Java Compiler - javac

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The 'javac' command - getting started

Simple example

Assuming that the "TutorJoes.java" contains the following Java source:

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

(For an explanation of the above code, please refer to Getting started with Java Language .

We can compile the above file using this command:

javac TutorJoes.java

This produces a file called "TutorJoes.class", which we can then run as follows:

java TutorJoes.java
Tutor Joes!

The key points to note from this example are:

• The source filename "TutorJoes.java" must match the class name in the source file ... which is TutorJoes. If they don't match, you will get a compilation error.
• The bytecode filename "TutorJoes.class" corresponds to the classname. If you were to rename the "TutorJoes.class", you would get an error when your tried to run it.
• When running a Java application using java, you supply the classname NOT the bytecode filename.

Example with packages

Most practical Java code uses packages to organize the namespace for classes and reduce the risk of accidental class name collision.

If we wanted to declare the TutorJoes class in a package call com.example, the "TutorJoes.java" would contain the following Java source:

package com.example;
 
public class TutorJoes
{
	public static void main(String[] args)
	{
		System.out.println("Tutor Joes!");
	}
}

This source code file needs to stored in a directory tree whose structure corresponds to the package naming.

# the current directory (for this example)
|
----com
	|
	----example
		|
		----TutorJoes.java

We can compile the above file using this command:

javac com/example/TutorJoes.java	

This produces a file called "com/example/TutorJoes.class"; i.e. after compilation, the file structure should look like this:

# the current directory (for this example)
|
----com
	|
	----example
		|
		----TutorJoes.java
		----TutorJoes.class

We can then run the application as follows:

java com.example.TutorJoes
Tutor Joes!	

Additional points to note from this example are:

• The directory structure must match the package name structure.
• When you run the class, the full class name must be supplied; i.e. "com.example.TutorJoes" not "TutorJoes".
• You don't have to compile and run Java code out of the current directory. We are just doing it here for illustration.

Compiling multiple files at once with 'javac'.

If your application consists of multiple source code files (and most do!) you can compile them one at a time. Alternatively, you can compile multiple files at the same time by listing the pathnames:

javac Foo.java Bar.java

or using your command shell's filename wildcard functionality ...

$ javac *.java
$ javac com/example/*.java
$ javac */**/*.java #Only works on Zsh or with globstar enabled on your shell

This will compile all Java source files in the current directory, in the "com/example" directory, and recursively in child directories respectively. A third alternative is to supply a list of source filenames (and compiler options) as a file. For example:

javac @sourcefiles

where the sourcefiles file contains:

Foo.java
Bar.java
com/example/TutorJoes.java

Note : compiling code like this is appropriate for small one-person projects, and for once-off programs. Beyond that, it is advisable to select and use a Java build tool. Alternatively, most programmers use a Java IDE (e.g. NetBeans, eclipse, IntelliJ IDEA) which offers an embedded compiler and incremental building of "projects".

Commonly used 'javac' options

Here are a few options for the javac command that are likely to be useful to you

• The -d option sets a destination directory for writing the ".class" files.
• The -sourcepath option sets a source code search path.
• The -cp or -classpath option sets the search path for finding external and previously compiled classes. For more information on the classpath and how to specify it, refer to the The Classpath Topic.
• The -version option prints the compiler's version information.

A more complete list of compiler options will be described in a separate exampl

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Compiling for a different version of Java

The Java programming language (and its runtime) has undergone numerous changes since its release since its initial public release. These changes include

• Changes in the Java programming language syntax and semantics
• Changes in the APIs provided by the Java standard class libraries.
• Changes in the Java (bytecode) instruction set and classfile format.

With very few exceptions (for example the enum keyword, changes to some "internal" classes, etc), these changes are backwards compatible.

• A Java program that was compiled using an older version of the Java toolchain will run on a newer version Java platform without recompilation.
• A Java program that was written in an older version of Java will compile successfully with a new Java compiler.

