Introduction
For this hamcrest tutorial, i want to show multiple matches, which are very powerful. Therefore you can fully express your assertions. It provides a fluent and expressive way to define custom matchers for complex conditions and enhances the readability and maintainability of test code. Like always you can see all examples in my example repository.
Matchers
1. Basic Matchers
There are several types of matchers, like Object-matchers, Collection-matchers, Text-matchers and even Custom-matchers. Let’s just start with some very simple basic-matchers
String expected = "Hello";
String actual = "Hello";
assertThat(actual, equalTo(expected));Like example show, we just check if actual and expected are equal.
Other examples of some Basic matchers are as following:
| Matcher for Type | Operation |
| String | startsWith, endsWith, containsString |
| Number | greaterThan, lessThan, closeTo |
| List | hasSize, contains, hasItem |
int x = 10;
double y = 3.14;
// Number
assertThat(x, greaterThan(5));
assertThat(x, lessThan(20));
assertThat(y, closeTo(3.1, 0.1));
String sentence = "The quick brown fox jumps over the lazy dog";
// String
assertThat(sentence, startsWith("The"));
assertThat(sentence, endsWith("dog"));
assertThat(sentence, containsString("fox"));
List<String> fruits = Arrays.asList("apple", "banana", "orange");
// List
assertThat(fruits, hasSize(3));
assertThat(fruits, contains("apple", "banana", "orange"));
assertThat(fruits, hasItem("banana"));2. Object Matchers
Let’s say we have a PersonObject
@Getter
@Setter
@AllArgsConstructor
public class Person {
private String name;
}
@Test
void objectMatcher() {
Person johnDoe = new Person("John Doe");
assertThat(johnDoe, hasProperty("name", equalTo("John Doe")));
Person johnDoe2 = new Person("John Doe");
assertThat(johnDoe, samePropertyValuesAs(johnDoe2));
}With this test, we can instantly look for property name and check it’s value and don’t have to use the getter. Also a small sidenote. We can also only use hasProperty, then it only checks that the value exists.
3. Custom Matcher
Right now there are 69 Implementations of matchers that you can use. Right now i want to show you some custom-matcher for palindromes.
package org.codenest;
import org.hamcrest.Description;
import org.hamcrest.Matcher;
import org.hamcrest.TypeSafeMatcher;
public class IsPalindromeMatcher {
public static Matcher<String> isPalindrome() {
return new TypeSafeMatcher<String>() {
@Override
protected boolean matchesSafely(String item) {
String reversed = new StringBuilder(item).reverse().toString();
return item.equals(reversed);
}
@Override
public void describeTo(Description description) {
description.appendText("is a palindrome");
}
};
}
}
String palindrome = "racecar";
String notPalindrome = "hello";
assertThat(palindrome, isPalindrome());
assertThat(notPalindrome, isPalindrome()); // This assertion will fail