Add unit tests for me.zhyd.oauth.utils.StringUtils

These tests were written using Diffblue Cover.

src/test/java/me/zhyd/oauth/utils/StringUtilsTest.java

+package me.zhyd.oauth.utils;
+
+import org.junit.Assert;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+public class StringUtilsTest {
+    @Rule public final ExpectedException thrown =
+        ExpectedException.none();
+
+    @Test
+    public void isEmptyNonEmptyInput() {
+        Assert.assertFalse(StringUtils.isEmpty("non-empty string"));
+    }
+
+    @Test
+    public void isEmptyEmptyInput() {
+        Assert.assertTrue(StringUtils.isEmpty(""));
+    }
+
+    @Test
+    public void isEmptyInputNull() {
+        Assert.assertTrue(StringUtils.isEmpty(null));
+    }
+
+    @Test
+    public void isNotEmptyNonEmptyInput() {
+        Assert.assertTrue(StringUtils.isNotEmpty("non-empty string"));
+    }
+
+    @Test
+    public void isNotEmptyEmptyInput() {
+        Assert.assertFalse(StringUtils.isNotEmpty(""));
+    }
+
+    @Test
+    public void isNotEmptyInputNull() {
+        Assert.assertFalse(StringUtils.isNotEmpty(null));
+    }
+
+    @Test
+    public void appendIfNotContainAppendedStringNotPresent() {
+        // (Check the case where appendStr doesn't occur in str)
+        final String str = "Prefix ";
+        final String appendStr = "suffix";
+        final String otherwise = "should be discarded";
+
+        final String result =
+            StringUtils.appendIfNotContain(str, appendStr, otherwise);
+
+        Assert.assertEquals("Prefix suffix", result);
+    }
+
+    @Test
+    public void appendIfNotContainAppendedStringPresent() {
+        // (Check the case where appendStr occurs in str)
+        final String str = "Prefix ";
+        final String appendStr = "Prefix";
+        final String otherwise = "should be appended";
+
+        final String result =
+            StringUtils.appendIfNotContain(str, appendStr, otherwise);
+
+        Assert.assertEquals("Prefix should be appended", result);
+    }
+
+    @Test
+    public void appendIfNotContainEmptyString() {
+        // (Check the special-case for str being empty)
+        final String str = "";
+        final String appendStr = "should not be appended";
+        final String otherwise = "should also not be appended";
+
+        final String result =
+            StringUtils.appendIfNotContain(str, appendStr, otherwise);
+
+        Assert.assertEquals("", result);
+    }
+
+    @Test
+    public void appendIfNotContainAppendingEmptyString() {
+        // (Check the special-case for appendStr being empty)
+        final String str = "should be kept";
+        final String appendStr = "";
+        final String otherwise = "should also not be appended";
+
+        final String result =
+            StringUtils.appendIfNotContain(str, appendStr, otherwise);
+
+        Assert.assertEquals("should be kept", result);
+    }
+}