Compiling old Java with a newer compiler

If you need to (re-)compile older Java code on a newer Java platform to run on the newer platform, you generally don't need to give any special compilation flags. In a few cases (e.g. if you had used enum as an identifier) you could use the -source option to disable the new syntax. For example, given the following class:

public class OldSyntax
{
	private static int enum; // invalid in Java 5 or later
}

the following is required to compile the class using a Java 5 compiler (or later):

 javac -source 1.4 OldSyntax.java

Compiling for an older execution platform

If you need to compile Java to run on an older Java platforms, the simplest approach is to install a JDK for the oldest version you need to support, and use that JDK's compiler in your builds.

You can also compile with a newer Java compiler, but there are complicated. First of all, there some important preconditions that must be satisfied:

• The code you are compiling must not use Java language constructs that were not available in the version of Java that you are targeting.
• The code must not depend on standard Java classes, fields, methods and so on that were not available in the older platforms.
• Third party libraries that the code depends must also be built for the older platform and available at compile-time and run-time.

Given the preconditions are met, you can recompile code for an older platform using the -target option. For example,

javac -target 1.4 SomeClass.java
 

will compile the above class to produce bytecodes that are compatible with Java 1.4 or later JVM. (In fact, the - source option implies a compatible -target, so javac -source 1.4 ... would have the same effect. The relationship between -source and -target is described in the Oracle documentation.)

Having said that, if you simply use -target or -source, you will still be compiling against the standard class libraries provided by the compiler's JDK. If you are not careful, you can end up with classes with the correct bytecode version, but with dependencies on APIs that are not available. The solution is to use the -bootclasspath option. For example:

javac -target 1.4 --bootclasspath path/to/java1.4/rt.jar SomeClass.java

will compile against an alternative set of runtime libraries. If the class being compiled has (accidental) dependencies on newer libraries, this will give you compilation errors.

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Basic Programs

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• Creating Your First Java Program
• Type Conversion
• Getters and Setters
• Reference Data Types
• Java Compiler - javac
• Documenting Java Code
• Command line Argument Processing
• Java Command - java and javaw
• Literals
• Primitive Data Types

Strings in Java

• Comparing Strings
• Changing the case of characters within a String
• Finding a String Within Another String
• String pool and heap storage
• Splitting Strings
• Joining Strings with a delimiter and Reversing Strings
• String concatenation and StringBuilders
• Substrings and Platform Independent new line Separator
• Adding toString() method for custom objects
• Remove Whitespace from the Beginning and End of a String and Case insensitive switch
• Replacing parts of Strings and Getting the nth character in a String
• Counting occurrences of a substring or character in a string

StringBuffer and StringBuilder

• String Buffer
• String Builder
• String Tokenizer and Splitting a string into fixed length parts

Date Class

• Convert java.util.Date to java.sql.Date and Basic date
• LocalDate and LocalDateTime objects
• Creating a Specific Date and Converting Date to String format
• Convert formatted string representation of date to Date object
• Comparing Date objects
• Converting String into Date and Time Zones and java.util.Date

Dates and Time (java.time.*)

• Dates and Time
• Date Time Formatting and Date Manipulations
• LocalTime

BigDecimal

• Comparing and Using BigDecimals
• BigDecimal.valueOf()
• Mathematical operations with BigDecimal

BigInteger

• Initialization and Generating random BigIntegers
• BigInteger Mathematical Operations Examples
• NumberFormat

Bit Manipulation

• Checking, setting, clearing, and toggling individual bits. Using long as bit mask
• Checking if a number is a power of 2
• Signed vs unsigned shift and Expressing the power of 2

Arrays

• Creating and Initializing Arrays
• Creating a List from an Array
• Multidimensional and Jagged Arrays
• Array Covariance and Arrays to Stream
• Iterating over arrays
• Sorting arrays and Length of an Array
• Finding an element in an array nd change the size of an array
• Primitives and boxed types and Remove an element from an array
• Comparing arrays for equality and Copying arrays

Collections

• Removing items from a List within a loop
• Constructing collections from existing data
• ArrayList and adding objects and Iterating over Collections
• Immutable Empty , Sub and Unmodifiable Collection
• Removing matching items from Lists and Join lists
• Creating your own Iterable structure for use with Iterator or for-each loop

