Language Constructs #

Many Python fundamentals have a Java equivalent, such as loops and if statements. This section shows a direct comparison of the syntax.

Variable Declaration #

i = 0

int i = 0;

• Just like Python, Java variables have types. In Java, to declare a variable, we have to explicitly say what type it is. A variable’s declared type can never change. Refer to Lecture 1 for more on “static typing.”
• We also have to put a semi-colon at the end of the statement.

Types #

Comments #

# This is a single line comment.

// This is a single line comment.

Java also has multi-line comments that are started by /* and ended by */.

while Loop #

i = 0
while i < 10:
    print(i)
    i += 1

int i = 0;
while (i < 10) {
    System.out.println(i);
    i++;
}

• The parentheses, ( and ) around the condition are required.
• In Java, ++ is often used instead of += 1.
• We really do use System.out.println to print in Java. Sorry.
• Instead of indenting, we use curly braces, { and } to wrap the code that is part of the while loop. Java doesn’t require indenting, but it’s good style!

for Loop (counting up) #

for i in range(10):
    print(i)

for (int i = 0; i < 10; i ++) {
    System.out.println(i);
}

In Java, the for loop has the syntax:

for (initialization; termination; increment) {
    // loop body
}

This is roughly equivalent to the while loops:

initialization
while termination:
    # loop body
    increment

initialization
while (termination) {
    // loop body
    increment
}

The while loops and the for loop exit when the termination condition is false. The for loops in the comparison table go “until” i = 10.

for Loop (counting down) #

for i in range(9, -1, -1):
  print(i)

for (int i = 9; i >= 0; i --) {
  System.out.println(i);
}

• Note the different “initialization”, “termination”, and “increment” blocks in the Java for loop.
• Similarly to ++, -- is often used instead of -= 1.
• The for loops in the comparison table go “until” i < 0.

Conditionals #

if i % 3 == 0 and i % 5 == 0:
    print("FizzBuzz")
elif i % 3 == 0:
    print("Fizz")
elif i % 5 == 0:
    print("Buzz")
else:
    print(i)

if (i % 3 == 0 && i % 5 == 0) {
    System.out.println("FizzBuzz");
} else if (i % 3 == 0) {
    System.out.println("Fizz");
} else if (i % 5 == 0) {
    System.out.println("Buzz");
} else {
    System.out.println(i);
}

The boolean operators are as follows:

• Note the difference between elif and else if.

Exponentiation #

x = 2**10

int x = Math.pow(2, 10);

Note that ^ in Java is the “XOR” operator, not the exponentiation operation. That is, 2 ^ 10 is valid code, but it will return 8, not 1024.

Function Declaration and Usage #

def greet(name):
    return "Hello, " + name

# Elsewhere...
print(greet("Josh"))

public static String greet(String name) {
    return "Hello, " + name;
}
// Elsewhere...
System.out.println(greet("Josh"));

• In Java, functions have a specific return type that comes before the function name. Functions also specify their arguments’ types.
  • When a function returns nothing, it has a return type of void.
• For now, all our functions will have public static in front. We’ll learn what these mean later.
• Calling a function looks the same as in Python.

Strings #

s = "hello"
s += " world"
s += str(5)
s_length = len(s)
substr = s[1:5]
c = s[2]
if "hello" in s:
    print("\"hello\" in s")

for letter in s:
    print(letter)

String s = "hello";
s += " world";
s += 5;
int sLength = s.length();
String substr = s.substring(1, 5);
char c = s.charAt(2);
if (s.indexOf("hello") != -1) {
    System.out.println("\"hello\" in s");
}
for (int i = 0; i < s.length(); i++) {
    char letter = s.charAt(i);
    System.out.println(letter);
}

• In Java, Strings are not directly iterable. We either iterate over an index and use charAt, or we convert it to an array (coming soon).
• In Java, you can add anything to a Strings, and it will be implicitly converted to a String without needing to explicitly cast.

Programs #

Now that we’ve covered individual language constructs, let’s look at some Java programs that use them. Here are some simple ones that you might find yourself referring to if you forget how to do something.

Hello World #

print("Hello World")

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

• All Java code must be in a class. We’ll learn more about classes later.
• When a Java program is executed, it runs the public static void main(String[] args) method. This is different from Python, where code can be executed outside of a function.