开源软件名称： in28minutes/java-tutorial-for-beginners开源软件地址： https://github.com/in28minutes/java-tutorial-for-beginners开源编程语言： 开源软件介绍： Welcome to this book on "Learning Java In 150 Steps" .
I am Ranga Karanam , and I have more than two decades of programming experience.
I love Programming. One of the aims I had when I started in28minutes was to make learning programming easy. Thanks for helping us provide amazing courses to 300,000 learners across the world.
At In28Minutes , we ask ourselves one question every day: "How do we create awesome learning experiences?"
In this book, you will learn to write object oriented code with Java. You will be exposed to a lot of examples, exercises and tips. We will take up a lot of examples, and try and see how to write code for those in Java.
Help us improve this guide - Fork, Pull Requests, Shares and Likes are recommended !
We did a study on why students give up on programming?
The popular answer was
Difficulty in writing their first program
Put yourselves in the shoes of a beginner and look at this typical Java Hello World Example.
package com .in28minutes .firstjavaproject ;
public class HelloWorld
{
public static void main (String [] args ) {
System .out .println ("Hello World" );
}
}A Programming Beginner will be overwhelmed by this. I remember how I felt when I saw this almost 20 years back. Stunned.
Why?
There are a number of keywords and concepts - package, public, class, static, void, String[] and a lot more..
What if the programmer makes a typo? Will he be able to fix it?
We believe that there has to be a better way to learn programming.
Why don't we learn programming step by step?
Why should it not be a lot of fun?
Why don't we solve a lot of problems and learn programming as a result?
This is the approach we took to writing this guide and develop our introductory programming courses for Java and Python.
Do you know? The first 3 hours of our Java Course is available here .
PMT-Challenge )Learning to program is a lot like learning to ride a bicycle. The first few steps are the most challenging ones.
Once you use this stepwise approach to solve a few problems, it becomes a habit.
In this book, we will introduce you to Java programming by taking on a few simple problems to start off.
Having fun along the way is what we will aim to do.
Are you all geared up, to take on your first programming challenge? Yes ? Let's get started then!
Our first programming challenge aims to do, what every kid does in math class: reading out a multiplication table.
PMT-Challenge
Compute the multiplication table for 5, with entries from 1 to 10.
Display this table.
5 * 1 = 5
5 * 2 = 10
5 * 3 = 15
5 * 4 = 20
5 * 5 = 25
5 * 6 = 30
5 * 7 = 35
5 * 8 = 40
5 * 9 = 45
5 * 10 = 50
As part of solving the multiplication table problem, you will be introduced to:
JShell Statements Expressions Variables Literals Conditionals Loops Methods
In this step, we:
Stated our first programming challenge, PMT-Challenge
Identified basic Java concepts to learn, to solve this challenge
JShellJShell is a programming tool, introduced in Java SE 9. JShell is a REPL REPL refers to this:
'R' R ead; Read the input Java code'E' E val; Evaluate the source code'P' P rint; Print out the result'L' L oop; Loop around, and wait for the next input
How about starting off exploring Java? Are you game?
You can use https://tryjshell.org/ to run the code for the first 25 steps. Or you can Install Java 12+ . Here's the troubleshooting section if you face problems.
Launch up command prompt or Terminal.
Let type in java -version on the terminal and press enter.
in28minutes$>java -version
java version "x.0.1"
Java(TM) SE Runtime Environment (build x.0.1+11)
Java HotSpot(TM) 64-bit Server VM (build x.0.1+11, mixed mode)
in28minutes$>
A successful execution displays the version of Java installed your system. You need to have atleast Java 9 to pursue this book.
You can launch JShell by typing jshell at your terminal.
in28minutes$ >jshell
| Welcome to JShell version x .0.1
| For an introduction type : /help intro
jshell > When run, this command displays basic information about the installed JShell program. A jshell prompt then appears, which waits for your input.
The JShell command /help, with a parameter intro, gives you basic guidelines on how you use the tool.
jshell > /help intro
|
| intro
| The jshell tool allows you to execute Java code , getting immediate results .
| You can enter a Java definition (variable , method , class , etc ), like : int x =8
| or a Java expression , like : x + x
| or a Java statement or import .
| These little chunks of Java code are called 'snippets' .
| There are also jshell commands that allow you to understand and
| control what you are doing , like : /list
|
| For a list of commands : /help
jshell > Type 3+4 at the Jshell prompt
jshell > 3 + 4
$1 ==> 7
jshell > This was your first real REPL cycle! When you type in 3 + 4, JShell evaluates it and prints the result.
The entity $1 is actually a variable name assigned to the result. We will talk about this later.