Lists

• Sorting a generic list
• Classes implementing List
• Replacement of a List element and Moving objects around in the list
• Creating, Adding and Removing element from an ArrayList
• Positional Access Operations and Iterating over elements in a list

Sets

• Initialization and Basics of Set
• Types and Usage of Sets

Maps

• Iterating Map Entries Effciently
• Using Default Methods of Map from Java 8
• Usage of HashMap and Iterating through the contents of a Map
• Merging, combine and composing Maps and Add multiple items
• Creating and Initializing Maps
• Check if key exists, Add an element and Clear the map
• LinkedHashMap, WeakHashMap and SortedMap

TreeMap and TreeSet

• TreeMap and TreeSet of a simple Java type
• TreeMap and TreeSet of a custom Java type
• TreeMap and TreeSet Thread Safety

Queues and Deques

• Usage of the Queue and Deque
• Stacks and BlockingQueue
• LinkedList as a FIFO Queue and Queue Interface
• Dequeue Interface

Enums

• Declaring and using a basic enum
• Enums with constructors and Abstract Methods
• Implements Interface and Singleton pattern
• Using methods and static blocks and Zero instance enum
• Enum constant specific body and Getting the values of an enum
• Compare and Contains for Enum values and Get enum constant by name
• Convert enum to String and Enums with static fields
• Enum Map , EnumSet class, Enum starting with number and Hashtable

Operators

• Increment/Decrement and Conditional Operator
• Bitwise, Logical and String Concatenation Operators
• Arithmetic Operators
• Shift and Instanceof Operators
• Assignment Operators
• Conditional-and ,Conditional-or and Relational Operators
• Equality and Lambda operator

Constructors

• Default and Call parent Constructor

Object Class Methods and Constructor

• hashCode() method
• toString() and finalize() method
• equals() method
• wait(), notify(), getClass(), clone() methods
• Object constructor

Annotations

• Defining annotation types
• Built-in annotations
• Compile time processing using annotation processor
• Repeating and Inherited Annotations
• Getting Annotation values at run-time and Annotations for 'this' and receiver parameters

Immutable Class

• What is the advantage of immutability and Rules and Example

Immutable Objects

• Creating an immutable version of a type using defensive copying
• Typical design flaws which prevent a class from being immutable

Visibility (controlling access to members of a class)

• Private ,Public ,Package and Protected Visibility

Generics

• Creating a Generic Class
• Diamond and Declaring a Generic Method
• Generic parameter and Benefits of Generic class and interface
• Creating a Bounded Generic Class
• Generics to auto-cast and Use of instanceof with Generics

Classes and Objects

• Method Overloading and Overriding
• Constructors and Initializing static final
• Basic Object Construction and Use

Nested and Inner Classes

• Static vs Non Static Nested Classes
• Anonymous Inner Classes and non-static inner class

The java.util.Objects Class

• Basic use for object null check and Objects.nonNull() method

Default Methods

• Why Use, Usage and Accessing overridden of default methods
• interface methods, multiple inheritance and Abstract class

Packages

• Using Packages to create classes and Protected Scope

Inheritance

• What is Inheritance
• Abstract Classes
• Abstract class and Interface usage
• Programming to an interface
• Overriding in Inheritance, Variable shadowing and Narrowing and Widening

Reference Types

• Different Reference Types

Console I/O

• Reading user input from the console
• Aligning strings in console and Implementing Basic Command-Line Behavior

Streams

• Using Streams
• Creating a Frequency Map, Infinite and Consuming Streams
• Collect Elements of a Stream into a Collection
• Flatten Streams, Parallel Stream and Implement Mathematical Functions
• Creating a Stream and Finding Statistics about Numerical Streams
• Concatenate Streams and Reduction with Streams
• IntStream to String and Finding the First Element that Matches a Predicate
• Create a Map based on a Stream and Joining a stream to a single String
• Streams of Primitives and Stream operations categories

