Working helloworld with test

src/main/java/eu/mateuszpiela/helloworld/Main.java

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+package eu.mateuszpiela.helloworld;
+
+public class Main {
+    public static void main(String[] args) {
+
+        System.out.println("Hello world from java");
+    }
+}

src/test/java/eu/mateuszpiela/helloworld/MainTest.java

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+package eu.mateuszpiela.helloworld;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.lang.reflect.Array;
+
+import static org.junit.Assert.assertEquals;
+
+public class MainTest {
+    private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
+    private final ByteArrayOutputStream errContent = new ByteArrayOutputStream();
+    private final PrintStream originalOut = System.out;
+    private final PrintStream originalErr = System.err;
+
+    @Before
+    public void setUpStreams() {
+        System.setOut(new PrintStream(outContent));
+        System.setErr(new PrintStream(errContent));
+    }
+
+    @Test
+    public void AssertConsoleOutputIsHelloWorld() {
+        String[] args = {};
+        eu.mateuszpiela.helloworld.Main.main(args);
+        assertEquals("Hello world from java" + System.lineSeparator(), outContent.toString());
+    }
+
+    @After
+    public void restoreStreams() {
+        System.setOut(originalOut);
+        System.setErr(originalErr);
+    }
+}