Add Java implementation

.gitignore

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+.DS_Store
+/ref/java/out
+/ref/java/build
+/ref/java/.gradle

ref/java/README.md

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+# Bech32 Java
+
+## Usage
+
+This [Bech32](https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki) implementation is 
+designed to be a standalone drop-in library for your Java and Android projects. <br/>
+
+Based entirely on the Java Standard library, it allows either to validate a **Bech32 String** or a **Bech32 Segwit Address**. <br/>
+In the latter case it's possible to decode, with the **SegwitAddress** class, an address in a way that throws an exception with 
+the releated issue if it's an invalid one. Otherwise it's possible to get back a *null* object if the
+address is invalid. 
+
+
+```java
+Bech32 bech32 = new Bech32();
+
+// Returns a valid Bech32Decoded object
+mBech32.decode("A12UEL5L");
+
+// Throws a Bech32ValidationException
+mBech32.decode("x1b4n0q5v"); 
+```
+```java
+SegwitAddress segwitAddress = new SegwitAddress();
+
+// Returns a valid SegwitAddress object
+segwitAddress.decode("bc1zw508d6qejxtdg4y5r3zarvaryvg6kdaj", "bc");
+
+// Returns a null object
+segwitAddress.decode("bc1zw508d6qejxtdg4y5r3zarvaryvqyzf3du", "bc");
+
+// Throws a Bech32ValidationException
+segwitAddress.decodeThrowing("bc1qw508d6qejxtdg4y5r3zarvary0c5xw7kv8f3t5", "bc");
+```
+
+## Running tests
+
+The project is integrated with Gradle, but thanks to its design can be built using your favourite build system. </br>
+It depends on JUnit's `org.junit.Assert` class in order to assert the test results. <br/> 
+In order to run the tests launch via the command line these two gradle tasks. <br/> 
+Note that a **gradlew** binary is already included when cloning this repository.
+
+```
+$ ./gradlew --refresh-dependencies
+$ ./gradlew test
+
+BUILD SUCCESSFUL in 0s
+3 actionable tasks: 3 up-to-date
+```
+

ref/java/build.gradle

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+plugins {
+    id 'java'
+}
+
+group 'com.conio.wallet'
+version '1.0'
+
+sourceCompatibility = 1.8
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    testCompile group: 'junit', name: 'junit', version: '4.12'
+}

ref/java/gradle/wrapper/gradle-wrapper.properties

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.10-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists