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Using underscore to improve readability

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Since Java 7 it has been possible to use one or more underscores (_) for separating groups of digits in a primitive number literal to improve their readability.

For instance, these two declarations are equivalent: 

int i1 = 123456;
int i2 = 123_456;
System.out.println(i1 == i2); // true

This can be applied to all primitive number literals as shown below: 

byte color = 1_2_3;
short yearsAnnoDomini= 2_016;
int socialSecurtyNumber = 999_99_9999;
long creditCardNumber = 1234_5678_9012_3456L;
float piFourDecimals =  3.14_15F;
double piTenDecimals =  3.14_15_92_65_35;

This also works using prefixes for binary, octal and hexadecimal bases: 

short binary= 0b0_1_0_1;
int octal = 07_7_7_7_7_7_7_7_0;
long hexBytes = 0xFF_EC_DE_5E;

There are a few rules about underscores which forbid their placement in the following places:

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Primitive Data Types:
* Primitive Data Types
* Comparing floating point values
* The char primitive
* Primitive Types Cheatsheet
* Using underscore to improve readability
* The float primitive
* Converting Primitives
* The int primitive
* Memory consumption of primitives vs. boxed primitives
* The double primitive
* The byte primitive
* The boolean primitive
* The long primitive
* Hexadecimal Octal and Binary literals
* Negative value representation
* The short primitive
Table Of Contents
0 Inheritance
1 Getting started with Java
2 Streams
3 Exceptions
4 Collections
5 Lambda Expressions
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25 JAXB
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27 Networking
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84 The Classpath
85 Bytecode Modification
86 XML Parsing using JAXP
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89 JAX-WS
90 XML XPath Evaluation
91 Performance Tuning
92 Parallel programming with ForkJoin framework
93 Common Pitfalls
94 Non-Access Modifiers
95 Java Compiler
96 XJC
97 Installing Java Standard Edition
98 Process
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100 Dates and Time using java.time
101 Fluent Interface
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103 Just in Time JIT compiler
104 FTP File Transfer Protocol
105 Java Native Access JNI
106 Modules
107 Pitfalls - exceptions
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109 ServiceLoader
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111 Object References
112 Pitfalss - performance
113 Creating Images Programmatically
114 Applets
115 NIO Networking
116 New File I/O
117 Secure objects
118 Pitfalls - threads and concurrency
119 Splitting a string into fixed length parts
120 Pitfalls - nulls and NullPointerException
121 SecurityManager
122 JNDI
123 super keyword
124 java.util.Objects class
125 Java command: java and javaw
126 Atomic types
127 Floating Point Operations
128 Converting to and from strings
129 sun.misc.Unsafe
130 Java Memory Model
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133 Queues and deques
134 Runtime commands
135 NumberFormat
136 Security and Cryptograhy
137 Java Virtual Machine JVM
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139 JavaBean
140 Expressions
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142 Java SE 8 Features
143 Java SE 7 Features
144 Packages
145 Concurrency and Money
146 Concurrent Collections
147 Using ThreadPoolExecutor in multi-threaded applications
148 Java Editions, versions, releases and distributions
149 Dynamic method dispatch
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151 Security and cryptography
152 Generating Java Code
153 JShell
154 Benchmarks
155 Collection Factory Methods
156 Multi-release JAR files
157 Stack-Walking API
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159 Sockets
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161 Using other scripting languages in Java
162 Functional interfaces
163 List vs. set
164 2D graphics
165 Reflection
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167 Enum Map
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169 Local Inner class
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171 Immutable class
172 String Tokenizer
173 File upload to S3 / AWS
174 Readers and writers
175 Hashtable
176 Enum starting with a number
177 SortedMap
178 WeakHashMap
179 LinkedHashMap
180 StringBuffer
181 Choosing Collections
182 C++ Comparison
183 CompletableFuture
184 Contributors