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Tom Hermann
committedMar 2, 2021
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‎README.md

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### Problem
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When writing JUnit tests, you want your test cases to be as descriptive and concise as possible. File I/O complicates
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matters because one must set up any necessary state, invoke functionality of the class under test, and clean up
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afterwards.
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### Options
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- JUnit4: The `@TemporaryFolder` rule
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- JUnit5: The `@TempDir` annotation
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- Mocking `File`/`Path`: Avoid this whenever possible
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### Demo Time!
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To test the different approaches, we created a class called `TextFileTranformer`, its responsibility is to stream each
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line of an input file, apply a transformation, and write the transformed line to an output file.
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Let's start off with a JUnit5 test. As with all tests, we need an instance of our class under test. So we'll go ahead
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and new one up as a field.
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The class under test happens to use a `Function<String, String>` that it will apply to each
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line of the input file. For the sake of this test we'll simply uppercase the input.
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```java
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import ...
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@DisplayName("Test file IO with JUnit5")
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class TextFileTransformerTest {
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private final TextFileTransformer testObject = new TextFileTransformer(String::toUpperCase);
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}
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```
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Now that we have a instance to test, lets start using it.
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```java
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@Test
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@DisplayName("Should transform multiline source to destination")
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void transformManyLines(@TempDir Path tempDir) throws IOException {
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Path destination = tempDir.resolve("destination.txt");
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Path source = tempDir.resolve("source.txt");
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Files.write(source, List.of("Larry", "Curly", "Moe"));
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testObject.transform(source, destination);
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assertEquals(List.of("LARRY", "CURLY", "MOE"), Files.readAllLines(destination));
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}
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```
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### Additional Resources

‎build.gradle

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plugins {
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id 'java'
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}
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group 'com.wwt'
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version '1.0.0'
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repositories {
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mavenCentral()
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}
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java {
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toolchain {
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languageVersion = JavaLanguageVersion.of(11)
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}
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}
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dependencies {
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implementation 'ch.qos.logback:logback-classic:1.2.3'
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testImplementation(platform('org.junit:junit-bom:5.7.1'))
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testImplementation('org.junit.jupiter:junit-jupiter')
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// The following dependencies are included to demonstrate LEGACY JUnit4 @Rule's
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// Do not include unless you are stuck on JUnit4!
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testImplementation("org.junit.jupiter:junit-jupiter-migrationsupport")
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testRuntimeOnly("org.junit.vintage:junit-vintage-engine") {
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because 'Allows JUnit4 tests to run'
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}
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}
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test {
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useJUnitPlatform()
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testLogging {
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events "passed", "skipped", "failed"
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showStandardStreams = true
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}
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}

‎gradle.properties

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kotlin.code.style=official

‎gradle/wrapper/gradle-wrapper.jar

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distributionBase=GRADLE_USER_HOME
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distributionPath=wrapper/dists
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distributionUrl=https\://services.gradle.org/distributions/gradle-6.8.3-bin.zip
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zipStoreBase=GRADLE_USER_HOME
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zipStorePath=wrapper/dists

