XmlValidator.java

import java.util.Stack;

public class XmlValidator {

	public boolean isValid(String xml) {

		Stack<String> stack = new Stack<>();

		boolean opening = false;
		boolean closing = false;
		String currentTag = "";

		for (char c : xml.toCharArray()) {

			if (c == '<') {

				if (opening == true) {
					return false;
				}

				opening = true;

			} else if (c == '/') {

				if (closing == true) {
					return false;
				}

				closing = true;

			} else if (c == '>') {

				if (!closing) {
					stack.push(currentTag);
				} else {
					if (stack.isEmpty()) {
						return false;
					}

					String lastTag = stack.pop();

					if (!currentTag.equals(lastTag)) {
						return false;
					}
				}

				currentTag = "";
				opening = false;
				closing = false;
			} else {
				currentTag += c;
			}
		}

		return stack.isEmpty();
	}

}

XmlValidatorTest.java

public class XmlValidatorTest {
	
	XmlValidator validator = new XmlValidator();
	
	@Test
	public void testNormal() {
		Assert.assertTrue(validator.isValid("<a></a>"));
	}

	@Test
	public void testTwo() {
		Assert.assertTrue(validator.isValid("<a></a><b></b>"));
	}

	@Test
	public void testEmbedded() {
		Assert.assertTrue(validator.isValid("<a><b></b></a>"));
	}

	@Test
	public void testTwoEmbedded() {
		Assert.assertTrue(validator.isValid("<a><b><c></c><d></d></b></a>"));
	}
	
	@Test
	public void testInvalid0() {
		Assert.assertFalse(validator.isValid("<a><"));
	}

	@Test
	public void testInvalid1() {
		Assert.assertFalse(validator.isValid("<a>"));
	}
	
	@Test
	public void testInvalid2() {
		Assert.assertFalse(validator.isValid("<a></"));
	}

	@Test
	public void testInvalid3() {
		Assert.assertFalse(validator.isValid("<a></a"));
	}
	
	@Test
	public void testInvalid4() {
		Assert.assertFalse(validator.isValid("<a></b>"));
	}

	@Test
	public void testInvalid5() {
		Assert.assertFalse(validator.isValid("<a><b></a>"));
	}
	
}