The /exit command terminates the JShell program, and we are back to the terminal prompt.
jshell > /exit
| Goodbye
in28minutes$ > You can now effortlessly launch, feed code to, and exit from JShell!
in28minutes$ > jshell
| Welcome to JShell version 9.0 .1
| For an introduction type : /help intro
jshell > /exit
| Goodbye
in28minutes$ > In this step, we learned:
How to launch JShell from our terminal, and run a few commands on it
How to run Java code on the JShell prompt
Lets try to break down the PMT-Challenge problem to be able to solve it.
5 * 1 = 5
5 * 2 = 10
5 * 3 = 15
5 * 4 = 20
5 * 5 = 25
5 * 6 = 30
5 * 7 = 35
5 * 8 = 40
5 * 9 = 45
5 * 10 = 50
Here is how our draft steps look like
Calculate 5 * 3 and print result as 15
Print 5 * 3 = 15 (15 is result of previous calculation)
Do this ten times, once for each table entry (going from 1 to 10)
In this step, we:
Broke down the PMT-Challenge problem into sub-problems
The first part of solving our PMT-Challenge is to calculate the product of 5 and another number, say 3.
Let's start up jshell and type 5 X 3.
in28minutes$ > jshell
| Welcome to JShell version x .0.1
| For an introduction type : /help intro
jshell > 5 X 3
| Error :
| ';' expected
| 5 X 3
| ^
| Error :
| not a statement
| 5 X 3
| ^
| Error :
| ';' expected
| 5 X 3
| ^
| Error :
| missing return statement
| 5 X 3
| ^---^ You probably look at the symbol 'X' as a multiplier, remembering your school days.
Java does not identify 'X' as the multiplication operator! Java supports multiplication, but only if you use its predefined operator, *.
Let's type in code shown below:
jshell > 5 * 3
$1 ==> 15
jshell > Success!
Let's look at some terminology:
5 * 3 is an expression.5 and 3 are operands. They are also called literals or constant values.* is an operator.
Java also has built-in operators to perform other numerical tasks, such as:
Addition: +
Subtraction: -
Division: /
Modulo arithmetic: %
The following examples illustrate how to use them.
jshell > 5 * 10
$2 ==> 50
jshell > 5 + 10
$3 ==> 15
jshell > 5 - 10
$4 ==> -5
jshell > 10 / 2
$5 ==> 5
jshell > Your school math memories are still fresh, and the operators don't seem to disappoint either! +, - and / are your bread-and-butter operators.
% is the modulo operator, which gives you the remainder when integer division is performed.
jshell > 9 % 2
$6 ==> 1
jshell > 8 % 2
$7 ==> 0
jshell > Java allows you to use more than one operator within an expression.
Let's look at some simple expressions with multiple operators.
jshell > 5 + 5 + 5
$8 ==> 15
jshell > 5 + 10 - 15
$9 ==> 0
jshell > 5 * 5 + 5
$10 ==> 30
jshell > 5 * 15 / 3
$11 ==> 25 Each of above expressions have two operators and three operands.
In this step, we:
Learned how to construct numeric expressions
Discovered that operators are predefined symbols
Combined several operators to form larger expressions
At this stage, your smile tells us that you enjoy evaluating Java expressions. What if we tickle your mind a bit, to make sure it hasn't fallen asleep?
Okay, here comes a couple of programming exercises.
Write an expression to calculate the number of minutes in a day.
Write an expression to calculate the number of seconds in a day.
60 (minutes in an hour) multipled by 24 (hours in a day)
jshell > 60 * 24
$1 ==> 1440 60 (seconds in a minute) multipled by 60 (minutes in an hour) multipled by 24 (hours in a day)
$jshell >60 * 60 * 24
$1 ==> 86400 Lets look at a few more examples to understand operators.
Let's type in 5 ** 6 followed by 5 $ 6 in JShell
jshell > 5 ** 6
| Error :
| Illegal start of expression
| 5 ** 6
| ^
jshell > 5 $ 6
| Error :
| ';' expected
| 5 $ 6
| ^
| Error :
| not a statement
| 5 $ 6
| ^
| Error :
| ';' expected
| 5 $ 6
| ^
jshell > 5 */ 6
| Error :
| Illegal start of expression
| 5 */ 6
| ^
JShell was not impressed with our efforts at all!
Java has a set of grammar rules, called its syntax . Code that does not follow these rules throw errors. The compiler informs you by listing the errors, and also provides hints on how you can correct them.
Now, why is an error being thrown?
Operators in Java are all predefined, and limited in number. * is a valid operator, whereas ** and $ were rejected, with error messages.
Let's look at another example:
jshell > 5 / 2
$1 ==> 2
jshell > Surprise, Surprise! JShell seems to evaluate 5 / 2 to 2 instead of 2.5. Where did we go wrong?