‎gradlew

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#!/usr/bin/env sh
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#
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# Copyright 2015 the original author or authors.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# https://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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#
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##############################################################################
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##
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## Gradle start up script for UN*X
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##
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##############################################################################
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# Attempt to set APP_HOME
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# Resolve links: $0 may be a link
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PRG="$0"
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# Need this for relative symlinks.
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while [ -h "$PRG" ] ; do
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ls=`ls -ld "$PRG"`
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link=`expr "$ls" : '.*-> \(.*\)$'`
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if expr "$link" : '/.*' > /dev/null; then
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PRG="$link"
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else
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PRG=`dirname "$PRG"`"/$link"
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fi
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done
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SAVED="`pwd`"
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cd "`dirname \"$PRG\"`/" >/dev/null
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APP_HOME="`pwd -P`"
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cd "$SAVED" >/dev/null
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APP_NAME="Gradle"
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APP_BASE_NAME=`basename "$0"`
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# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
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DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'
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# Use the maximum available, or set MAX_FD != -1 to use that value.
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MAX_FD="maximum"
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warn () {
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echo "$*"
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}
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die () {
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echo
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echo "$*"
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echo
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exit 1
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}
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# OS specific support (must be 'true' or 'false').
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cygwin=false
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msys=false
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darwin=false
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nonstop=false
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case "`uname`" in
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CYGWIN* )
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cygwin=true
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;;
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Darwin* )
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darwin=true
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;;
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MINGW* )
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msys=true
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;;
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NONSTOP* )
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nonstop=true
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;;
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esac
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CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
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# Determine the Java command to use to start the JVM.
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if [ -n "$JAVA_HOME" ] ; then
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if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
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# IBM's JDK on AIX uses strange locations for the executables
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JAVACMD="$JAVA_HOME/jre/sh/java"
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else
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JAVACMD="$JAVA_HOME/bin/java"
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fi
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if [ ! -x "$JAVACMD" ] ; then
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die "ERROR: JAVA_HOME is set to an invalid directory: $JAVA_HOME
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Please set the JAVA_HOME variable in your environment to match the
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location of your Java installation."
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fi
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else
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JAVACMD="java"
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which java >/dev/null 2>&1 || die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
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Please set the JAVA_HOME variable in your environment to match the
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location of your Java installation."
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fi
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# Increase the maximum file descriptors if we can.
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if [ "$cygwin" = "false" -a "$darwin" = "false" -a "$nonstop" = "false" ] ; then
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MAX_FD_LIMIT=`ulimit -H -n`
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if [ $? -eq 0 ] ; then
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if [ "$MAX_FD" = "maximum" -o "$MAX_FD" = "max" ] ; then
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MAX_FD="$MAX_FD_LIMIT"
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fi
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ulimit -n $MAX_FD
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if [ $? -ne 0 ] ; then
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warn "Could not set maximum file descriptor limit: $MAX_FD"
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fi
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else
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warn "Could not query maximum file descriptor limit: $MAX_FD_LIMIT"
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fi
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fi
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# For Darwin, add options to specify how the application appears in the dock
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if $darwin; then
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GRADLE_OPTS="$GRADLE_OPTS \"-Xdock:name=$APP_NAME\" \"-Xdock:icon=$APP_HOME/media/gradle.icns\""
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fi
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# For Cygwin or MSYS, switch paths to Windows format before running java
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if [ "$cygwin" = "true" -o "$msys" = "true" ] ; then
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APP_HOME=`cygpath --path --mixed "$APP_HOME"`
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CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
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JAVACMD=`cygpath --unix "$JAVACMD"`
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# We build the pattern for arguments to be converted via cygpath
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ROOTDIRSRAW=`find -L / -maxdepth 1 -mindepth 1 -type d 2>/dev/null`
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SEP=""
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for dir in $ROOTDIRSRAW ; do
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ROOTDIRS="$ROOTDIRS$SEP$dir"
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SEP="|"
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done
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OURCYGPATTERN="(^($ROOTDIRS))"
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# Add a user-defined pattern to the cygpath arguments
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if [ "$GRADLE_CYGPATTERN" != "" ] ; then
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OURCYGPATTERN="$OURCYGPATTERN|($GRADLE_CYGPATTERN)"
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fi
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# Now convert the arguments - kludge to limit ourselves to /bin/sh
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i=0
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for arg in "$@" ; do
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CHECK=`echo "$arg"|egrep -c "$OURCYGPATTERN" -`
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CHECK2=`echo "$arg"|egrep -c "^-"` ### Determine if an option
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if [ $CHECK -ne 0 ] && [ $CHECK2 -eq 0 ] ; then ### Added a condition
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eval `echo args$i`=`cygpath --path --ignore --mixed "$arg"`
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else
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eval `echo args$i`="\"$arg\""
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fi
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i=`expr $i + 1`
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done
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case $i in
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0) set -- ;;
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1) set -- "$args0" ;;
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2) set -- "$args0" "$args1" ;;
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3) set -- "$args0" "$args1" "$args2" ;;
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4) set -- "$args0" "$args1" "$args2" "$args3" ;;
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5) set -- "$args0" "$args1" "$args2" "$args3" "$args4" ;;
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6) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" ;;
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7) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" ;;
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8) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" ;;
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9) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" "$args8" ;;
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esac
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fi
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# Escape application args
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save () {
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for i do printf %s\\n "$i" | sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/' \\\\/" ; done
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echo " "
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}
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APP_ARGS=`save "$@"`
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# Collect all arguments for the java command, following the shell quoting and substitution rules
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eval set -- $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS "\"-Dorg.gradle.appname=$APP_BASE_NAME\"" -classpath "\"$CLASSPATH\"" org.gradle.wrapper.GradleWrapperMain "$APP_ARGS"
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exec "$JAVACMD" "$@"

