Comments

• Single-line comments

  Python	Java
  # This is a comment x = 3 # This is also a comment	// This is a comment int x = 3; // This is also a comment

• Multiline comments

  Python	Java
  """ This is another comment. It extends over more than one line. """	/* This is another comment. It extends over more than one line. */

Statements

• Declaration statements, e.g.:

  int x;

• Expression statements: assignment, calling a method,..., e.g.:

  x = 3;
  double y = 12.3;
  x = x + 1;
  java.util.Scanner sc = new java.util.Scanner(System.out);
  System.out.println(x);
  String z = sc.nextLine();
  System.out.println(z);

• Control flow statements: if, while, for, break,...
• In Java, most statements are terminated by semi-colons

while loops

while x < y:
    x = x + 1
    print(x)
  

while(x < y) {
    x = x + 1;
    System.out.println(x);
}

In the Java, note:

• the round parentheses for the Boolean expression;
• the curly braces for the blocks

for loops

• Using a loop to 'visit' each item in a list or array:

  Python	Java
  groceries = ['eggs', 'bread', 'milk'] for item in groceries: print(item)	We're not ready for lists and arrays in Java yet!

• Using a loop to count, e.g. from 0 to 9 inclusive:

  Python	Java
  for i in range(10): print(i)	for (int i = 0; i < 10; i = i + 1) { System.out.println(i); }

• In the Java,
  • variable i is referred to as the loop variable (or 'loop counter')
  • the expression int i = 0 declares and initialises the loop variable
  • the expression i < 10 is called the loop test (or 'loop condition')
  • the block is called the loop body

Other control flow statements

• Ones that are common to Java and Python
  • Ones you already know
    • break, return, import
    • try...catch...finally (Java), try...except...else...finally (Python)
  • Ones you may not know
    • continue
    • throw (Java), raise (Python)
• Ones that Java has and Python doesn't
  • switch, do...while
• Ones that Python has that Java doesn't
  • pass, yield,...

Class exercise: convert to Java

from random import choice

secret_num = choice( range( 1, 101 ) )

while True:
    guess = int( input( "What's your guess: " ) )
    if guess < secret_num :
        print( "    ", guess, "is too low" )
    elif guess > secret_num :
        print( "    ", guess, "is too high" )
    else :
        print( "    ", guess, "is correct" )
        break

Initialization of local variables

• You must explicitly initialize all local variables
• In Java, compile-time error if you do not! (In Python, run-time error if you do not)
• Either declare and initialize in a single statement (generally better), e.g.

  int n = 3;

  or declare first and initialize later, e.g.:

  int n;
  ...
  n = 3;

Class exercise

Which lines give compile-time errors?

class InitializationExercise {

    public static void main(String[] args) {
    
        java.util.Scanner sc = new java.util.Scanner(System.in);
        
        int x = 3;
        int y;
        int z;
        
        y = sc.nextInt();
        System.out.println(x + y + z);
        
        z = sc.nextInt();
        int i;
        if (z < 999) {
            i = 100;
        }
        System.out.println(i);
        
        int j;
        if (z < 999) {
            j = 100;
        } else {
            j = x;
        }
        System.out.println(j);
        
    }
}

Java exercise

Draw the scope and find the compile-time errors

public static void main(String[] args) {
    java.util.Scanner sc = new java.util.Scanner(System.in);
    int x = sc.nextInt();
    if (x < 0) {
        int y = sc.nextInt();
        x = x + y;
    } 
    System.out.println(x);
    System.out.println(y);
    
    if (x < 100) {
        int z = sc.nextInt();
        x = x + z;
    } else {
        double z = sc.nextDouble();
        x = x * z;
    }
    System.out.println(z);
}

Python exercise

Draw the scope. Are there errors?

def stupid_function :
    x = int( input() )
    if x < 0 :
        y = int( input() )
        x = x + y
    print(x)
    print(y)
    
    if x < 100 :
        z = int( input() )
        x = x + z
    else :
        z = float( input() )
        x = x * z
    print(z)