Read what follows, with the biggest magnifying lens you can find:
The result of an expression when evaluated, depends on the operator context . This context is determined by the operands passed to it
There are two kinds of numbers typically used in programming : integer (1,2,3,...) and floating-point (1.1,2.3, 56.7889 etc). These values are represented by different types in Java. Integers are commonly of type int, and the floating-point numbers are double by default.
In the expression 5/2, both 5 and 2 are of type int. So, the result is also of type int.
Let's try with a few floating point numbers:
jshell > 5.0 / 2.0
$2 ==> 2.5 Both 5.0 and 2.0 are of type double, the result is double. We get a result of 2.5, as expected.
Let's do a mixed operation - using a floating point number and integer.
jshell > 5.0 / 2
$3 ==> 2.5
jshell > Among the types int and double, double is considered to be a wider type . When you perform a numeric operation between two types, the result will be of the wider type.
Let's look at few complex examples of expressions with more than one operator.
jshell > 5 + 5 * 6
$1 ==> 35
jshell > 5 - 2 * 2
$2 ==> 1
jshell > 5 - 2 / 2
$3 ==> 4
jshell > Surprised with the results? You might expect 5 + 5 * 6 evaluate to 10 * 6 i.e. 60. Howeever, we got 35!
We write English left-to-right, and carry this habit to calculations as well.
In expressions with multiple operators, the order of sub-expression evaluation depends on operator precedence .
The basic rules for operator precedence are actually quite simple (we will look at other rules a little later).
The operators in the set {*, /, %} have higher precedence than the operators in the set {+, -}.
In the expression 5 + 5 * 6 : 5*6 is evaluated first. So, 5 + 5 * 6 becomes 5 + 30 i.e. 35.
5 - 2 * 2 and 5 - 2 / 2 are evaluated by following the same rules.
Java provides syntax elements, called parentheses ( and ), to group parts of an expression.
jshell > (5 - 2 ) * 2
$4 ==> 6
jshell > 5 - (2 * 2 )
$5 ==> 1
jshell > When you put an expression in parenthesis, it is evaluated first. (5 - 2) * 2 => 3 * 2 => 6.
Parentheses lead to better readability as they reduce confusion and help avoid errors.
The old adage A stitch in time saves nine
In this step, we:
Discovered that operators are all predefined
Learned that result of operation depends on operand types
Understood what operator precedence means
Used parentheses to group parts of an expression
We have computed the product of two literals (as in 5 * 3) in earlier steps.
The next step is to print this result in a customized format - 5 * 3 = 15.
How do we do this?
Let try typing it in into JShell
jshell > 5 * 3 = 15
| Error :
| unexpected type
| required : variable
| found : value
| 5 * 3 = 15
| ^---^Hmm! Error.
How do we print text?
Java has a built-in utility method called System.out.println(), that displays text on the console.
jshell > System .out .println (3 *4 )
12 We formed an expression, 3*4, and passed it to System.out.println(), which is a built-in Java method .
System.out.println(3*4) is an example of a statement . It is a method call .
The syntax rules for method calls are quite strict, and all its parts are mandatory.
jshell > System .out .println3 *4 )
| Error :
| ';' expected
| System .out .println3 *4 )
|___________________ ^
| Error :
| cannot find symbol
| symbol : variable println3
| System .out .println3 *4 )
| ^---------------------^
jshell > System .out .println 3 *4
| Error :
| ';' expected
| System .out .println 3 *4
| ^
| Error :
| cannot find symbol
| symbol : variable println
| System .out .println 3 *4
| ^----------------What if we want to print an entry of the Multiplication Table, as part of our solution to PMT-Challenge ? In other words, how do we print the exact text 5 * 2 = 10 on the console?
jshell > System .out .println (5 * 2 = 10 )
| Error :
| unexpected type
| required : variable
| found : value
| System .out .println (5 * 2 = 10 )
|____________________ ^--------^ You wanted to print 5 * 2 = 10 on the console. However, we see that we cannot pass 5 * 2 = 10 as an argument to System.out.println().
5 * 2 = 10 is not a single value. It is a piece of text with numeric characters, * and =.
In Java, we represent text using String. A String literal is a sequence of characters, enclosed within double quotes : " and ".
jshell > System .out .println ("5 * 2 = 10" )
| 5 * 2 = 10 Congratulations! You have now figured out how to display not just numbers on the console, but text as well!
In this step, we:
Were introduced to the System.out.println() method for console output
Used this utility to print a single PMT-Challenge table entry
Try and solve the following exercises:
Print Hello World onto the console.
Print 5 * 3, as is.
Print the calculated value of 5 * 3.
Print the number of seconds in a day, using the System.out.println method.
Do a syntax revision for the code that you write for each of the above exercises. In your code, identify the following elements:
Numeric and string literals
Expressions
Operators
Operands
Method calls
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