDouble();

            if (operator.equals("+")) {
                System.out.println("[" + count + "]  " + left + right);
            } else if (operator.equals("*")) {  // we can have any number of else ifs between if and else
                System.out.println("[" + count + "]  " + left * right);
            } else {
                throw new UnsupportedOperationException(operator + " not an implemented operator");
            }
            count++;
            // calling hasNext will cause the program to wait for input, 
            // so we need to print our prompt before that
            System.out.print("Input your expression: ");
        }
        System.out.println("Nice computing with you! :)");  // never hurts to be nice
    }
}

• This version has a bug too!

Input your expression: 8 + 6
[0]  8.06.0

• We can use + with a number and a String, and Java will convert the number to a string and concatenate them
• In System.out.println("[" + count + "] " + left + right);, the additions are evaluated from left to right, so it adds a string and left, and adds the result and right, giving us a concatenated mess
• We should use parentheses to make sure left + right happens first:

System.out.println("[" + count + "]  " + (left + right));

class Mean {

    // args is an array of Strings, each a token entered on the command line 
    // as part of the command starting the program
    public static void main(String[] args) {
        if (args.length == 0) {  // use .length to get the number of elements in an array
            // the user did not provide the command-line arguments we expected
            // print out a helpful message and then exit
            System.out.println("usage: java Mean arg1 [arg2 ...]");
            System.exit(1);
        }

        int total = 0;
        int index = 0;
        while (index < args.length) {
            total += Integer.parseInt(args[index]);
            index++;  // equivalent to index += 1 (but with slightly fewer keystrokes!)
        }
        System.out.println("the mean is " + total / args.length);
    }
}

• Arrays in Java are objects (like Strings), but have several special properties/quirks
  • Unlike all other data structures in Java, array length is a field, not a method
    • e.g., args.length to get the length of an array, s.length() to get the length of a String s
  • An array type is given by add square brackets to the type of data in the array (e.g., String[] for an array of Strings)
  • Arrays are fixed length and can only contain a single type of data
    • You can change array elements, but you can't add or remove them (unlike Python lists)
    • Can't mix numbers and strings in a single array (unlike Python lists)
  • Arrays are the only thing in Java that can be indexed with square brackets
    • They are zero-indexed
  • java.util.Arrays provides a lot of useful array-related functions
• One problem with the program above:
  • It's only printing the integer part of the mean (should be 2.5)

    $ java Mean 1 2 3 4
    the mean is 2
    

  • This is because in Java division between two integers returns an integer
  • If either or both of the numbers are real numbers (float or double), the result will be a real number
  • Two solutions: declare total a double instead of an int, or cast one of the quantities in the division to a double:

    System.out.println("the mean is " + (double)total / args.length);
    

• Finally, the while loop works fine here, but Java has two other kinds of loops that would be more appropriate
  • C-style for loop
    • Using syntax borrowed from the C programming language, you can stuff the initialization and updating of the loop variable into the loop statement:

      for (int index = 0; index < args.length; index++) {
          total += Integer.parseInt(args[index]);
      }
      

    • Use thing kind of loop in the same situations (e.g., looping over indexes) that you would this style of Python for loop:

      for index in range(len(args)):
          total += int(args[index])
      

  • The for-each loop
    • Modern Java supports a for loop that iterates over the elements in an array:

      for (String arg : args) {
          total += Integer.parseInt(arg);
      }
      

    • Use thing kind of loop in the same situations (e.g., looping over elements) that you would this style of Python for loop:

      for arg in args:
          total += int(arg)