InputStreams and OutputStreams

• Reading,Writing,Copying,Closing,Wrapping and InputStream

Readers and Writers

• BufferedReader and StringWriter Example

Preferences

• Using preferences, Adding event listeners and Getting sub-nodes of Preferences
• Exporting, Importing preferences and Getting, Setting preferences values

Collection Factory Methods

• List<E>, Set<E> and Map<K, V> Factory Method Examples

Alternative Collections

• Multimap in Guava, Apache and Eclipse Collections
• Apache HashBag, Guava HashMultiset and Eclipse HashBag
• Compare operation with collections - Create collections

Concurrent Collections

• Insertion into ConcurrentHashMap, Thread-safe and Concurrent Collections

super keyword

• Super keyword use with examples

Serialization

• Basic and Custom Serialization
• Serialization with Gson, Versioning and serialVersionUID
• Custom JSON Deserialization with Jackson

Optional

• Map,Return default value and Throw an exception
• Filter, FlatMap andUsing Optional containers for primitive number types

Object References

• Object References as method parameters

Exceptions and exception handling

• Catching an exception with try-catch
• The try-with-resources statement
• Custom Exceptions and Handling InterruptedException
• Introduction and Return statements in try catch block
• The Java Exception Hierarchy - Unchecked and Checked Exceptions
• Creating and reading stacktraces
• Throwing an exception and Advanced features of Exceptions
• try-catch-finally statements and throws clause in a method declaration

Calendar and its Subclasses

• Creating, Subtracting and Finding AM/PM Calendar

Using the static keyword

• Reference to non-static member from static context

Properties Class

• Reference to non-static member from static context

Lambda Expressions

• Introduction to Java lambdas
• Using Lambda Expressions to Sort a Collection
• Implementing multiple interfaces, Listener and Java Closures with lambda expressions
• Memory utilization, own functional interface and Traditional style to Lambda
• return only returns from the lambda and Execute-around Pattern

Basic Control Structures

• Switch, do while, For Each, Continue Statement in Java
• If, Else If, Else Control, For Loops and Ternary Operator
• Try-Catch-Finally, Break, While Loops, If Else and Nested break / continue

BufferedWriter

• Write a line of text to File

New File I/O

• Creating paths, Manipulating, Retrieving, Reading and Writing files

File I/O

• Migrating from java.io.File to Java 7 NIO
• Reading an image, File Read/Write, Reading all bytes and Copying a file
• Stream vs Writer/Reader API and Reading a file with a Scanner
• Copying a file using InputStream and OutputStream
• Blocking or redirecting standard output / error
• Reading a file using BufferedInputStream

Scanner

• General Pattern that does most commonly asked about tasks
• Custom delimiters, Reading system input and Reading file input using Scanner

Interfaces

• Implementing multiple interfaces, Declaring and Implementing an Interface
• Usefulness of interfaces and Default methods
• Using Interfaces with Generics and Strengthen bounded type parameters

Regular Expressions

• Using capture groups and Escape Characters
• Matching a backslash, Not matching string and Matching with a regex literal

Comparable and Comparator

• Sorting a List using Comparable<T> or a Comparator<T>
• compareTo, compare Methods and Natural vs explicit sorting

Java Floating Point Operations

• Comparing floating point values
• OverFlow and UnderFlow, Formatting the floating point values

Currency and Money

• Add custom currency

Object Cloning

• Cloning performing a deep copy and copy factory
• Cloning using a copy constructor and implementing Clonable interface

Recursion

• Basic idea of Recursion
• StackOverflowError & recursion to loop
• Computing the Nth Fibonacci Number and power of a number

Converting to and from Strings

• Converting String to other datatypes
• Converting other datatypes to String

Random Number Generation

• Pseudo Random Numbers in Specific Range
• Generating Random Numbers with a Specified Seed

Singletons

• Enum Singleton and Singleton without use of Enum, Thread safe Singleton
• Extending singleton (singleton inheritance)

Autoboxing

• Using int and Integer interchangeably and Auto-unboxing may lead to NullPointerException
• Different Cases When Integer and int can be used interchangeably

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