Intro to Java

Python vs Java

• Which is better?

It depends…

Python vs. Java

Python:

• Wants to get out of your way and let you work
• Do more with less
• Never talk about types
• Intentional effort needs to be put on to prevent bugs
• You can build big if you want

Python vs. Java

Java:

• Wants you to build REALLY BIG things
• Cares deeply about types
• Will fight you over some bugs python will ignore
• Large scale organization is the default

Java

• Descendant of C++, a lot of syntax and data type overlap/reuse
• Compiled and Interpreted mix
  • advanced error-checking from compiled
  • run-everywhere from interpreted
• The de-facto object-oriented programming language
• Built for large-scale programming
  • Makes it a bit wordy
  • Has lots of “engineering” features

Terms to Know

• static typing vs. dynamic typing
• strong typing vs. weak typing
• compiling (javac) vs. interpreting (java)
• compile time vs. runtime
• running a java program requires 2 steps:
  • MyProgram.java → javac → MyProgram.class → java → output

Java – Hello World!

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

Difference between System.out.println() and System.out.print()

javac Hello.java

java Hello

Java – method signature

The main method looks like this:

public static void main(String[] args) {

    }

• public is an access modifier (private is another option)
• static – the method belongs to the class itself rather than to an instance of that class
• void is what the method returns (nothing)
• String[] args is the parameter, an array of String

Java Program Structure

• Everything is in a class – what is a class? (Well, there are packages too, but later)
• No class, no program.
• Every program must have a main() method.
• Why? main() is the execution entry point
• Programs typically consist of many classes
• Each class could have a main() method
• So, then, which main() runs?

Typing

• Python does dynamic typing

word = "car"
number = 3
new_list = []
names = ["Pedro", "Melissa", "Jessica"]

• Static typing in Java

String word = "car";
int number = 3;
int[] numbers = new int[10];
String[] names = new String[]{"Pedro", "Melissa", "Jessica"};

Try it out

Modify your main method in your HelloWorld class to assign the string literal "Hello World!" to a String variable name, and then print out that variable instead.

Questions:

• Can you use both double and single strings to create the variable?

• Does concatenation with the + operator work?

• Can we concatenate (glue together) String, char and int (or a combination of these)?

Java variables

Declare any time before use, and strongly-typed

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

Java variables

• Declare any time before use
• Strongly-typed
• If you don’t assign a value it’s probably a bug
  • in some context a type-specific default is used
• Two “flavors” of variable: primitive (often called scaler or base) and object (reference)

Java primitive types

• boolean
• char (unicode) – single quotes
• int
  • short and byte for smaller ints
  • long for longer ints
• double
  • float for shorter floats

Primitive type names are lower-case

Java primitive types

These act like c++ variables

• variable literally and directly stores value

Java Data Types and Memory Space

1 byte is 8 binary digits

All Objects (everything else) (4 bytes)

Mathematical Operations

All operators as you would expect:

• mathematical: + * - / %
  • No // in Java. Division type will be based on input types
  • int divided by an int is an int
  • Only works for number types
• comparison: < > <= >=
  • numbers only

Mathematical Operations

• equality: == !=
  • behave as expected for primitives
  • behave differently for objects
• logical: && || !
  • back to the symbols (instead of words in Python)

Style and documentation

• variable names often camelCase
• white space (new lines, indendation) means nothing, but should be used for style (to make code easier to read)
• Java requires a lot of code to express a simple concept.
• You need to be much more explicit when writing Java than Python (more words to type in Java)

Style and documentation

// one line comment

/*
  multi-line
  comment
 */
    
/**
 * JavaDoc comments for external documentation
 * @returns description of what returns   
 */