‎gradlew.bat

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@rem
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@rem Copyright 2015 the original author or authors.
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@rem
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@rem Licensed under the Apache License, Version 2.0 (the "License");
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@rem you may not use this file except in compliance with the License.
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@rem You may obtain a copy of the License at
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@rem
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@rem https://www.apache.org/licenses/LICENSE-2.0
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@rem
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@rem Unless required by applicable law or agreed to in writing, software
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@rem distributed under the License is distributed on an "AS IS" BASIS,
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@rem WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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@rem See the License for the specific language governing permissions and
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@rem limitations under the License.
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@rem
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@if "%DEBUG%" == "" @echo off
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@rem ##########################################################################
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@rem
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@rem Gradle startup script for Windows
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@rem
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@rem ##########################################################################
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@rem Set local scope for the variables with windows NT shell
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if "%OS%"=="Windows_NT" setlocal
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set DIRNAME=%~dp0
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if "%DIRNAME%" == "" set DIRNAME=.
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set APP_BASE_NAME=%~n0
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set APP_HOME=%DIRNAME%
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@rem Resolve any "." and ".." in APP_HOME to make it shorter.
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for %%i in ("%APP_HOME%") do set APP_HOME=%%~fi
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@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
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set DEFAULT_JVM_OPTS="-Xmx64m" "-Xms64m"
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@rem Find java.exe
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if defined JAVA_HOME goto findJavaFromJavaHome
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set JAVA_EXE=java.exe
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%JAVA_EXE% -version >NUL 2>&1
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if "%ERRORLEVEL%" == "0" goto execute
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echo.
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echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
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echo.
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echo Please set the JAVA_HOME variable in your environment to match the
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echo location of your Java installation.
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goto fail
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:findJavaFromJavaHome
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set JAVA_HOME=%JAVA_HOME:"=%
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set JAVA_EXE=%JAVA_HOME%/bin/java.exe
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if exist "%JAVA_EXE%" goto execute
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echo.
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echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME%
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echo.
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echo Please set the JAVA_HOME variable in your environment to match the
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echo location of your Java installation.
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goto fail
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:execute
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@rem Setup the command line
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set CLASSPATH=%APP_HOME%\gradle\wrapper\gradle-wrapper.jar
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@rem Execute Gradle
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"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -classpath "%CLASSPATH%" org.gradle.wrapper.GradleWrapperMain %*
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:end
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@rem End local scope for the variables with windows NT shell
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if "%ERRORLEVEL%"=="0" goto mainEnd
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:fail
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rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
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rem the _cmd.exe /c_ return code!
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if not "" == "%GRADLE_EXIT_CONSOLE%" exit 1
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exit /b 1
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:mainEnd
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if "%OS%"=="Windows_NT" endlocal
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:omega

‎settings.gradle

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rootProject.name = 'testing-file-io-junit'
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package com.wwt.testing.files;
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import java.util.function.Predicate;
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class Preconditions {
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static <T> void checkArgument(T argument, Predicate<T> check, String message) {
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if (!check.test(argument)) {
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throw new IllegalArgumentException(message);
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}
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}
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private Preconditions() {
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}
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}

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