Java Tips for Beginners (1): Data Types

Background

I just started taking a Java course this week. It is an introduction-level undergrad course to computer science from my university. It is taught online though.

To start, download and install Java 8 and Eclipse.

This course uses two online platforms: Piazza as a message board for Q&A, and Bitbucket as a git solution for teamwork.

Course page: http://www.cse.wustl.edu/~dshook/cse131/

Textbook online version: http://introcs.cs.princeton.edu/java/home/

I typed a lot of notes in Word documents, but here I am (obviously) not going to write down everything. Just some tips.

Table of Contents

Before we get started...
  Tip 1: How to undo typing (go back) in Eclipse?
  Tip 2: How to type System.out.println() fast?
Module 1: Data Types
  Tip 3: Review built-in data types
  Tip 4: Do the math carefully
         / and % for integers
         / for doubles
         evaluation tree
         operator cannot be chained
  Tip 5: Useful functions and values
         Transcendental functions
         Return random numbers within the range [0,1)
         Constants π and e
         Scientific notation
  Tip 6: Rounding
         How to round numbers to two decimal places?
  Tip 7: Prompt the user for input values
  Tip 8: How to convert types?

Before we get started…

Tip 1: How to undo typing (go back) in Eclipse?

right click – choose “undo typing”

Or

press “Ctrl + Z”

(I did not realize that until the end of module 1. It’s not used very often, but good to know.)

Tip 2: How to type System.out.println() fast?

It’s pretty long! But you can type it fast by content assist.

Steps:

(1) Type “Sysout”

(2) Press “Ctrl+Space”

It’s done! System.out.println() will show up automatically.

Module 1: Data Types

Tip 3: Review built-in data types

// Below is what I learnt from this module:

char

String   *Note that String starts with the capital letter S

int

long

double   *Doubles are real numbers.

boolean

// Others data types (byte, short, float):

see https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

Tip4: Do the math carefully

/ and % for integers

3/2 = 1; 5/2 = 2. Everything after the decimal point will be dropped.

The % operator computes the remainder after division

3%2 = 1, 5%2 = 1

/ for doubles

However, the slash “/”  works differently for doubles.

3.0/2.0 = 1.5

Evaluation tree

Note that the data type is converted before dividing by 2.

(Screenshot. Reference: course website http://www.cse.wustl.edu/~dshook/cse131/)

Q. Why doesn’t the statement if (a <= b <= c) work?

A. The <= operator cannot be chained. Instead, use if (a <= b && b <= c).

Tip 5: Useful functions and values

Transcendental functions

• Math.sin( ), Math.cos( ), Math.tan( )
• Math.log( )
• Math.exp( )
• Math.sqrt( )

Return random numbers within the range [0,1)

syntax:

double r = Math.random();

0 ≤ r < 1

Constants π and e

• Math.PI – close to π
• Math.E – close to e

Scientific notation

• We can use scientific notation
  • The E character represents “times 10 to the”
  • You can also specify the E as lower-case e

  “double val = 3E-9” is the same with “double val = .000000003”

Tip 6: Rounding

Math.round ()

• round numbers to the nearest integer

How to round numbers to two decimal places?

Below is the answer from Jonik:

(Source: stackoverflow)

double time = 200.3456;
DecimalFormat df = new DecimalFormat("#.##");      
time = Double.valueOf(df.format(time));

System.out.println(time); // 200.35

“Note that this will actually do the rounding for you, not just formatting.”

Below is the rationale behind this.

(Source: http://mathbits.com/MathBits/Java/DataBasics/money.htm)

“The pattern establishes how the output will look – “00.00” – the zeroes in front of the point will be the minimum number of digits displayed.  The zeros after the point will be the maximum number of digits displayed.”

I noticed that some people use “#.##” while some use “0.00”. I tried both and both worked fine.

—-

Only one problem! I would get an output value such as 90.5 instead of 90.50.

How to keep exactly two decimal places? (That is, show “90.50”, for example)

I have asked my instructor. I will update this section when he answers.

Update: OK he said it’s fine because the assignment requires rounding to two decimal points and having 0s dropped off is fine. If I want to keep exactly two decimal points, I can use string manipulation.

—–

Update 2: I later realized that there’s an easier way to do this with just Math.round().

For example (source: http://stackoverflow.com/questions/13210491/math-round-java):

double inches = Math.round( (centimeters / 2.54) * 100.0 ) / 100.0;

Tip 7: Prompt the user for input values

// example

ArgsProcessor ap = new ArgsProcessor(args);

int aliceCarrots = ap.nextInt(“Starting number of carrots?”);

String name = ap.nextString(“What’s the name of the person?”);
double quiz = ap.nextDouble(“What’s the number of quiz points received?”);

Tip 8: How to convert types?

Type casting: There are two types – implicit and explicit

This website provides clear instructions about this:

http://www.studytonight.com/java/type-casting-in-java

Examples:

*Note that casting such as (int) will remove all decimals, e.g., turning 20.74 to 20. So it is different from rounding, which turns 20.74 to 21.

      int i = 100;	
      long l = i;	//no explicit type casting required  
      float f = l;	//no explicit type casting required

      double d = 100.04;  
      long l = (long)d;  //explicit type casting required  
      int i = (int)l;	//explicit type casting required