!84 support substring by code point

Merge pull request !84 from Ease/v5-dev

hutool-core/src/main/java/cn/hutool/core/util/StrUtil.java

@@ -1579,6 +1579,33 @@ public class StrUtil {
 		return str.toString().substring(fromIndex, toIndex);
 	}
 
+	/**
+	 * 通过CodePoint截取字符串，可以截断Emoji
+	 *
+	 * @param str String
+	 * @param fromIndex 开始的index（包括）
+	 * @param toIndex 结束的index（不包括）
+	 * @return 字串
+	 */
+	public static String subByCodePoint(CharSequence str, int fromIndex, int toIndex) {
+		if (isEmpty(str)) {
+			return str(str);
+		}
+
+		if (fromIndex < 0 || fromIndex > toIndex) {
+			throw new IllegalArgumentException();
+		}
+
+		if (fromIndex == toIndex) {
+			return EMPTY;
+		}
+
+		StringBuilder sb = new StringBuilder();
+		int subLen = toIndex - fromIndex;
+		str.toString().codePoints().skip(fromIndex).limit(subLen).forEach(v -> sb.append(Character.toChars(v)));
+		return sb.toString();
+	}
+
 	/**
 	 * 截取部分字符串，这里一个汉字的长度认为是2
 	 *

hutool-core/src/test/java/cn/hutool/core/util/StrUtilTest.java

@@ -219,6 +219,20 @@ public class StrUtilTest {
 		Assert.assertEquals("ghigh", pre);
 	}
 
+	@Test
+	public void subByCodePointTest() {
+		// 🤔👍🍓🤔
+		String test = "\uD83E\uDD14\uD83D\uDC4D\uD83C\uDF53\uD83E\uDD14";
+
+		// 不正确的子字符串
+		String wrongAnswer = StrUtil.sub(test, 0, 3);
+		Assert.assertNotEquals("\uD83E\uDD14\uD83D\uDC4D\uD83C\uDF53", wrongAnswer);
+
+		// 正确的子字符串
+		String rightAnswer = StrUtil.subByCodePoint(test, 0, 3);
+		Assert.assertEquals("\uD83E\uDD14\uD83D\uDC4D\uD83C\uDF53", rightAnswer);
+	}
+	
 	@Test
 	public void subBeforeTest() {
 		String a = "abcderghigh";